LED driving circuit

Abstract

In an LED driving circuit according to the present invention, a control signal (PS1) repeats, in an alternating pattern, a first period (T1) during which a control signal line (CL1) is at a ground potential and a second period (T2) during which the control signal line (CL1) is at a control potential. The LED driving circuit is provided with a diode (D1) and a capacitor (C1) connected sequentially in series between a power supply potential and the control signal line (CL1). A connection point (A) between the diode (D1) and the capacitor (C1) is connected to an anode terminal (90a). A switching element (Q1) connected between a cathode terminal (90b) and the ground potential is turned off when the control signal line (CL1) is at the ground potential, and is turned on when the control signal line (CL1) is at the control potential. A resistive element (R1) for current limitation is interposed between the connection point (A) and the switching element (Q1) in series with an LED (90).

Description

LED Driving Circuit

[0001] The present invention relates to an LED (Light Emitting Diode) driving circuit, and more particularly, to an LED driving circuit configured to drive an LED such as a blue LED or a white LED as a load.

[0002] In recent years, in addition to red, orange, yellow, and green LEDs with a forward voltage of about 2.1 V, blue LEDs and white LEDs with a forward voltage of about 3.5 V have come to be used in various devices. When an LED is used as an indicator in a medical device, the function of the indicator is defined by color according to the medical device standard (IEC60601-1). For example, red, yellow, green, and cyan (blue) LEDs are defined to have the function of indicating warnings, cautions, completion of preparation for use, etc., and are prohibited from being used for other general purposes. Therefore, when using an LED as a general-purpose indicator, there are also situations where a white LED has to be used.

[0003] Conventionally, in a device with a power supply voltage of less than 3.5 V, in order to drive a white LED, (1) for example, as disclosed in Patent Document 1 (Japanese Patent Application Laid-Open No. 2011-100837), a boost circuit including an inductor as a component is configured to generate a voltage for lighting the white LED, (2) a commercially available boost IC (Integrated Circuit) integrating the above functions is adopted to generate a voltage for lighting the white LED, or (3) a commercially available boost IC using a charge pump is adopted to generate a voltage for lighting the white LED. In any of these ways, a voltage for lighting the white LED is generated.

[0004] Japanese Patent Application Laid-Open No. 2011-100837

[0005] However, in method (1) above, the inductor included in the boost circuit does not have a standardized shape as a surface-mount component, unlike other common circuit components (resistors, capacitors, transistors, etc.). Therefore, if a manufacturer discontinues production of that inductor, it becomes difficult to procure a compatible replacement, and the design of the copper foil pattern on the electrical circuit board must be changed to accommodate the replacement, resulting in problems in terms of continued production. In methods (2) and (3) above, there are often no commercially available boost ICs that can be replaced without changing the copper foil pattern on the electrical circuit board, and similarly, problems in terms of continued production arise.

[0006] Therefore, the object of this invention is to provide an LED driving circuit that can be composed of general circuit components (resistors, capacitors, transistors, etc.) that do not include inductor elements, and that can drive LEDs, including white LEDs and / or blue LEDs, as loads.

[0007] To solve the above problems, in the first aspect, the LED driving circuit of this disclosure is an LED driving circuit configured to drive an LED by applying a voltage between the anode terminal and the cathode terminal of an LED as a load, comprising a first control signal supply unit that supplies a first control signal through a first control signal line, wherein the first control signal alternately repeats a first period in which the first control signal line is at ground potential and a second period in which the first control signal line is at a control potential higher than the ground potential, comprising a first charge pump circuit including a first diode and a first capacitor connected in series between the power supply potential and the first control signal line, wherein the first diode allows current to flow in the forward direction from the power supply potential to the first capacitor while blocking current in the reverse direction, the first connection point between the first diode and the first capacitor is connected to the anode terminal, comprising a switching element connected between the cathode terminal and the ground potential, wherein the switching element is turned off when the first control signal line is at ground potential and turned on when the first control signal line is at the control potential. A first current-limiting resistor element is interposed in series with the LED between the first connection point and the switching element. During the first period, a charging current flows from the power supply potential through the first diode and the first capacitor towards the first control signal line, charging the first capacitor. As a result, the first connection point and the anode terminal reach a positive charging potential equal to the power supply potential minus the forward voltage of the first diode. At the same time, the cathode terminal is isolated from the ground potential by the off-state switching element, and no driving current flows to the LED. During the second period described above, the cathode terminal becomes ground potential due to the switched element being turned on, while the first connection point and the anode terminal temporarily reach a boosted potential, which is increased from the charging potential by the control potential amount via the first capacitor. As a result, the drive current flows from the first control signal line through the first capacitor, the first resistor, the LED, and the switched element in the turned-on state toward the ground potential.The capacitance of the first capacitor is set such that the above-mentioned drive current continues to flow for more than half the length of the second period.

[0008] In this specification, the "first control signal supply unit" may include an input terminal that receives a "control signal" from outside the load drive circuit and "supply the "first control signal" received at this input terminal through a "first control signal line". Alternatively, the "first control signal supply unit" may include a circuit that creates (generates) the "first control signal" and "supply the "first control signal" it has created through a "first control signal line". The same applies to the "second control signal supply unit" and the "third control signal supply unit" described later.

[0009] "Ground potential" corresponds to zero potential. "Control potential" can take on a potential higher than zero potential up to the power supply potential, and is typically the power supply potential. The potential difference between the ground potential and the power supply potential corresponds to the power supply voltage.

[0010] "Connected" is not limited to physically direct connections, but also includes, for example, connections that allow for indirect conductivity through a conductive material.

[0011] When the anode terminal, cathode terminal, first connection point, etc. are described as "reaching a certain potential," it means that the potentials of the anode terminal, cathode terminal, first connection point, etc. are substantially at a certain potential, including when explicitly stated as "substantially" and when not explicitly stated. In this specification, "substantially" includes slight differences that do not affect the procedure (method) by which this LED driving circuit drives the LED. For example, when the switching element is composed of an NPN bipolar transistor, the collector-emitter voltage (saturation voltage) exhibited by the ON-state NPN bipolar transistor may be considered a slight difference that does not affect the procedure by which this LED driving circuit drives the LED.

[0012] In the LED driving circuit of this disclosure, the first control signal supplied by the first control signal supply unit through the first control signal line alternately repeats a first period in which the first control signal line is at ground potential and a second period in which the first control signal line is at a control potential higher than ground potential.

[0013] During the first period described above, a charging current flows from the power supply potential through the first diode and the first capacitor towards the first control signal line, charging the first capacitor. As a result, the first connection point and the anode terminal reach a positive charging potential equal to the power supply potential minus the forward voltage of the first diode. Simultaneously, the switching element, which is in the off state, isolates the cathode terminal from the ground potential, preventing any drive current from flowing to the LED.

[0014] During the second period described above, the cathode terminal becomes ground potential (zero potential) due to the switched element being turned on. Meanwhile, the first connection point and the anode terminal temporarily reach a boosted potential, which is increased from the charging potential by the control potential via the first capacitor. Here, the boosted potential is the sum of the charging potential and the control potential. Therefore, even if the power supply voltage is less than 3.5V, at the start of the second period, the potential difference between the anode terminal and the cathode terminal (which is at zero potential) can exceed 3.5V, which is necessary to drive the white LED (the specific voltage value reached will be described later). As a result, a drive current flows from the first control signal line through the first capacitor, the first resistor, the LED, and the switched element in the ON state towards the ground potential. This drive current decreases from the start of the second period as the charge stored in the first capacitor discharges.

[0015] In this LED driving circuit, it is desirable that the capacitance of the first capacitor be set such that the driving current exceeds half the length of the second period and typically continues to flow until the end. As a result, this LED driving circuit can drive LEDs, including white LEDs and / or blue LEDs, as loads.

[0016] Furthermore, the switching element may consist, for example, of an NPN bipolar transistor or an N-channel field-effect transistor and a resistive element. Therefore, this LED driving circuit may be composed of general circuit components (resistors, capacitors, transistors, etc.) that do not include an inductive element.

[0017] In one embodiment of the LED driving circuit, the resistance value of the first resistor element is set such that, at the start of the second period, the driving current becomes less than a predetermined upper limit of the forward current of the LED and less than a predetermined upper limit of the output current of the first control signal supply unit.

[0018] The "predetermined upper limit of the LED's forward current" refers, for example, to the absolute maximum rating of the pulse forward current of the LED. Similarly, the "predetermined upper limit of the output current of the first control signal supply unit" refers, for example, to the absolute maximum rating of the output current of the CPU (Central Processing Unit) output port. If the LED and CPU are common components (commercially available products, etc.), then such absolute maximum ratings are predetermined as upper limits.

[0019] In this embodiment of the LED driving circuit, the resistance value of the first resistor element is set such that, at the start of the second period, the driving current becomes less than a predetermined upper limit of the forward current of the LED and less than a predetermined upper limit of the output current of the first control signal supply unit. Therefore, during the second period, it is possible to prevent the LED and / or the first control signal supply unit from being damaged by the driving current.

[0020] In one embodiment of the LED driving circuit, a second current-limiting resistor is interposed between the first diode and the first connection point.

[0021] In this embodiment of the LED driving circuit, a second resistive element is interposed between the first diode and the first connection point. The resistance value of the second resistive element can be set, for example, so that the current flowing from the power supply potential to the first control signal line (i.e., the charging current to the first capacitor) at the start of the first period is less than a predetermined upper limit of the output current of the first control signal supply unit. This prevents the first control signal supply unit from being damaged or otherwise destroyed by the charging current to the first capacitor during the first period.

[0022] In one embodiment of the LED driving circuit, the first resistive element is interposed in series with the first capacitor between the first connection point and the first control signal line, instead of between the first connection point and the switching element.

[0023] In this embodiment of the LED driving circuit, the first resistor element is interposed in series with the first capacitor between the first connection point and the first control signal line, instead of between the first connection point and the switching element. The resistance value of the first resistor element can be set so that, at the start of the first period, the current flowing from the power supply potential to the first control signal line (i.e., the charging current to the first capacitor) is less than a predetermined upper limit of the output current of the first control signal supply unit. This prevents the first control signal supply unit from being damaged by the charging current to the first capacitor during the first period. Furthermore, the resistance value of the first resistor element can be set so that, at the start of the second period, the driving current is less than a predetermined upper limit of the forward current of the LED and less than a predetermined upper limit of the output current of the first control signal supply unit. This prevents the LED and / or the first control signal supply unit from being damaged by the driving current during the second period.

[0024] In one embodiment of the LED driving circuit, the combined period of the first control signal, including the first period and the second period, is in the range of 100 microseconds to 10 milliseconds, and the second period has a length corresponding to a range of 1 / 2 to 1 / 20 of the period.

[0025] In this embodiment of the LED driving circuit, the combined period of the first and second periods of the first control signal is within the range of 100 microseconds to 10 milliseconds, and the second period is preferably within the range of half to 1 / 20 of the period, typically corresponding to 1 / 10. This allows the LED to be perceived as lit while it is being driven (conversely, it is not perceived as the LED blinking). If the generation and non-generation of the first control signal are repeated with a longer period, for example, from 0.5 seconds to 1.0 second, the LED may be perceived as blinking. Furthermore, if the length of the second period is less than 1 / 20 of the period, it becomes difficult to ensure sufficient brightness of the LED, but if the length of the second period is 1 / 20 or more of the period, sufficient brightness of the LED can be ensured.

[0026] In one embodiment of the LED driving circuit, the control potential is equal to the power supply potential, the first diode is a Schottky barrier diode, the switching element is an NPN bipolar transistor or an N-channel field-effect transistor, and when the switching element is an NPN bipolar transistor, a base resistor is interposed between the first control signal line and the base terminal of the NPN bipolar transistor, while when the switching element is an N-channel field-effect transistor, a gate-source resistor is connected between the source terminal and the gate terminal of the N-channel field-effect transistor.

[0027] In this embodiment of the LED driving circuit, since the control potential is equal to the power supply potential, the boosted potential can be further increased compared to the case where the control potential is between the power supply potential and the ground potential. As a result, the load can be driven at a higher voltage. Specifically, when the ground potential is zero, the power supply potential is Vcc, and the forward voltage of the diode is Vf, the boosted potential can be substantially set to (2Vcc - Vf).

[0028] Furthermore, Schottky barrier diodes generally have a lower forward voltage than PN junction diodes. Typically, the forward voltage of a PN junction diode is 0.5 to 0.6 V, while the forward voltage of a Schottky barrier diode is Vf ≈ 0.2 V. Therefore, if the first diode is a Schottky barrier diode, the boosted potential can be further increased.

[0029] For example, if the power supply potential and the control potential are Vcc = 3V (i.e., equivalent to two 1.5V dry cell batteries), and the forward voltage of the first diode (in this case, a Schottky barrier diode) is Vf ≈ 0.2V, then at the start of the second period, the boosted potential reached by the anode terminal will be (2Vcc - Vf) ≈ 5.8V. In other words, at the start of the second period, the potential difference between the anode terminal and the cathode terminal (which is at zero potential) can exceed the 3.5V required to drive the white LED.

[0030] Furthermore, in this embodiment of the LED driving circuit, when the first control signal line is at ground potential, the NPN bipolar transistor or N-channel field-effect transistor, which acts as a switching element, is stably turned off. On the other hand, when the first control signal line is at control potential, the NPN bipolar transistor or N-channel field-effect transistor, which acts as a switching element, is stably turned on. The collector-emitter voltage (saturation voltage) exhibited by the NPN bipolar transistor in the ON state, the drain-source voltage exhibited by the N-channel field-effect transistor in the ON state, etc., can be ignored.

[0031] Furthermore, since the switching element consists of an NPN bipolar transistor or an N-channel field-effect transistor and a resistive element, this LED driving circuit can be composed of general circuit components (resistors, capacitors, transistors, etc.) that do not include an inductive element.

[0032] In one embodiment of the LED driving circuit, the load includes a plurality of LEDs, the first connection point is connected to the anode terminals of the plurality of LEDs, the first resistive element is interposed between the first connection point and the anode terminals of the plurality of LEDs, each switching element is interposed between the cathode terminals of the plurality of LEDs and the ground potential, each switching element is provided with a third control signal line corresponding to each switching element, each switching element is turned off when the corresponding third control signal line is at the ground potential instead of the first control signal line, and turned on when the corresponding third control signal line is at the control potential, and the circuit is characterized by having a third control signal supply unit that performs potential control, keeping all of the third control signal lines at the ground potential during the first period, and keeping the third control signal line corresponding to the LED to be lit at the control potential and the third control signal line corresponding to the LED to be turned off at the ground potential during the second period.

[0033] In this embodiment of the LED driving circuit, the load includes a plurality of LEDs. The first connection point is connected to the anode terminals of the plurality of LEDs. The first resistive element is interposed between the first connection point and the anode terminals of the plurality of LEDs. Separate switching elements are interposed between the cathode terminals of the plurality of LEDs and the ground potential. A separate third control signal line is provided corresponding to each switching element. Each switching element is configured to turn off when the corresponding third control signal line is at the ground potential, and to turn on when the corresponding third control signal line is at the control potential, instead of the first control signal line. The third control signal supply unit maintains all of the third control signal lines at the ground potential during the first period, and during the second period, performs potential control to maintain the third control signal line corresponding to the LED to be lit at the control potential and the third control signal line corresponding to the LED to be turned off at the ground potential.

[0034] During the first period described above, a charging current flows from the power supply potential through the first diode and the first capacitor towards the first control signal line, charging the first capacitor. As a result, the first connection point and the anode terminals of the multiple LEDs reach a positive charging potential equal to the power supply potential minus the forward voltage of the first diode. Simultaneously, the third control signal line is kept at ground potential (zero potential), so each switching element turns off. With each switching element in the off state, the cathode terminals of the multiple LEDs are isolated from ground potential, and no drive current flows to any of the multiple LEDs.

[0035] During the second period described above, the third control signal line corresponding to the LED to be turned off among the multiple LEDs is kept at ground potential (zero potential), and as a result, the switching elements corresponding to that / their third control signal lines are all kept off. Therefore, no drive current flows to the LEDs corresponding to the switching elements that are kept off (i.e., the LEDs to be turned off).

[0036] On the other hand, during the second period, the third control signal line corresponding to the LED to be lit among the plurality of LEDs is maintained at the control potential, thereby turning on the switching element corresponding to that / their third control signal line. The cathode terminal of the LED corresponding to the turned-on switching element becomes the ground potential (zero potential). On the other hand, the first connection point and the anode terminals of the plurality of LEDs temporarily reach a boosted potential, which is increased from the charging potential by the amount of the control potential via the first capacitor. For example, if the power supply potential and the control potential are Vcc = 3V (i.e., equivalent to two 1.5V dry cell batteries) and the forward voltage of the first diode (here, a Schottky barrier diode) is Vf ≈ 0.2V, then at the start of the second period, the boosted potential reached by the anode terminal of the LED to be lit is (2Vcc - Vf) ≈ 5.8V. Therefore, at the start of the second period, the potential difference between the anode terminal and the cathode terminal (which is at zero potential) of the LED to be lit is (2Vcc - Vf) ≈ 5.8V. In other words, at the start of the second period, the potential difference between the anode terminal and the cathode terminal of the LED to be lit may exceed the 3.5V required to drive the white LED. As a result, a drive current flows from the first control signal line through the first capacitor, the first resistor, the LED to be lit, and the switching element in the ON state toward the ground potential. This drive current decreases from the start of the second period as the charge stored in the first capacitor discharges.

[0037] In this LED driving circuit, it is desirable that the capacitance of the first capacitor be set such that the driving current exceeds half the length of the second period and typically continues to flow until the end. As a result, according to this embodiment of the LED driving circuit, multiple LEDs, including white LEDs and / or blue LEDs, can be driven as a load while selecting which LEDs to light up.

[0038] Furthermore, each switching element may consist of, for example, an NPN bipolar transistor and a resistive element. Therefore, the LED driving circuit of this embodiment may be composed of general circuit components (resistors, capacitors, transistors, etc.) that do not include inductive elements.

[0039] In the second aspect, the LED driving circuit of this disclosure is an LED driving circuit configured to drive an LED by applying a voltage between the anode terminal and the cathode terminal of an LED as a load, comprising: a first control signal supply unit that supplies a first control signal through a first control signal line, wherein the first control signal alternates between a first period in which the first control signal line is at ground potential and a second period in which the first control signal line is at a control potential higher than ground potential; a second control signal supply unit that supplies a second control signal through a second control signal line, wherein the second control signal alternates between a first period in which the second control signal line is at the control potential and a second period in which the second control signal line is at ground potential, in the opposite phase to the first control signal; and a first charge pump circuit including a first diode and a first capacitor connected in series between the power supply potential and the first control signal line, wherein the first diode allows current to flow in the forward direction from the power supply potential to the first capacitor, while blocking current in the reverse direction. The first connection point between the first diode and the first capacitor is connected to the anode terminal, and the second charge pump circuit includes a second diode and a second capacitor connected in series between the ground potential and the second control signal line, wherein the second diode allows current to flow in the forward direction from the second capacitor to the ground potential while blocking current in the reverse direction, the second connection point between the second diode and the second capacitor is connected to the cathode terminal, and the first current-limiting resistor element is interposed in series with respect to the LED between the first connection point and the second connection point.During the first period described above, a charging current flows from the power supply potential through the first diode and the first capacitor towards the first control signal line, charging the first capacitor. As a result, the first connection point and the anode terminal reach a positive charging potential equal to the power supply potential minus the forward voltage of the first diode. Simultaneously, a charging current flows from the second control signal line through the second capacitor and the second diode towards the ground potential, charging the second capacitor. As a result, the second connection point and the cathode terminal reach a voltage equal to the forward voltage of the second diode. During the second period described above, the first connection point and the anode terminal temporarily reach a boosted potential obtained by raising the charge potential by the control potential amount via the first capacitor, and the second connection point and the cathode terminal temporarily reach a step-down potential obtained by lowering the forward voltage of the second diode by the control potential amount via the second capacitor, thereby causing the drive current to flow from the first control signal line through the first capacitor, the first resistor element, the LED, and the second capacitor to the second control signal line, and the capacitances of the first and second capacitors are set such that the drive current continues to flow for more than half the length of the second period described above.

[0040] In the LED driving circuit of this disclosure, the first control signal supplied by the first control signal supply unit through the first control signal line alternately repeats a first period in which the first control signal line is at ground potential and a second period in which the first control signal line is at a control potential higher than ground potential. The second control signal supplied by the second control signal supply unit through the second control signal line alternately repeats a first period in which the second control signal line is at the control potential and a second period in which the second control signal line is at ground potential, in the opposite phase to the first control signal.

[0041] During the first period described above, a charging current flows from the power supply potential through the first diode and the first capacitor towards the first control signal line, charging the first capacitor. As a result, the first connection point and the anode terminal reach a positive charging potential equal to the power supply potential minus the forward voltage of the first diode. Simultaneously, a charging current flows from the second control signal line through the second capacitor and the second diode towards the ground potential, charging the second capacitor. As a result, the second connection point and the cathode terminal reach a voltage equal to the forward voltage of the second diode.

[0042] During the second period described above, the second connection point and the cathode terminal temporarily reach a stepped-down potential obtained by stepping down the forward voltage of the second diode by the control potential amount via the second capacitor. At the same time, the first connection point and the anode terminal temporarily reach a boosted potential obtained by stepping up the charging potential by the control potential amount via the first capacitor. For example, if the power supply potential and the control potential are Vcc = 1.5V (i.e., equivalent to one 1.5V dry cell battery), and the forward voltages of both the first diode and the second diode are Vf ≈ 0.2V, then at the start of the second period, the stepped-down potential reached by the cathode terminal will be (Vf - Vcc) ≈ -1.3V. On the other hand, the boosted potential reached by the anode terminal will be (2Vcc - Vf) ≈ 2.8V. Therefore, at the start of the second period, the potential difference between the anode terminal and the cathode terminal is (2Vcc - Vf) - (Vf - Vcc) = (3Vcc - 2Vf) ≈ 4.1V. In other words, at the start of the second period, the potential difference between the anode terminal and the cathode terminal may exceed 3.5V, which is necessary to drive the white LED. As a result, the drive current flows from the first control signal line through the first capacitor, the first resistor, the LED, and the second capacitor to the second control signal line. This drive current decreases from the start of the second period as the charge stored in the first and second capacitors discharges.

[0043] Here, in this LED drive circuit, it is desirable that the capacitance values of the first capacitor and the second capacitor are set such that the drive current continues to flow beyond half the length of the second period, typically until the end. Thereby, according to this LED drive circuit, an LED including a white LED and / or a blue LED can be driven as a load.

[0044] Also, this LED drive circuit can be composed of general circuit components (resistive elements, capacitors, transistors, etc.) that do not include an inductor element.

[0045] In the LED drive circuit of one embodiment, the second control signal supply unit includes an inverter controlled by the first control signal and inserted between the power supply potential and the ground potential, and is configured to supply the output of this inverter as the second control signal through the second control signal line.

[0046] In the LED drive circuit of this one embodiment, the second control signal supply unit includes an inverter controlled by the first control signal and inserted between the power supply potential and the ground potential, and supplies the output of this inverter as the second control signal through the second control signal line. Therefore, control is not performed by the two units of the first control signal supply unit and the second control signal supply unit, but substantially only by the first control signal supply unit. Therefore, the control can be simplified.

[0047] In the LED driving circuit according to one embodiment, the load includes a plurality of LEDs. The first connection point is connected to the anode terminals of the plurality of LEDs. The first resistor element is inserted between the first connection point and the anode terminals of the plurality of LEDs. Different switching elements are respectively inserted between the cathode terminals of the plurality of LEDs and the second connection point. Different third control signal lines are provided corresponding to each switching element. Each switching element is turned off when the corresponding third control signal line is at the ground potential and is turned on when the corresponding third control signal line is at the control potential. In the first period, all the third control signal lines are kept at the ground potential, and in the second period, a third control signal supply unit is provided which performs potential control to keep the third control signal line corresponding to the LED to be lit among the plurality of LEDs at the control potential and keep the third control signal line corresponding to the LED to be turned off at the ground potential.

[0048] In the LED driving circuit according to this one embodiment, the load includes a plurality of LEDs. The first connection point is connected to the anode terminals of the plurality of LEDs. The first resistor element is inserted between the first connection point and the anode terminals of the plurality of LEDs. Different switching elements are respectively inserted between the cathode terminals of the plurality of LEDs and the second connection point. Different third control signal lines are provided corresponding to each switching element. Each switching element is turned off when the corresponding third control signal line is at the ground potential and is turned on when the corresponding third control signal line is at the control potential. The third control signal supply unit keeps all the third control signal lines at the ground potential in the first period and performs potential control to keep the third control signal line corresponding to the LED to be lit among the plurality of LEDs at the control potential and keep the third control signal line corresponding to the LED to be turned off at the ground potential in the second period.

[0049] During the first period described above, a charging current flows from the power supply potential through the first diode and the first capacitor towards the first control signal line, charging the first capacitor. As a result, the first connection point and the anode terminals of the multiple LEDs reach a positive charging potential equal to the power supply potential minus the forward voltage of the first diode. Simultaneously, a charging current flows from the second control signal line through the second capacitor and the second diode towards the ground potential, charging the second capacitor. As a result, the second connection point reaches a voltage equal to the forward voltage of the second diode. Simultaneously, since all three control signal lines are maintained at ground potential (zero potential), each switching element turns off. With each switching element in the off state, the cathode terminals of the multiple LEDs are electrically isolated from the second connection point, and no drive current flows to any of the multiple LEDs.

[0050] During the second period described above, the third control signal line corresponding to the LED to be turned off among the multiple LEDs is kept at ground potential (zero potential), and as a result, the switching elements corresponding to that / their third control signal lines are all kept off. Therefore, no drive current flows to the LEDs corresponding to the switching elements that are kept off (i.e., the LEDs to be turned off).

[0051] Meanwhile, during the second period, the third control signal line corresponding to the LED to be lit among the plurality of LEDs is maintained at the control potential, thereby turning on the switching element corresponding to that / their third control signal line. The cathode terminal of the LED corresponding to the turned-on switching element becomes equal to the potential of the second connection point. At the same time, the second connection point (and the cathode terminal of the LED to be lit) temporarily reaches a stepped-down potential, which is obtained by stepping down the forward voltage of the second diode by the amount of the control potential via the second capacitor. Meanwhile, both the first connection point and the anode terminals of the plurality of LEDs temporarily reach a stepped-up potential, which is obtained by stepping up the charge potential by the amount of the control potential via the first capacitor. For example, if the power supply potential and the control potential are Vcc = 3V (i.e., equivalent to two 1.5V dry cell batteries), and the forward voltages of the first diode and the second diode are both Vf ≈ 0.3V, then at the start of the second period, the step-down potential reached by the second connection point and the cathode terminal of the LED to be lit will be (Vf - Vcc) ≈ -2.7V. On the other hand, the step-up potential reached by the anode terminal will be (2Vcc - Vf) ≈ 5.7V. Therefore, at the start of the second period, the potential difference between the anode terminal and the cathode terminal of the LED to be lit will be (2Vcc - Vf) - (Vf - Vcc) = (3Vcc - 2Vf) ≈ 8.4V. In other words, at the start of the second period, the potential difference between the anode terminal and the cathode terminal of the LED to be lit may exceed 3.5V, which is necessary to drive the white LED. As a result, a drive current flows from the first control signal line through the first capacitor, the first resistor, the LED to be lit, and the second capacitor to the second control signal line. This drive current decreases from the start of the second period as the charge stored in the first capacitor discharges.

[0052] In this LED driving circuit, it is desirable that the capacitances of the first and second capacitors are set such that the driving current exceeds half the length of the second period and typically continues to flow until the end. As a result, this LED driving circuit can drive multiple LEDs, including white LEDs and / or blue LEDs, as a load while selecting which LEDs to light up.

[0053] Furthermore, each switching element may consist of, for example, an NPN field-effect transistor and a resistive element. Therefore, the LED driving circuit of this embodiment may be composed of general circuit components (resistors, capacitors, transistors, etc.) that do not include inductive elements.

[0054] In the first or second aspect, in the LED driving circuit of one embodiment, the third control signal supply unit comprises a multiplexer including a plurality of input terminals and a number of output terminals greater than the number of input terminals, and the multiplexer is configured to receive the third control signals supplied to the third control signal lines at the corresponding input terminals and to control the on / off state of a number of switching elements greater than the number of third control signal lines through the output terminals.

[0055] In this embodiment of the LED driving circuit, the multiplexer included in the third control signal supply unit receives the third control signals supplied to the third control signal lines at their corresponding input terminals and controls the on / off state of a number of switching elements greater than the number of third control signal lines through the output terminals. Therefore, it is possible to save wiring space compared to the case where the same number of third control signal lines as the number of switching elements are provided.

[0056] As is clear from the above, the LED driving circuit of this disclosure may consist of common circuit components (such as resistors, capacitors, and transistors) that do not include inductor elements, and can drive LEDs, including white LEDs and / or blue LEDs, as loads.

[0057] Figure 1 shows the configuration of an LED driving circuit according to the first embodiment of this invention. Figure 2 shows the change in the potential of the first control signal in the LED driving circuit of Figure 1 over time (hereinafter referred to as the "potential waveform"). Figure 3 shows the operation of the LED driving circuit of Figure 1 during the first period when the first control signal line is at ground potential. Figure 4 shows the operation of the LED driving circuit of Figure 1 during the second period when the first control signal line is at control potential. Figure 5(A) is a diagram that specifically illustrates the potential waveform of the first control signal in the LED driving circuit of Figure 1. Figure 5(B) is a diagram that specifically illustrates the charging current to the first capacitor included in the LED driving circuit of Figure 1. Figure 6(A) is a diagram that specifically illustrates the terminal voltage of the first capacitor included in the LED driving circuit of Figure 1. Figure 6(B) is a diagram that specifically illustrates the potential of the first connection point in the LED driving circuit of Figure 1. Figure 7(A) is a diagram that specifically illustrates the driving current flowing to the LED as a load by the LED driving circuit of Figure 1. Figure 7(B) is a diagram illustrating the relationship between the drive current flowing through the LED and the capacitance of the first capacitor in the LED drive circuit of Figure 1. This figure shows the configuration of the LED drive circuit of the second embodiment of the present invention. This figure illustrates the operation of the LED drive circuit of Figure 8 during the first period when the first control signal line is at ground potential. This figure illustrates the operation of the LED drive circuit of Figure 8 during the second period when the first control signal line is at control potential. Figure 11(A) is a diagram illustrating the potential waveform of the first control signal in the LED drive circuit of Figure 8. Figure 11(B) is a diagram illustrating the waveform of the charging current to the first capacitor included in the LED drive circuit. Figure 12(A) is a diagram illustrating the waveform of the terminal voltage of the first capacitor included in the LED drive circuit of Figure 8. Figure 12(B) is a diagram illustrating the potential waveform of the first connection point in the LED drive circuit of Figure 8. Figure 13(A) is a diagram illustrating the waveform of the drive current flowing through the LED as a load by the LED drive circuit of Figure 8. Figure 13(B) is a diagram illustrating the relationship between the drive current flowing through the LED and the capacitance of the first capacitor in the LED drive circuit of Figure 8. This diagram shows the configuration of the LED drive circuit of the third embodiment of this invention.Figure 14 is a diagram comparing the potential waveform of the first control signal and the potential waveform of the second control signal in the LED driving circuit of Figure 14. This diagram explains the operation of the LED driving circuit of Figure 14 during the first period when the first control signal line is at ground potential and the second control signal line is at control potential. This diagram explains the operation of the LED driving circuit of Figure 14 during the second period when the first control signal line is at control potential and the second control signal line is at ground potential. Figure 18(A) is a diagram specifically illustrating the potential waveform of the first control signal in the LED driving circuit of Figure 14. Figure 18(B) is a diagram specifically illustrating the waveform of the charging current to the first capacitor included in the LED driving circuit of Figure 14. Figure 19(A) is a diagram specifically illustrating the potential waveform of the second control signal in the LED driving circuit of Figure 14. Figure 19(B) is a diagram specifically illustrating the waveform of the charging current to the second capacitor included in the LED driving circuit of Figure 14. Figure 20(A) is a diagram specifically illustrating the waveform of the terminal voltage of the first capacitor included in the LED driving circuit of Figure 14. Figure 20(B) is a diagram specifically illustrating the waveform of the terminal voltage of the second capacitor included in the LED driving circuit of Figure 14. Figure 21(A) is a diagram specifically illustrating the waveform of the potential difference between the first connection point and the second connection point in the LED driving circuit of Figure 14. Figure 21(B) is a diagram specifically illustrating the waveform of the drive current flowing through the LED as a load by the LED driving circuit of Figure 14. This is a diagram showing the configuration of the LED driving circuit of the fourth embodiment of this invention. This is a diagram showing the configuration of the LED driving circuit of the fifth embodiment of this invention. This is a diagram illustrating the operation of the LED driving circuit of Figure 23 during the first period when the first control signal line is at ground potential. This is a diagram illustrating the operation of the LED driving circuit of Figure 23 during the second period when the first control signal line is at control potential. This is a diagram showing the configuration of the LED driving circuit of the sixth embodiment of this invention. This is a diagram showing the configuration of the LED driving circuit of the seventh embodiment of this invention. Figures 28(A) to 28(D) show the potential waveforms of the first and third control signals in the LED driving circuit of Figure 26. Figures 28(E) to (G) show the potential waveforms of the input control signals for the multiplexer included in the LED driving circuit of Figure 27. This figure shows the configuration of the LED driving circuit of the eighth embodiment of this invention.This figure illustrates a specific comparison of the gate-source voltage waveforms of the NMOSFETs corresponding to the LEDs to be lit and the gate-source voltage waveforms of the NMOSFETs corresponding to the LEDs to be turned off, as included in the LED driving circuit shown in Figure 29. This figure shows the configuration of the LED driving circuit according to the ninth embodiment of this invention.

[0058] The embodiments of this invention will now be described in detail with reference to the drawings.

[0059] (First Embodiment) (Circuit Configuration) Figure 1 shows the configuration of the LED driving circuit 100 according to the first embodiment of the present invention. This LED driving circuit 100 is a circuit for driving an LED 90 by applying a voltage between the anode terminal 90a and cathode terminal 90b of the LED 90 which is a load. In this example, the LED 90 is a white LED, and the forward voltage required for driving is assumed to be about 3.5V. However, a blue LED or an LED of another color may also be used.

[0060] This LED driving circuit 100 includes a first control signal supply unit 11 and a first charge pump circuit 21.

[0061] In this example, the first control signal supply unit 11 is an input terminal that receives the first control signal PS1 from outside the LED driving circuit 100 (in this example, consisting of an output port of a CPU, not shown in the figure), and supplies the first control signal PS1 received at this input terminal through the first control signal line CL1. In this example, as shown in Figure 2, as time t progresses, the first control signal PS1 is a rectangular wave signal that alternately repeats a first period T1 in which the first control signal line CL1 is at ground potential GND (corresponding to zero potential) and a second period T2 in which the first control signal line CL1 is at a control potential H higher than ground potential GND. In this example, the control potential H is set to the power supply potential Vcc.

[0062] The first charge pump circuit 21 shown in Figure 1 includes a diode D1 connected in series between the power supply potential Vcc and the first control signal line CL1, a second resistor R2 for current limiting, and a capacitor C1. Diode D1 allows current to flow when biased forward from the power supply potential Vcc towards capacitor C1, and blocks current when biased in the reverse direction. In this example, diode D1 is a Schottky barrier diode. Therefore, diode D1 has a smaller forward voltage Vf compared to a PN junction diode. Typically, Vf ≈ 0.2V (however, the forward voltage Vf depends on the drive current).

[0063] In this example, the first connection point A between the second resistor R2 and the first capacitor C1 is connected to the anode terminal 90a of the LED 90 via the first resistor R1 for current limiting. In this example, the first resistor R1 is interposed between the first connection point A and the anode terminal 90a of the LED 90, but it may instead be interposed between the cathode terminal 90b of the LED 90 and the transistor Q1.

[0064] An NPN bipolar transistor Q1 (hereinafter referred to as "transistor Q1" as appropriate) is provided as a switching element between the cathode terminal 90b of the LED 90 and the ground potential GND. In this example, the collector terminal Q1c, emitter terminal Q1e, and base terminal Q1b of transistor Q1 are connected to the cathode terminal 90b of the LED 90, the ground potential GND, and the first control signal line CL1, respectively. As a result, transistor Q1 is turned off when the first control signal line CL1 is at the ground potential GND, and turned on when the first control signal line CL1 is at the control potential H.

[0065] In this example, transistor Q1 is a digital transistor with a base-emitter resistor Qr2 connected between its emitter terminal Q1e and base terminal Q1b, and a base resistor Qr1 interposed between the base terminal Q1b and the first control signal line CL1. This stabilizes the switching operation of transistor Q1 and allows for a simple configuration of the switching element.

[0066] (Circuit Operation) When the LED driving circuit 100 is in operation, as shown in Figure 2, the first control signal PS1 supplied by the first control signal supply unit 11 through the first control signal line CL1 alternately repeats a first period T1 in which the first control signal line CL1 is at ground potential GND (=0) and a second period T2 in which the first control signal line CL1 is at a control potential H (=Vcc) higher than ground potential GND. In this example, the length of the first period T1 is 90 μsec, and the length of the second period T2 is 10 μsec. For simplicity, below, the sign of a capacitor and its capacitance value will be represented by the same sign, and the sign of a resistor and its resistance value will be represented by the same sign.

[0067] Figure 3 shows the operation of the LED driving circuit 100 during a first period T1 when the first control signal line CL1 is at ground potential GND (=0). During this first period T1, no base current flows through the transistor Q1, which acts as a switching element, and the transistor Q1 is turned off. During this first period T1, a charging current CH flows from the power supply potential Vcc through the first diode D1, the second resistor R2, and the first capacitor C1 in sequence towards the first control signal line CL1, charging the first capacitor C1. At this time, the time constant formed by the capacitance value of the first capacitor C1 and the resistance value of the second resistor R2 is assumed to be sufficiently smaller than the length of the first period T1. As a result, at the end of the first period T1, the first connection point A, the first resistor R1, and the anode terminal 90a of the LED 90 reach a positive charging potential (Vcc - Vf) equal to the power supply potential Vcc minus the forward voltage Vf of the first diode D1. The charge across the terminals of the first capacitor C1 is C1 × (Vcc - Vf). At the same time, the off-state transistor Q1 isolates the cathode terminal 90b of the LED 90 from the ground potential GND, so no drive current flows through the LED 90.

[0068] Figure 4 shows the operation of the LED driving circuit 100 during the second period T2, when the first control signal line CL1 is at the control potential H (=Vcc). During the second period T2, a base current Ib flows through the transistor Q1, which acts as a switching element, and the cathode terminal 90b of the LED 90 is turned on by the transistor Q1, bringing it to ground potential GND (=0). On the other hand, due to the boosting effect of the first charge pump circuit 21, at the start of the second period T2, the first connection point A, the first resistor R1, and the anode terminal 90a of the LED 90 temporarily reach a boosted potential (Vcc - Vf + H) = (2Vcc - Vf), which is increased by the control potential H via the first capacitor C1 from the charging potential (Vcc - Vf). Therefore, even if the power supply voltage is less than 3.5V, at the start of the second period T2, the potential difference between the anode terminal 90a and the cathode terminal 90b (which is at zero potential) may exceed 3.5V, which is necessary to drive the white LED 90 (the specific voltage value reached will be described later). At this time, the first diode D1 is reverse-biased, blocking the current from the first connection point A to the power supply potential Vcc. As a result, the drive current DH flows from the first control signal line CL1 through the first capacitor C1, the first resistor R1, the LED 90, and the on-state transistor Q1 towards the ground potential GND. This drive current DH decreases from the start of the second period T2 as the charge stored in the first capacitor C1 discharges.

[0069] In this LED driving circuit 100, it is desirable that the capacitance of the first capacitor C1 is set such that the driving current DH exceeds half the length of the second period T2 and continues to flow until the end of this example. As a result, this LED driving circuit 100 can drive an LED 90 including a white LED and / or a blue LED as a load. In this example, the length of the first period T1 is 90 μsec and the length of the second period T2 is 10 μsec, so the LED 90 is perceived as being lit while it is being driven (conversely, it is not perceived as the LED 90 blinking). However, if the generation and non-generation of the first control signal PS1 are repeated at a longer period, for example, from 0.5 seconds to 1.0 seconds, the LED 90 may be perceived as blinking.

[0070] Furthermore, in this example, the transistor Q1, which acts as a switching element, can be composed of an NPN bipolar transistor and a resistive element (a base-emitter resistor Qr2 and a base resistor Qr1 that form a digital transistor). Therefore, this LED driving circuit 100 can be composed of general circuit components (resistors, capacitors, transistors, etc.) that do not include an inductive element.

[0071] (Specific Example) For example, the power supply potential Vcc is set to Vcc = 3V, which is the voltage of two 1.5V dry cell batteries. The LED 90 used as a load is a white LED (part number NSPW500BS) manufactured by Nichia Corporation. The first diode D1 is a Schottky diode (part number RB160MM-60) manufactured by Rohm Co., Ltd. Transistor Q1 is a digital transistor (part number DTC043E) manufactured by Rohm Co., Ltd. The capacitance of the first capacitor C1 is 0.47μF.

[0072] The first resistor R1 is set to 47Ω. The second resistor R2 is set to 270Ω. The values ​​of these first and second resistors R1 and R2 are set as follows. That is, during the first period T1, a maximum charging current CH flows (Vcc - Vf) / R2. The resistance value of the second resistor R2 is selected from commercially available products so that this maximum current value is less than the absolute maximum rating of the output current of the first control signal supply unit 11 (typically 10mA). This prevents the first control signal supply unit 11 from being damaged by the charging current CH to the first capacitor C1 during the first period T1. During the second period T2, a maximum drive current DH flows {(2Vcc - Vf) - Vfled} / R1 (where Vfled is the forward voltage of the LED 90 when energized). The resistance value of the first resistor R1 is selected from commercially available products so that this maximum current value is less than the absolute maximum rating (approximately several tens of mA) of the forward current of the LED 90 (in this example, the pulse forward current) and less than the absolute maximum rating (typically 10 mA) of the output current of the first control signal supply unit 11. This prevents the LED 90 and / or the first control signal supply unit 11 from being damaged by the drive current DH during the second period T2.

[0073] Figure 5(A) specifically illustrates the potential waveform of the first control signal PS1 of the LED driving circuit 100. Figure 5(B) specifically illustrates the waveform of the charging current CH for the first capacitor C1 included in the LED driving circuit 100. The charging current CH rises sharply to approximately 10 mA at the start of the first period T1, and gradually approaches zero and saturates at the end of the first period T1.

[0074] Figure 6(A) specifically illustrates the waveform of the terminal voltage VC1 of the first capacitor C1 in the LED driving circuit 100. At the start of the first period T1, the terminal voltage VC1 drops from approximately 3.0V to zero, then immediately rises from zero, and gradually approaches (Vcc - Vf) and saturates at the end of the first period T1. In this example, (Vcc - Vf) ≈ 2.8V.

[0075] Figure 6(B) specifically illustrates the waveform of the potential VA1 at the first connection point A in the LED driving circuit 100. The potential VA1 at the first connection point A rises from zero at the start of the first period T1, gradually approaches (Vcc - Vf) and saturates at the end of the first period T1, and at the start of the second period T2, it temporarily reaches the boosted potential (2Vcc - Vf). In this example, (2Vcc - Vf) ≈ 5.8V. In other words, the boosted potential (2Vcc - Vf) exceeds 3.5V, which is required to drive the white LED 90.

[0076] Figure 7(A) specifically illustrates the waveform of the drive current DH supplied to the LED 90 by the LED drive circuit 100. The drive current DH rises sharply to approximately 42 mA at the start of the second period T2, and then decreases as the charge stored in the first capacitor C1 discharges. In this example, it is desirable that the capacitance of the first capacitor C1 be set such that the drive current DH continues to flow for more than half the length of the second period T2, and continues until the end of this example. In this example, the drive current DH remains even at the end of the second period T2, continuing to flow at approximately 2 mA.

[0077] Figure 7(B) specifically illustrates the relationship between the drive current DH flowing through the LED 90 and the capacitance of the first capacitor C1 in the LED drive circuit 100. For example, as the capacitance value of the first capacitor C1 increases sequentially from 0.47 μF to 1 μF to 2.2 μF, the magnitude of the remaining drive current DH at the end of the second period T2 increases, as shown by arrow E1 in Figure 7(B). The waveform of the drive current DH of the LED 90 changes from a triangular wave to a trapezoidal wave and gradually approaches a square wave. In this way, by appropriately setting the capacitance of the first capacitor C1, the drive current DH can be made to continue flowing until the end of the second period T2.

[0078] Furthermore, instead of an NPN bipolar transistor, an N-channel field-effect transistor may be used as the switching element transistor Q1. In that case, a gate-source resistor is connected between the source and gate terminals of the N-channel field-effect transistor. This enables stable on / off switching.

[0079] (Second Embodiment) (Circuit Configuration) Figure 8 shows the configuration of the LED driving circuit 200 of the second embodiment of the present invention. This LED driving circuit 100 is a circuit for driving an LED 90 by applying a voltage between the anode terminal 90a and cathode terminal 90b of the LED 90 which is a load, similar to the first embodiment. In this example, the LED 90 is a white LED, and the forward voltage required for driving is assumed to be about 3.5V. However, a blue LED or an LED of another color may also be used.

[0080] In this LED driving circuit 200, the first resistive element (represented by the symbol R1') is interposed in series with the first capacitor C1 between the first connection point A and the first control signal line CL1, instead of between the first connection point A and the transistor Q1 as a switching element in the first embodiment. In this example, the first resistive element R1' is interposed between the first capacitor C1 and the first control signal line CL1, but it may also be interposed between the first connection point A and the first capacitor C1. The second resistive element R2 is omitted. In other words, the first resistive element R1 and the second resistive element R2 in the first embodiment are made common as a single component by the first resistive element R1'.

[0081] The other circuit configurations are the same as those in the first embodiment, and redundant explanations are omitted using the same reference numerals.

[0082] (Circuit Operation) When the LED driving circuit 200 is in operation, the first control signal PS1 supplied by the first control signal supply unit 11 through the first control signal line CL1 alternately repeats a first period T1 in which the first control signal line CL1 is at ground potential GND (=0) and a second period T2 in which the first control signal line CL1 is at a control potential H (=Vcc) higher than ground potential GND. In this example, the length of the first period T1 is 90 μsec and the length of the second period T2 is 10 μsec.

[0083] Figure 9 shows the operation of the LED driving circuit 200 during a first period T1 when the first control signal line CL1 is at ground potential GND (=0). During the first period T1, no base current flows through the transistor Q1, which acts as a switching element, and the transistor Q1 is turned off. During this first period T1, a charging current CH' flows from the power supply potential Vcc through the first diode D1, the first capacitor C1, and the first resistor R1' in sequence towards the first control signal line CL1, charging the first capacitor C1. At this time, the time constant formed by the capacitance value of the first capacitor C1 and the resistance value of the first resistor R1' is assumed to be sufficiently smaller than the length of the first period T1. As a result, at the end of the first period T1, the first connection point A and the anode terminal 90a of the LED 90 reach a positive charging potential (Vcc - Vf) equal to the power supply potential Vcc minus the forward voltage Vf of the first diode D1. The charge across the terminals of the first capacitor C1 is C1 × (Vcc - Vf). At the same time, the off-state transistor Q1 isolates the cathode terminal 90b of the LED 90 from the ground potential GND, so no drive current flows through the LED 90.

[0084] Figure 10 shows the operation of the LED driving circuit 200 during the second period T2, when the first control signal line CL1 is at the control potential H (=Vcc). During the second period T2, a base current Ib flows through the transistor Q1, which acts as a switching element, and the cathode terminal 90b of the LED 90 is turned on by the transistor Q1, bringing it to ground potential GND (=0). On the other hand, due to the boosting effect of the first charge pump circuit 21, at the start of the second period T2, the first connection point A and the anode terminal 90a of the LED 90 temporarily reach a boosted potential (Vcc - Vf + H) = (2Vcc - Vf), which is increased by the control potential H via the first capacitor C1 from the charging potential (Vcc - Vf). Therefore, even if the power supply voltage is less than 3.5V, at the start of the second period T2, the potential difference between the anode terminal 90a and the cathode terminal 90b (which is at zero potential) can exceed 3.5V, which is necessary to drive the white LED 90 (the specific voltage value reached will be described later). At this time, the first diode D1 is reverse-biased, blocking the current from the first connection point A to the power supply potential Vcc. As a result, the drive current DH' flows from the first control signal line CL1 through the first resistor R1', the first capacitor C1, the LED 90, and the on-state transistor Q1 towards the ground potential GND. This drive current DH' decreases from the start of the second period T2 as the charge stored in the first capacitor C1 discharges.

[0085] In this LED driving circuit 200, it is desirable that the capacitance of the first capacitor C1 be set such that the driving current DH' exceeds half the length of the second period T2 and continues to flow until the end in this example, similar to the first embodiment. As a result, this LED driving circuit 200 can drive LEDs 90 including white LEDs and / or blue LEDs as a load, similar to the first embodiment.

[0086] Furthermore, this LED driving circuit 200 can be composed of general circuit components (resistors, capacitors, transistors, etc.) that do not include inductors, similar to the first embodiment. Moreover, since the first resistor R1 and the second resistor R2 in the first embodiment are shared as a single component by the first resistor R1', the number of components can be reduced.

[0087] (Specific Example) For example, the power supply potential Vcc is set to Vcc = 3V, which is the voltage of two 1.5V dry cell batteries. The LED 90 used as a load is a white LED (part number NSPW500BS) manufactured by Nichia Corporation. The first diode D1 is a Schottky diode (part number RB160MM-40) manufactured by Rohm Co., Ltd. Transistor Q1 is a digital transistor (part number DTC043E) manufactured by Rohm Co., Ltd. The capacitance of the first capacitor C1 is 0.1μF.

[0088] The first resistor R1' is set to 270Ω. The value of this first resistor R1' is set as follows: During the first period T1, a maximum charging current CH' flows by (Vcc - Vf) / R1'. The first requirement for the resistance value of the first resistor R1' is imposed so that this maximum current value is less than the absolute maximum rating of the output current of the first control signal supply unit 11 (typically 10mA). During the second period T2, a maximum drive current DH' flows by {(2Vcc - Vf) - Vfled} / R1' (where Vfled is the forward voltage of the LED 90 while energized). The second requirement for the resistance value of the first resistor R1' is imposed so that this maximum current value is less than the absolute maximum rating of the forward current of the LED 90 (in this example, the pulse forward current) (approximately several tens of mA), and less than the absolute maximum rating of the output current of the first control signal supply unit 11 (typically 10mA). The resistance value of the first resistor R1' is selected from commercially available products so as to satisfy both the first and second requirements. This prevents the first control signal supply unit 11 from being damaged by the charging current CH' to the first capacitor C1 during the first period T1. Furthermore, it prevents the LED 90 and / or the first control signal supply unit 11 from being damaged by the driving current DH' during the second period T2.

[0089] Figure 11(A) specifically illustrates the potential waveform of the first control signal PS1 of the LED driving circuit 200. Figure 11(B) specifically illustrates the waveform of the charging current CH' for the first capacitor C1 included in the LED driving circuit 200. The charging current CH' rises sharply to approximately 10 mA at the start of the first period T1, and gradually approaches zero and saturates at the end of the first period T1.

[0090] Figure 12(A) specifically illustrates the waveform of the terminal voltage VC1' of the first capacitor C1 in the LED driving circuit 200. The terminal voltage VC1' rises from zero at the start of the first period T1 and gradually approaches (Vcc - Vf) and saturates at the end of the first period T1. In this example, (Vcc - Vf) ≈ 2.8V.

[0091] Figure 12(B) specifically illustrates the waveform of the potential VA1' at the first connection point A in the LED driving circuit 200. The potential VA1' at the first connection point A rises from approximately 2.8V at the start of the first period T1, gradually approaches (Vcc - Vf) and saturates at the end of the first period T1, and at the start of the second period T2, it temporarily reaches approximately 4.0V as a boosted potential. In this example, due to the relatively large resistance value of the first resistor R1', the same boosted potential (2Vcc - Vf) ≈ 5.8V as in the first embodiment cannot be obtained. However, since the potential VA1' at the first connection point A temporarily reaches approximately 4.0V, it exceeds the 3.5V required to drive the white LED 90.

[0092] Figure 13(A) specifically illustrates the waveform of the drive current DH' supplied to the LED 90 by the LED drive circuit 200. The drive current DH' rises sharply to approximately 9.5 mA at the start of the second period T2, and then decreases as the charge stored in the first capacitor C1 discharges. In this example, it is desirable that the capacitance of the first capacitor C1 be set such that the drive current DH' continues to flow for more than half the length of the second period T2, and continues until the end of this example. In this example, the drive current DH' remains even at the end of the second period T2, continuing to flow at approximately 2 mA.

[0093] Figure 13(B) specifically illustrates the relationship between the drive current DH' flowing through the LED 90 and the capacitance of the first capacitor C1 in the LED drive circuit 200. For example, as the capacitance value of the first capacitor C1 increases sequentially from 0.1 μF to 0.47 μF to 2.2 μF, the magnitude of the remaining drive current DH' at the end of the second period T2 increases, as indicated by the arrow E1' in Figure 13(B). The waveform of the drive current DH' of the LED 90 changes from a triangular wave to a trapezoidal wave, and gradually approaches a square wave. In this way, by appropriately setting the capacitance of the first capacitor C1, the drive current DH' can be made to continue flowing until the end of the second period T2.

[0094] (Third Embodiment) In the LED driving circuit 100 of the first embodiment and the LED driving circuit 200 of the second embodiment, if the power supply voltage (power supply potential) is 2V or higher, the LED 90 including white LEDs and / or blue LEDs can be driven stably. However, if the power supply voltage is less than 2V, there is insufficient margin in the driving voltage, so the brightness when the LED is lit may be affected by variations in the characteristics of the components, or the LED may not light up at all. Therefore, in this third embodiment, an LED driving circuit 300 is provided that can drive the LED 90 including white LEDs and / or blue LEDs stably even when the power supply voltage is less than 2V, typically equivalent to one 1.5V dry cell battery such as one alkaline or manganese dry cell battery.

[0095] (Circuit Configuration) Figure 14 shows the configuration of the LED driving circuit 300 of the third embodiment of this invention. Similar to the first and second embodiments, this LED driving circuit 300 is a circuit for driving an LED 90 by applying a voltage between the anode terminal 90a and cathode terminal 90b of the LED 90 which is a load. In this example, the LED 90 is a white LED, and the forward voltage required for driving is assumed to be about 3.5V. However, a blue LED or an LED of another color may also be used.

[0096] This LED driving circuit 300 includes a first control signal supply unit 11, a second control signal supply unit 12, a first charge pump circuit 21, and a second charge pump circuit 22. The same reference numerals are used for the same components as in the first embodiment.

[0097] The first control signal supply unit 11 is an input terminal that receives the first control signal PS1 from outside the LED drive circuit 300 (in this example, from the output port of a CPU, which is not shown), similar to the first embodiment, and supplies the first control signal PS1 received at this input terminal through the first control signal line CL1. In this example, as shown in the upper part of Figure 15, as time t progresses, the first control signal PS1 is a rectangular wave signal that alternately repeats a first period T1 in which the first control signal line CL1 is at ground potential GND (corresponding to zero potential) and a second period T2 in which the first control signal line CL1 is at a control potential H higher than ground potential GND. In this example, the control potential H is set to the power supply potential Vcc.

[0098] The second control signal supply unit 12 shown in Figure 14 is, in this example, an input terminal that receives the second control signal PS2 from outside the LED drive circuit 300 (in this example, consisting of an output port of a CPU, not shown), and supplies the second control signal PS2 received at this input terminal through the second control signal line CL2. As shown in the lower part of Figure 15, the second control signal PS2 is a rectangular wave signal that alternately repeats between ground potential GND and control potential H, with the opposite phase to the first control signal PS1.

[0099] The first charge pump circuit 21 shown in Figure 14 includes a first diode D1 connected in series between the power supply potential Vcc and the first control signal line CL1, a second resistor R21 for current limiting, and a first capacitor C1. The first diode D1 allows current to flow when biased forward from the power supply potential Vcc towards the first capacitor C1, and blocks current when biased in the reverse direction. In this example, the first diode D1 is a Schottky barrier diode. Therefore, the forward voltage Vf of the first diode D1 is smaller than that of a PN junction diode. Typically, Vf ≈ 0.2V (however, the forward voltage Vf depends on the drive current).

[0100] In this example, the first connection point A between the second resistor R21 and the first capacitor C1 is connected to the anode terminal 90a of the LED 90 via the first resistor R1 for current limiting. In this example, the first resistor R1 is interposed between the first connection point A and the anode terminal 90a of the LED 90, but it may instead be interposed between the cathode terminal 90b of the LED 90 and the transistor Q1.

[0101] The second charge pump circuit 22 includes a second diode D2, a second resistor R22, and a second capacitor C2, all connected in series between the ground potential GND (=0) and the second control signal line CL2. The second diode D2 allows current to flow in the forward direction from the second capacitor C2 to the ground potential GND, while blocking current in the reverse direction. In this example, the second diode D2 is a Schottky barrier diode, similar to the first diode D1. Therefore, the forward voltage Vf of the second diode D2 is smaller than that of a PN junction diode. Typically, Vf ≈ 0.2V (however, the forward voltage Vf depends on the drive current).

[0102] In this example, the second connection point B between the second resistor R22 and the second capacitor C2 is connected to the cathode terminal 90b of the LED 90.

[0103] (Circuit Operation) When the LED driving circuit 300 is in operation, as shown in Figure 15, the first control signal PS1 supplied by the first control signal supply unit 11 through the first control signal line CL1 alternately repeats a first period T1 in which the first control signal line CL1 is at ground potential GND (=0) and a second period T2 in which the first control signal line CL1 is at a control potential H (=Vcc) higher than ground potential GND. The second control signal PS2 supplied by the second control signal supply unit 12 through the second control signal line CL2 is in the opposite phase to the first control signal PS1, and alternately repeats a first period T1 in which the second control signal line CL2 is at control potential H (=Vcc) and a second period T2 in which the second control signal line CL2 is at ground potential GND (=0). In this example, the length of the first period T1 is 90 μsec, and the length of the second period T2 is 10 μsec.

[0104] Figure 16 shows the operation of the LED driving circuit 300 during a first period T1 in which the first control signal line CL1 is at ground potential GND (=0) and the second control signal line CL2 is at control potential H (=Vcc). During the first period T1, a charging current CH1 flows from the power supply potential Vcc through the first diode D1, the second resistor R21, and the first capacitor C1 in sequence towards the first control signal line CL1, thereby charging the first capacitor C1. At this time, the time constant formed by the capacitance value of the first capacitor C1 and the resistance value of the second resistor R21 is assumed to be sufficiently smaller than the length of the first period T1. As a result, at the end of the first period T1, the first connection point A, the first resistor R1, and the anode terminal 90a of the LED 90 reach a positive charging potential (Vcc - Vf) equal to the power supply potential Vcc minus the forward voltage Vf of the first diode D1. The charge across the terminals of the first capacitor C1 is C1 × (Vcc - Vf). Simultaneously, during the first period T1, a charging current CH2 flows from the second control signal line CL2 through the second capacitor C2, the second resistor R22, and the second diode D2 in sequence toward ground potential GND (=0), thereby charging the second capacitor C2. At this time, the time constant formed by the capacitance value of the second capacitor C2 and the resistance value of the second resistor R22 is assumed to be sufficiently smaller than the length of the first period T1. As a result, at the end of the first period T1, the voltage across the second connection point B, the second resistor R22, and the cathode terminal 90b of the LED 90 becomes equal to the forward voltage Vf of the second diode D2. The charge across the terminals of the second capacitor C2 is C2 × (Vcc - Vf).

[0105] Figure 17 shows the operation of the LED drive circuit 300 during the second period T2, when the first control signal line CL1 is at the control potential H (=Vcc) and the second control signal line CL2 is at the ground potential GND (=0). During the second period T2, the first connection point A, the first resistor R1, and the anode terminal 90a of the LED 90 temporarily reach a boosted potential (Vcc - Vf + H) = (2Vcc - Vf), which is increased from the charging potential (Vcc - Vf) by the control potential H via the first capacitor C1. At the same time, the second connection point B, the second resistor R22, and the cathode terminal 90b of the LED 90 temporarily reach a bucked potential (Vf - Vcc), which is decreased from the forward voltage Vf of the second diode D2 by the control potential H (=Vcc) via the second capacitor C2. In other words, the potential difference between the anode terminal 90a and the cathode terminal 90b can temporarily reach {(2Vcc - Vf) - (Vf - Vcc)} = (3Vcc - 2Vf). Therefore, even when the power supply voltage is less than 2.0V (1.5V in this example), at the start of the second period T2, the potential difference between the anode terminal 90a and the cathode terminal 90b can exceed 3.5V, which is necessary to drive the white LED 90 (the specific voltage value reached will be described later). At this time, the first diode D1 is reverse-biased, so it blocks the current from the first connection point A to the power supply potential Vcc. Similarly, the second diode D1 is reverse-biased, so it blocks the current from the ground potential GND (=0) to the second connection point B. As a result, the drive current DH2 flows from the first control signal line CL1 through the first capacitor C1, the first resistor R1, the LED 90, and the second capacitor C2 to the second control signal line CL2. This drive current DH2 decreases from the start of the second period T2 as the charges stored in the first capacitor C1 and the second capacitor C2 discharge.

[0106] In this LED driving circuit 300, it is desirable that the capacitances of the first capacitor C1 and the second capacitor C2 are set such that the driving current DH2 exceeds half the length of the second period T2 and continues to flow until the end in this example. In particular, the capacitance values ​​of the first capacitor C1 and the second capacitor C2 are set to be equal to each other so that the charge in only one of the first capacitor C1 and the second capacitor C2 is not depleted first. Therefore, with this LED driving circuit 300, even if the power supply voltage is less than 2V, typically the equivalent of one 1.5V dry cell battery, an LED 90 including a white LED and / or a blue LED can be driven as a load. In this example, the length of the first period T1 is 90 μsec and the length of the second period T2 is 10 μsec, so while the LED 90 is being driven, it is visually apparent that the LED 90 is lit (conversely, it is not perceived that the LED 90 is blinking). Furthermore, if the generation and non-generation of the first and second control signals PS1 and PS2 are repeated at a longer period, such as from 0.5 seconds to 1.0 second, the LED 90 can be visually perceived as blinking.

[0107] Furthermore, the LED driving circuit 300 may be composed of general circuit components (such as resistors, capacitors, and transistors) that do not include inductors, similar to those in the first and second embodiments.

[0108] (Specific Example) For example, the power supply potential Vcc is set to Vcc = 1.5V, which is the value of one 1.5V dry cell battery. The LED 90 used as a load is a white LED (model number NSPW500BS) manufactured by Nichia Corporation. The first diode D1 is a Schottky diode (model number RB160MM-40) manufactured by Rohm Co., Ltd. The capacitances of the first capacitor C1 and the second capacitor C2 are both 0.1μF.

[0109] The first resistor R1 is set to 82Ω. The second resistors R21 and R22 are both set to 120Ω. The values ​​of these first resistor R1 and second resistors R21 and R22 are set as follows. That is, during the first period T1, a maximum charging current CH1 flows by (Vcc - Vf) / R21, and a maximum charging current CH2 flows by (Vcc - Vf) / R22. The resistance values ​​of the second resistors R21 and R22 are selected from commercially available products so that these maximum current values ​​are each about 10mA. This prevents the first control signal supply unit 11 and the second control signal supply unit 12 from being damaged by the charging currents CH1 and CH2 to the first capacitor C1 and the second capacitor C1, respectively, during the first period T1. During the second period T2, a maximum drive current DH2 flows of {(3Vcc - 2Vf) - Vfled} / R1 (where Vfled is the forward voltage of the LED 90 while energized). The resistance value of the first resistor R1 is selected from commercially available products so that this maximum current value is less than the absolute maximum rating (approximately several tens of mA) of the forward current of the LED 90 (in this example, the pulse forward current) and less than the absolute maximum rating (typically 10 mA) of the output current of the first control signal supply unit 11. This prevents the LED 90 and / or the first control signal supply unit 11 from being damaged by the drive current DH2 during the second period T2.

[0110] Figure 18(A) specifically illustrates the potential waveform of the first control signal PS1 of the LED driving circuit 300. Figure 18(B) specifically illustrates the waveform of the charging current CH1 for the first capacitor C1 included in the LED driving circuit 300. The charging current CH1 rises sharply to approximately 4.8 mA at the start of the first period T1, and gradually approaches zero and saturates at the end of the first period T1.

[0111] Figure 19(A) specifically illustrates the potential waveform of the second control signal PS2 of the LED driving circuit 300. Figure 19(B) specifically illustrates the waveform of the charging current CH2 for the second capacitor C2 included in the LED driving circuit 300. The charging current CH2 rises sharply to approximately 4.0 mA at the start of the first period T1, and gradually approaches zero and saturates at the end of the first period T1.

[0112] Figure 20(A) specifically illustrates the waveform of the terminal voltage VC1 of the first capacitor C1 in the LED driving circuit 300. The terminal voltage VC1 rises from approximately 7.5V at the start of the first period T1 and gradually approaches (Vcc - Vf) and saturates at the end of the first period T1. In this example, (Vcc - Vf) ≈ 1.3V.

[0113] Figure 20(B) specifically illustrates the waveform of the terminal voltage VC2 of the second capacitor C2 in the LED driving circuit 300. The terminal voltage VC2 shows a sharp fluctuation at the start of the first period T1, then falls from approximately -0.8V, and gradually approaches (Vf - Vcc) and saturates at the end of the first period T1. In this example, (Vf - Vcc) ≈ -1.3V.

[0114] Figure 21(A) specifically illustrates the waveform of the potential difference VAB between the first connection point A and the second connection point in the LED driving circuit 300. The potential difference VAB between the first connection point A and the second connection point drops from approximately 3.1V to zero at the start of the first period T1, rises from zero, gradually approaches (Vcc - 2Vf) and saturates at the end of the first period T1, and at the start of the second period T2 it reaches the boosted potential (3Vcc - 2Vf). In this example, (Vcc - 2Vf) ≈ 1.2V and (3Vcc - 2Vf) ≈ 4.1V. In other words, the boosted potential (3Vcc - 2Vf) exceeds 3.5V, which is necessary to drive the white LED 90. In this example, the potential difference VAB between the first connection point A and the second connection point remains at approximately 3.1V at the end of the second period T2.

[0115] Figure 21(B) specifically illustrates the waveform of the drive current DH2 supplied to the LED 90 by the LED drive circuit 300. The drive current DH2 rises sharply to approximately 11.5 mA at the start of the second period T2, and then decreases as the charge stored in the first capacitor C1 and the second capacitor C2 discharges. In this example, it is desirable that the capacitances of the first capacitor C1 and the second capacitor C2 are set so that the drive current DH2 continues to flow for more than half the length of the second period T2, and continues until the end of this example. As previously mentioned, C1 = C2 = 0.1 μF. In this example, the drive current DH2 remains even at the end of the second period T2, continuing to flow at approximately 2.5 mA.

[0116] Furthermore, in this LED driving circuit 300, it is also possible to design it so that the power supply potential is increased from Vcc = 1.5V to Vcc = 3V, equivalent to two 1.5V dry cell batteries. In that case, a boosted potential of (3Vcc - 2Vf) ≈ 8.6V can be obtained. Therefore, it becomes possible to use it, for example, to drive multiple (two) white LEDs connected in series, which are included in the backlight light source of a liquid crystal display.

[0117] (Fourth Embodiment) Figure 22 shows the configuration of an LED driving circuit 400 according to the fourth embodiment of the present invention. In this LED driving circuit 400, the second control signal supply unit 12 in the LED driving circuit 300 of the third embodiment is omitted, and instead, a CMOS inverter 16 (in this example, a Toshiba CMOS 04) shown in Figure 22 is provided.

[0118] The CMOS inverter 16 is connected between the power supply potential Vcc and the ground potential GND (=0), and has an input terminal 16i and an output terminal 16o. The CMOS inverter 16 receives a first control signal PS1 from the first control signal supply unit 11 at the input terminal 16i, and supplies a signal with the opposite phase to the first control signal PS1 (i.e., a second control signal PS2) from the output terminal 16o through the second control signal line CL2. The other circuit configurations are the same as in the third embodiment, and redundant explanations are omitted using the same reference numerals.

[0119] With this LED driving circuit 400, control is performed substantially by the first control signal supply unit 11 alone, rather than by both the first control signal supply unit 11 and the second control signal supply unit 12. Therefore, control can be simplified.

[0120] (Fifth Embodiment) (Circuit Configuration) Figure 23 shows the configuration of the LED driving circuit 500 according to the fifth embodiment of the present invention. In this LED driving circuit 500, the second control signal supply unit 12 in the LED driving circuit 300 of the third embodiment is omitted, and instead, an NMOS inverter 17 shown in Figure 23 is provided. In addition, the second resistor element R22 is omitted and is used interchangeably by the resistor element R6 that makes up the NMOS inverter 17.

[0121] More specifically, the NMOS inverter 17 comprises a resistor R6 connected in series between the power supply potential Vcc and the ground potential GND (=0), and an NMOS transistor FT. More specifically, the drain terminal FTd of the NMOS transistor FT is connected to the resistor R6, and the source terminal FTs is connected to the ground potential GND. A gate-source resistor R5 is connected between the gate terminal FTg and the source terminal FTs of the NMOS transistor FT to enable stable on / off switching.

[0122] The gate terminal FTg of the NMOS transistor FT is connected to the first control signal supply unit 11. Therefore, the NMOS transistor FT is turned off when the first control signal line CL1 is at ground potential GND, and turned on when the first control signal line CL1 is at control potential H. As a result, the NMOS inverter 17 receives the first control signal PS1 from the first control signal supply unit 11 to the gate terminal FTg of the NMOS transistor FT, and supplies a signal with the opposite phase to the first control signal PS1 (i.e., the second control signal PS2) from the drain terminal FTd of the NMOS transistor FT through the second control signal line CL2.

[0123] The other circuit configurations are the same as those in the third embodiment, and redundant explanations are omitted using the same reference numerals.

[0124] (Circuit Operation) When the LED driving circuit 500 is in operation, the first control signal PS1 supplied by the first control signal supply unit 11 through the first control signal line CL1 alternately repeats a first period T1 in which the first control signal line CL1 is at ground potential GND (=0) and a second period T2 in which the first control signal line CL1 is at a control potential H (=Vcc) higher than ground potential GND. Accordingly, the second control signal PS2 supplied by the NMOS transistor FT through the second control signal line CL2 alternately repeats a first period T1 in which the second control signal line CL2 is at control potential H (=Vcc) and a second period T2 in which the second control signal line CL2 is at ground potential GND (=0), in the opposite phase to the first control signal PS1. In this example, the length of the first period T1 is 90 μsec, and the length of the second period T2 is 10 μsec.

[0125] Figure 24 shows the operation of the LED driving circuit 500 during a first period T1 when the first control signal line CL1 is at ground potential GND (=0). During the first period T1, ground potential GND is applied to the gate terminal FTg of the NMOS transistor FT, and the NMOS transistor FT is turned off. During this first period T1, a charging current CH1 flows from the power supply potential Vcc through the first diode D1, the second resistor R21, and the first capacitor C1 in sequence towards the first control signal line CL1, and the first capacitor C1 is charged. At this time, the time constant formed by the capacitance value of the first capacitor C1 and the resistance value of the second resistor R21 is assumed to be sufficiently smaller than the length of the first period T1. As a result, at the end of the first period T1, the first connection point A, the first resistor R1, and the anode terminal 90a of the LED 90 reach a positive charging potential (Vcc - Vf) equal to the power supply potential Vcc minus the forward voltage Vf of the first diode D1. The charge between the terminals of the first capacitor C1 is C1 × (Vcc - Vf). At the same time, during the first period T1, a charging current CH2' flows from the power supply potential Vcc through the resistor R6, the second capacitor C2, and the second diode D2 in sequence toward the ground potential GND (=0), thereby charging the second capacitor C2. At this time, the time constant formed by the capacitance value of the second capacitor C2 and the resistance value of the resistor R6 is assumed to be sufficiently smaller than the length of the first period T1. As a result, at the end of the first period T1, the voltage at the second connection point B and the cathode terminal 90b of LED 90 becomes equal to the forward voltage Vf of the second diode D2. The charge across the terminals of the second capacitor C2 is C2 × (Vcc - Vf).

[0126] Figure 25 shows the operation of the LED driving circuit 500 during the second period T2, when the first control signal line CL1 is at the control potential H (=Vcc). During the second period T2, the control potential H (=Vcc) is applied to the gate terminal FTg of the NMOS transistor FT, and the NMOS transistor FT is turned on. During this second period T2, the first connection point A, the first resistor R1, and the anode terminal 90a of the LED 90 temporarily reach a boosted potential (Vcc - Vf + H) = (2Vcc - Vf), which is increased from the charging potential (Vcc - Vf) by the control potential H via the first capacitor C1. At the same time, the second connection point B and the cathode terminal 90b of the LED 90 temporarily reach a bucked potential (Vf - Vcc), which is decreased from the forward voltage Vf of the second diode D2 by the control potential H (=Vcc) via the second capacitor C2. In other words, the potential difference between the anode terminal 90a and the cathode terminal 90b can temporarily reach {(2Vcc - Vf) - (Vf - Vcc)} = (3Vcc - 2Vf). Therefore, even when the power supply voltage is less than 2.0V (1.5V in this example), at the start of the second period T2, the potential difference between the anode terminal 90a and the cathode terminal 90b can exceed 3.5V, which is necessary to drive the white LED 90. At this time, the first diode D1 is reverse-biased, thus blocking the current from the first connection point A to the power supply potential Vcc. Similarly, the second diode D1 is reverse-biased, thus blocking the current from the ground potential GND (=0) to the second connection point B. As a result, the drive current DH3 flows from the first control signal line CL1 through the first capacitor C1, the first resistor R1, the LED 90, the second capacitor C2, and the ON-state NMOS transistor FT to the second control signal line CL2. This drive current DH3 decreases from the start of the second period T2 as the charges stored in the first capacitor C1 and the second capacitor C2 discharge.

[0127] In this LED driving circuit 500, it is desirable that the capacitances of the first capacitor C1 and the second capacitor C2 are set such that the driving current DH3 exceeds half the length of the second period T2 and continues to flow until the end of this example. In particular, the capacitance values ​​of the first capacitor C1 and the second capacitor C2 are set to be equal to each other so that the charge in only one of the first capacitor C1 and the second capacitor C2 is not depleted first. Therefore, with this LED driving circuit 500, even if the power supply voltage is less than 2V, typically the equivalent of one 1.5V dry cell battery, an LED 90 including a white LED and / or a blue LED can be driven as a load. In this example, the length of the first period T1 is 90 μsec and the length of the second period T2 is 10 μsec, so while the LED 90 is being driven, it is visually apparent that the LED 90 is lit (conversely, it is not perceived that the LED 90 is blinking). Furthermore, if the generation and non-generation of the first control signal PS1 are repeated at a longer period, for example, from 0.5 seconds to 1.0 second, the LED 90 can be visually perceived as blinking.

[0128] Furthermore, the LED driving circuit 500 may be composed of general circuit components (such as resistors, capacitors, and transistors) that do not include inductors, similar to the third embodiment.

[0129] However, as shown in Figure 25, this LED driving circuit 500 has the disadvantage that during the second period T2, a leakage current DH4 flows from the power supply potential Vcc through the resistor R6 and the ON-state NMOS transistor FT in sequence toward the ground potential GND (=0).

[0130] (Sixth Embodiment) (Circuit Configuration) Figure 26 shows the configuration of the LED driving circuit 600 of the sixth embodiment of the present invention. Unlike the LED driving circuit 100 of the first embodiment, which was directed to drive one LED 90, this LED driving circuit 600 is adapted to drive multiple (three in this example) LEDs 90-1, 90-2, and 90-3. In this example, each LED is a white LED, and the forward voltage required for driving is approximately 3.5V. However, blue LEDs or LEDs of other colors may also be used.

[0131] This LED driving circuit 600 includes a first control signal supply unit 11 and a first charge pump circuit 21, as well as a third control signal supply unit 13. Note that the same reference numerals are used for components identical to those in the first embodiment, and redundant explanations are omitted.

[0132] In this example, the third control signal supply unit 13 is an input terminal that receives third control signals SL3-1, SL3-2, and SL3-3 from outside the LED drive circuit 600 (in this example, consisting of an output port of a CPU, not shown in the diagram). The third control signals received at this input terminal are supplied through separate third control signal lines CL3-1, CL3-2, and CL3-3.

[0133] In this LED driving circuit 600, the first connection point A in the first charge pump circuit 21 is connected to the anode terminals 90a of LEDs 90-1, 90-2, and 90-3, respectively, via the first resistor element R1.

[0134] Between the cathode terminals 90b of LEDs 90-1, 90-2, and 90-3 and the ground potential GND (=0), the same NPN bipolar transistors Q1, Q2, and Q3 (hereinafter referred to as "transistors Q1, Q2, and Q3" as appropriate) as in the LED driving circuit 100 of the first embodiment are interposed as separate switching elements. In this example, transistors Q1, Q2, and Q3 are each digital transistors.

[0135] Third control signal lines CL3-1, CL3-2, and CL3-3 are connected to the base terminals of transistors Q1, Q2, and Q3, respectively. As a result, each transistor Q1, Q2, and Q3 is turned off when its corresponding third control signal line CL3-1, CL3-2, and CL3-3 are at ground potential GND (=0), and turned on when its corresponding third control signal line CL3-1, CL3-2, and CL3-3 are at control potential H (=Vcc).

[0136] (Circuit Operation) As shown in Figure 28(A), the first control signal PS1 supplied by the first control signal supply unit 11 through the first control signal line CL1 alternately repeats a first period T1 in which the first control signal line CL1 is at ground potential GND (=0) and a second period T2 in which the first control signal line CL1 is at a control potential H (=Vcc) higher than ground potential GND. As shown in Figures 28(B) to 28(D), the third control signals SL3-1, SL3-2, and SL3-3 supplied by the third control signal supply unit 13 through the third control signal lines CL3-1, CL3-2, and CL3-3 all maintain ground potential GND (=0) during the first period T1. On the other hand, during the second period T2, the third control signals SL3-1, SL3-2, and SL3-3 maintain the third control signal line corresponding to the LED to be lit among LEDs 90-1, 90-2, and 90-3 at a control potential H (=Vcc), and the third control signal line corresponding to the LED to be turned off at ground potential GND (=0). For the sake of explanation, in the following, we will assume that the LED to be lit is LED 90-2, and the LEDs to be turned off are LEDs 90-1 and 90-3.

[0137] During the first period T1, a charging current CH (see Figure 3) flows from the power supply potential Vcc shown in Figure 26 through the first diode D1 and the first capacitor C1 towards the first control signal line CL1, charging the first capacitor C1. As a result, the anode terminals 90a of the first connection point A and LEDs 90-1, 90-2, and 90-3 reach a positive charging potential (Vcc - Vf) equal to the power supply potential Vcc minus the forward voltage of the first diode D1. Simultaneously, the third control signal lines CL3-1, CL3-2, and CL3-3 are all kept at ground potential GND (=0), so transistors Q1, Q2, and Q3 are turned off. With transistors Q1, Q2, and Q3 in the OFF state, the cathode terminals 90b of LEDs 90-1, 90-2, and 90-3 are all isolated from the ground potential GND (=0), and no drive current flows to LEDs 90-1, 90-2, and 90-3.

[0138] During the second period T2, the third control signal lines CL3-1 and CL3-3 corresponding to LEDs 90-1 and 90-3, which should be turned off, are kept at ground potential GND (=0). As a result, transistors Q1 and Q3, which correspond to these third control signal lines CL3-1 and CL3-3, are both kept off. Therefore, no drive current flows to the LEDs corresponding to the transistors Q1 and Q3 that are kept off (i.e., LEDs 90-1 and 90-3, which should be turned off).

[0139] Meanwhile, during this second period T2, the third control signal line CL3-2 corresponding to LED 90-2, which is to be lit among LEDs 90-1, 90-2, and 90-3, is kept at the control potential H (=Vcc), thereby turning on the transistor Q2 corresponding to the third control signal line CL3-2. The cathode terminal 90b of LED 90-2, which corresponds to the turned-on transistor Q2, becomes ground potential GND (=0). Meanwhile, the anode terminals 90a of the first connection point A and LEDs 90-1, 90-2, and 90-3 temporarily reach a boosted potential (2Vcc-Vf) which is increased by the control potential H via the first capacitor C1 from the charging potential. For example, if the power supply potential Vcc and control potential H (=Vcc) are Vcc = 3V (i.e., equivalent to two 1.5V dry cell batteries), and the forward voltage of the first diode D1 (here, a Schottky barrier diode) is Vf ≈ 0.2V, then at the start of the second period T2, the boosted potential reached by the anode terminal 90a of the LED 90-2 to be lit will be (2Vcc - Vf) ≈ 5.8V. Therefore, at the start of the second period T2, the potential difference between the anode terminal 90a and the cathode terminal 90b (which is at zero potential) of the LED 90-2 to be lit will be (2Vcc - Vf) ≈ 5.8V. In other words, at the start of the second period T2, the potential difference between the anode terminal 90a and the cathode terminal 90b of the LED 90-2 to be lit may exceed the 3.5V required to drive the white LED 90. As a result, the drive current DH (see Figure 4) flows from the first control signal line CL1 through the first capacitor C1, the first resistor R1, the LED 90-2 to be lit, and the ON transistor Q2, toward the ground potential GND (=0). This drive current DH decreases from the start of the second period T2 as the charge stored in the first capacitor C1 discharges.

[0140] In this LED driving circuit, it is desirable that the capacitance of the first capacitor C1 be set such that the driving current DH exceeds half the length of the second period T2 and continues to flow until the end in this example. As a result, according to the LED driving circuit of this embodiment, LEDs 90-1, 90-2, and 90-3, including white LEDs and / or blue LEDs, can be driven as a load while selecting which LED to light up.

[0141] Furthermore, each switching element may consist of NPN bipolar transistors Q1, Q2, and Q3 and an attached resistor. Therefore, this LED driving circuit 600 may be composed of general circuit components (resistors, capacitors, transistors, etc.) that do not include inductors.

[0142] In the example above, LED 90-2 is assumed to be the LED to be lit, and LEDs 90-1 and 90-3 are assumed to be the LEDs to be turned off, but this is not limited to this. LED 90-1 may be the LED to be lit, and LEDs 90-2 and 90-3 may be the LEDs to be turned off, or LED 90-3 may be the LED to be lit, and LEDs 90-1 and 90-2 may be the LEDs to be turned off.

[0143] (Seventh Embodiment) (Circuit Configuration) Figure 27 shows the configuration of the LED driving circuit 700 of the seventh embodiment of the present invention. While the LED driving circuit 600 of the sixth embodiment was directed to drive three LEDs 90-1, 90-2, and 90-3, this LED driving circuit 700 is adapted to drive seven LEDs 90-1, 90-2, ..., 90-7. In this example, each LED is a white LED, and the forward voltage required for driving is assumed to be about 3.5V. However, blue LEDs or LEDs of other colors may also be used.

[0144] This LED driving circuit 700 includes a first control signal supply unit 11, a first charge pump circuit 21, a third control signal supply unit 13, and a multiplexer 18 (in this example, an analog multiplexer CD4051 manufactured by Texas Instruments). Components identical to those in the sixth embodiment are denoted by the same reference numerals, and redundant explanations are omitted.

[0145] In this LED driving circuit 700, the first connection point A in the first charge pump circuit 21 is connected to the anode terminals 90a of LEDs 90-1, 90-2, ..., 90-7 via the first resistor R1.

[0146] Between the cathode terminals 90b of LEDs 90-1, 90-2..., 90-7 and the ground potential GND (=0), NPN bipolar transistors Q1, Q2, ..., Q7 (hereinafter referred to as "transistors Q1, Q2, ..., Q7" as appropriate) are interposed as separate switching elements, the same as those in the LED driving circuit 100 of the sixth embodiment. In this example, transistors Q1, Q2, ..., Q7 are each digital transistors. The output terminals X0, X1, ..., X6 of the multiplexer 18 are connected to the base terminals of transistors Q1, Q2, ..., Q7, respectively.

[0147] The multiplexer 18 has three input terminals A to C to which the third control signal lines CL3-1, CL3-2, and CL3-3 are connected, respectively, and eight output terminals X0, X1, ..., X7, which is more than the number of input terminals. Depending on the combination of logical values ​​(0 or 1) of the third control signals IN1, IN2, IN3 (see Figures 28(E) to 28(G)) supplied to the three input terminals A to C, which are either ground potential GND (=0) or control potential H (=Vcc), the multiplexer 18 selects one of the eight output terminals X0, X1, ..., X7 (represented by the symbol XX) and outputs the potential of that output terminal XX as control potential H (=Vcc) (logic 1). At the same time, the multiplexer 18 outputs the potential of the remaining output terminals as ground potential GND (=0).

[0148] In this example, as shown in Figures 28(E) to 28(G), the third control signals IN1, IN2, and IN3 supplied to the third control signal lines CL3-1, CL3-2, and CL3-3 take the following combinations of logical values: when selecting output terminal X0, the combination is (0,0,0); when selecting output terminal X1, the combination is (1,0,0); when selecting output terminal X2, the combination is (0,1,0); and when selecting output terminal X7, the combination is (1,1,1).

[0149] As a result, each transistor Q1, Q2, ..., Q7 shown in Figure 27 is turned off when its corresponding output terminal X0, X1, ..., X7 is at ground potential GND (=0), and turned on when its corresponding output terminal X0, X1, ..., X7 is at control potential H (=Vcc).

[0150] This LED driving circuit 700, similar to the LED driving circuit 600 of the sixth embodiment, can drive LEDs 90-1, 90-2, ..., 90-7, including white LEDs and / or blue LEDs, as a load while selecting which LED to light up. Moreover, it is possible to save wiring space compared to the case where the same number of third control signal lines CL3-1, CL3-2, CL3-3, ... are provided as the number of transistors Q1, Q2, ..., Q7 that serve as switching elements.

[0151] (Eighth Embodiment) Figure 29 shows the configuration of the LED driving circuit 800 according to the eighth embodiment of the present invention. Unlike the LED driving circuit 300 of the third embodiment, which was directed to drive one LED 90, this LED driving circuit 800 is adapted to drive multiple (three in this example) LEDs 90-1, 90-2, and 90-3. In this example, each LED is a white LED, and the forward voltage required for driving is approximately 3.5V. However, blue LEDs or LEDs of other colors may also be used.

[0152] This LED driving circuit 800 includes a first control signal supply unit 11, a second control signal supply unit 12, a first charge pump circuit 21, a second charge pump circuit 22, and a third control signal supply unit 13. Note that the same components as in the third embodiment are denoted by the same reference numerals, and redundant explanations are omitted.

[0153] The third control signal supply unit 13 is an input terminal that receives third control signals SL3-1, SL3-2, and SL3-3 from outside the LED drive circuit 800 (in this example, from an output port of a CPU, not shown), similar to the LED drive circuit 600 of the sixth embodiment. The third control signals received at this input terminal are supplied through separate third control signal lines CL3-1, CL3-2, and CL3-3.

[0154] In this LED driving circuit 800, the first connection point A in the first charge pump circuit 21 is connected to the anode terminals 90a of LEDs 90-1, 90-2, and 90-3, respectively, via the first resistor element R1.

[0155] Between the cathode terminals 90b of LEDs 90-1, 90-2, and 90-3 and the second connection point B, NMOS field-effect transistors FT1, FT2, and FT3 (hereinafter referred to as "transistors FT1, FT2, and FT3" as appropriate) are interposed as separate switching elements.

[0156] Third control signal lines CL3-1, CL3-2, and CL3-3 are connected to the gate terminals of transistors FT1, FT2, and FT3, respectively. As a result, each transistor FT1, FT2, and FT3 is turned off when its corresponding third control signal line CL3-1, CL3-2, and CL3-3 are at ground potential GND (=0), and turned on when its corresponding third control signal line CL3-1, CL3-2, and CL3-3 are at control potential H (=Vcc). Gate-source resistors R51, R52, and R53 are connected between the gate and source terminals of transistors FT1, FT2, and FT3, respectively, to enable stable on / off switching.

[0157] (Circuit Operation) In this example, as shown in Figure 28(A), the first control signal PS1 supplied by the first control signal supply unit 11 through the first control signal line CL1 alternates between a first period T1 in which the first control signal line CL1 is at ground potential GND (=0) and a second period T2 in which the first control signal line CL1 is at a control potential H (=Vcc) higher than ground potential GND. The second control signal PS2 supplied by the second control signal supply unit 12 through the second control signal line CL2 alternates between ground potential GND and control potential H in the opposite phase to the first control signal PS1. As shown in Figures 28(B) to 28(D), the third control signals SL3-1, SL3-2, and SL3-3 supplied by the third control signal supply unit 13 through the third control signal lines CL3-1, CL3-2, and CL3-3 all maintain ground potential GND (=0) during the first period T1. On the other hand, during the second period T2, the third control signals SL3-1, SL3-2, and SL3-3 maintain the third control signal line corresponding to the LED to be lit among LEDs 90-1, 90-2, and 90-3 at a control potential H (=Vcc), and maintain the third control signal line corresponding to the LED to be turned off at ground potential GND (=0). For the sake of explanation, in the following, we will assume that LED 90-2 is the LED to be lit, and LEDs 90-1 and 90-3 are the LEDs to be turned off.

[0158] During the first period T1, a charging current CH1 (see Figure 16) flows from the power supply potential Vcc shown in Figure 29 through the first diode D1 and the first capacitor C1 toward the first control signal line CL1, charging the first capacitor C1. As a result, the anode terminals 90a of the first connection point A and LEDs 90-1, 90-2, and 90-3 reach a positive charging potential (Vcc - Vf) equal to the power supply potential Vcc minus the forward voltage of the first diode D1. Simultaneously, during the first period T1, a charging current CH2 (see Figure 16) flows from the second control signal line CL2 through the second capacitor C2, the second resistor R22, and the second diode D2 toward the ground potential GND (=0), charging the second capacitor C2. As a result, at the end of the first period T1, the second connection point B, the second resistor R22, and the cathode terminal 90b of LED 90 become equal to the forward voltage Vf of the second diode D2. At the same time, the third control signal lines CL3-1, CL3-2, and CL3-3 are all kept at ground potential GND (=0), so each transistor FT1, FT2, and FT3 is turned off. With each transistor FT1, FT2, and FT3 in the off state, the cathode terminals 90b of LEDs 90-1, 90-2, and 90-3 are electrically isolated from the second connection point B, and no drive current flows to LEDs 90-1, 90-2, and 90-3.

[0159] During the second period T2, the third control signal lines CL3-1 and CL3-3 corresponding to LEDs 90-1 and 90-3, which should be turned off, are kept at ground potential GND (=0). As a result, the transistors FT1 and FT3 corresponding to these third control signal lines CL3-1 and CL3-3 are both kept off. Therefore, no drive current flows to the LEDs corresponding to the transistors FT1 and FT3 that are kept off (i.e., LEDs 90-1 and 90-3, which should be turned off).

[0160] Meanwhile, during this second period T2, the third control signal line CL3-2 corresponding to LED 90-2, which is to be lit among LEDs 90-1, 90-2, and 90-3, is maintained at the control potential H (=Vcc), thereby turning on the transistor FT2 corresponding to the third control signal line CL3-2. The cathode terminal 90b of LED 90-2, which is corresponding to the turned-on transistor FT2, becomes equal to the potential of the second connection point B. At the same time, the second connection point B (and the cathode terminal 90b of LED 90-2 to be lit) temporarily reaches a stepped-down potential (Vf - Vcc) obtained by stepping down the forward voltage Vf of the second diode D2 by the control potential H (=Vcc) via the second capacitor C2. On the other hand, the anode terminals 90a of the first connection point A and LEDs 90-1, 90-2, and 90-3 all temporarily reach a boosted potential (2Vcc-Vf) which is increased by the control potential H via the first capacitor C1 from the charging potential (Vcc-Vf). Therefore, at the start of the second period T2, the potential difference between the anode terminal 90a and the cathode terminal 90b of LED 90-2 to be lit is (2Vcc-Vf) - (Vf-Vcc) = (3Vcc-2Vf). For example, if the power supply potential Vcc and control potential H are Vcc = 1.5V (i.e., equivalent to one 1.5V dry cell battery), and the forward voltages of the first diode D1 and the second diode D2 are both Vf ≈ 0.2V, then at the start of the second period T2, the step-down potential reached by the cathode terminal 90b of the LED 90-2 to be lit will be (Vf - Vcc) ≈ -1.3V. On the other hand, the step-up potential reached by the anode terminal 90a will be (2Vcc - Vf) ≈ 2.8V. Therefore, at the start of the second period T2, the potential difference between the anode terminal 90a and the cathode terminal 90b of the LED 90-2 to be lit will be (2Vcc - Vf) - (Vf - Vcc) = (3Vcc - 2Vf) ≈ 4.1V. In other words, at the start of the second period T2, the potential difference between the anode terminal 90a and the cathode terminal 90b of the LED 90-2 to be lit may exceed 3.5V, which is necessary to drive the white LED 90. As a result, the drive current DH2 (see Figure 17) flows from the first control signal line CL1 through the first capacitor C1, the first resistor R1, the LED 90-2 to be lit, the on-state transistor FT2, and the second capacitor C2 to the second control signal line CL2.This drive current DH2 decreases from the start of the second period T2 as the charge stored in the first capacitor C1 discharges.

[0161] In this LED driving circuit, it is desirable that the capacitances of the first capacitor C1 and the second capacitor C2 be set such that the driving current DH2 exceeds half the length of the second period T2 and continues to flow until the end in this example. As a result, this LED driving circuit 800 can drive LEDs 90-1, 90-2, and 90-3, including white LEDs and / or blue LEDs, as a load while selecting which LED to light up.

[0162] Furthermore, each switching element may consist of NMOS field-effect transistors FT1, FT2, and FT3 and resistors R51, R52, and R53. Therefore, the LED driving circuit of this embodiment may be composed of general circuit components (resistors, capacitors, transistors, etc.) that do not include inductors.

[0163] Figure 30 provides a concrete example of comparing the waveforms of the gate-source voltages Vgs1 and Vgs3 of transistors FT1 and FT3, which correspond to LEDs 90-1 and 90-3 that should be turned off, and the waveform of the gate-source voltage Vgs2 of transistor FT2, which corresponds to LED 90-2 that should be lit, both included in this LED driving circuit 800. At the start of the second period T2, a potential of (Vcc - Vf) ≈ 1.3V is applied to the gate terminals of transistors FT1 and FT3, which correspond to LEDs 90-1 and 90-3 that should be turned off, via gate-source resistors R51 and R53, respectively, due to potential fluctuations within the circuit. On the other hand, a potential of (2Vcc - Vf) ≈ 2.8V is applied to the gate terminal of transistor FT2, which corresponds to LED 90-2 that should be lit, via gate-source resistor R52, due to potential fluctuations within the circuit. Therefore, in this example, each NMOS field-effect transistor FT1, FT2, and FT3 must not turn on at a gate-source voltage of 1.3V, but must turn on at a gate-source voltage of 2.8V. In other words, the gate threshold voltage ThG of each NMOS field-effect transistor FT1, FT2, and FT3 must be set to a level between (Vcc - Vf) and (2Vcc - Vf), as shown in Figure 30. In this example, ThG is set to approximately 1.7V. This prevents transistors FT1 and FT3 corresponding to LEDs 90-1 and 90-3, which should be turned off, from being mistakenly turned on at the start of the second period T2, while also allowing transistor FT2, which corresponds to LED 90-2, to be turned on appropriately.

[0164] Thus, when NMOS field-effect transistors FT1, FT2, and FT3 are used as switching elements, the on state of the transistor corresponding to LED 90-2, which should be lit, and the off state of the transistors corresponding to LEDs 90-1 and 90-3, which should be turned off, can be clearly distinguished by the threshold ThG, enabling appropriate operation.

[0165] Furthermore, during the first period T1, the gate terminals of each transistor FT1, FT2, and FT3 are at a negative or zero potential, so the NMOS field-effect transistors FT1, FT2, and FT3 can operate properly without the possibility of operational errors.

[0166] In the example above, as in the sixth embodiment, the LED to be lit is LED 90-2 and the LEDs to be turned off are LED 90-1 and 90-3, but it is not limited to this. The LED to be lit may be LED 90-1 and the LEDs to be turned off may be LED 90-2 and 90-3, or the LED to be lit may be LED 90-3 and the LEDs to be turned off may be LED 90-1 and 90-2. Alternatively, any two of LEDs 90-1, 90-2, and 90-3 may be lit and the remaining one may be turned off. Alternatively, all of LEDs 90-1, 90-2, and 90-3 may be lit.

[0167] (Ninth Embodiment) (Circuit Configuration) Figure 31 shows the configuration of the LED driving circuit 900 of the ninth embodiment of the present invention. While the LED driving circuit 800 of the eighth embodiment was directed to drive three LEDs 90-1, 90-2, and 90-3, this LED driving circuit 900 is adapted to drive seven LEDs 90-1, 90-2, ..., 90-7. In this example, each LED is a white LED, and the forward voltage required for driving is assumed to be about 3.5V. However, blue LEDs or LEDs of other colors may also be used.

[0168] This LED driving circuit 900 includes a first control signal supply unit 11, a second control signal supply unit 12, a first charge pump circuit 21, a second charge pump circuit 22, a third control signal supply unit 13, and a multiplexer 18. Note that the same components as in the eighth embodiment are denoted by the same reference numerals, and redundant explanations are omitted.

[0169] The multiplexer 18, similar to the LED drive circuit 700 of the seventh embodiment, has three input terminals A to C to which the third control signal lines CL3-1, CL3-2, and CL3-3 are connected, respectively, and eight output terminals X0, X1, ..., X7, which is more than the number of input terminals. The multiplexer 18 selects one of the eight output terminals X0, X1, ..., X7 (represented by the symbol XX) according to the combination of logical values ​​(0 or 1) of the third control signals IN1, IN2, IN3 (see Figures 28(E) to 28(G)) supplied to the three input terminals A to C, whether they are ground potential GND (=0) or control potential H (=Vcc), and outputs the potential of that output terminal XX as control potential H (=Vcc) (logic 1). At the same time, the multiplexer 18 outputs the potential of the remaining output terminals as ground potential GND (=0).

[0170] As a result, each transistor FT1, FT2, ..., FT7 shown in Figure 31 is turned off when its corresponding output terminal X0, X1, ..., X7 is at ground potential GND (=0), and turned on when its corresponding output terminal X0, X1, ..., X7 is at control potential H (=Vcc).

[0171] This LED driving circuit 900, similar to the LED driving circuit 700 of the seventh embodiment, can drive LEDs 90-1, 90-2, ..., 90-7, including white LEDs and / or blue LEDs, as a load while selecting which LED to light up. Moreover, it is possible to save wiring space compared to the case where the same number of third control signal lines CL3-1, CL3-2, CL3-3, ... are provided as the number of transistors FT1, FT2, ..., FT7 used as switching elements.

[0172] In the examples above, the length of the first period T1 of the first control signal PS1 (and the second control signal PS2) is assumed to be 90 μsec, and the length of the second period T2 is assumed to be 10 μsec, but it is not limited to this. The combined period of the first period T1 and the second period T2 is in the range of 100 microseconds to 10 milliseconds (i.e., frequency 100 Hz to 10 kHz), and the second period T2 may have a length ranging from half to 1 / 20 of the period, typically corresponding to 1 / 10 (i.e., a duty cycle of 10%). This allows the LED to be visually perceived as lit while it is being driven (conversely, it is not perceived that the LED 90 is blinking). Furthermore, many commercially available LEDs are defined with a duty cycle of 10%. Therefore, many commercially available LEDs can be driven within the range defined by this invention. As previously mentioned, if the generation and non-generation of the first control signal PS1 (and the second control signal PS2) are repeated at a longer period, such as from 0.5 seconds to 1.0 second, the LED can be visually perceived as blinking. Furthermore, if the length of the second period T2 is less than 1 / 20th of the above period, it becomes difficult to ensure sufficient LED brightness, but if the length of the second period T2 is 1 / 20th or more of the above period, sufficient LED brightness can be ensured.

[0173] The embodiments described above are illustrative, and various modifications are possible without departing from the scope of this invention. Each of the above embodiments can stand on its own, but they can also be combined. Furthermore, various features within different embodiments can stand on their own, but they can also be combined.

[0174] C1 First capacitor C2 Second capacitor D1 First diode D2 Second diode 11 First control signal supply unit 12 Second control signal supply unit 13 Third control signal supply unit 16 CMOS inverter 17 NMOS inverter 18 Multiplexer 21 First charge pump circuit 22 Second charge pump circuit 100, 200, 300, 400, 500, 600, 700, 800, 900 LED drive circuit

Claims

1. An LED driving circuit configured to drive an LED by applying a voltage between the anode terminal and cathode terminal of an LED as a load, comprising a first control signal supply unit that supplies a first control signal through a first control signal line, wherein the first control signal alternately repeats a first period in which the first control signal line is at ground potential and a second period in which the first control signal line is at a control potential higher than the ground potential, comprising a first charge pump circuit including a first diode and a first capacitor connected in series between the power supply potential and the first control signal line, wherein the first diode allows current to flow in the forward direction from the power supply potential to the first capacitor while blocking current in the reverse direction, the first connection point between the first diode and the first capacitor is connected to the anode terminal, and comprising a switching element connected between the cathode terminal and the ground potential, wherein the switching element is turned off when the first control signal line is at ground potential and turned on when the first control signal line is at the control potential, A first current-limiting resistor element is interposed in series with the LED between the first connection point and the switching element. During the first period, a charging current flows from the power supply potential through the first diode and the first capacitor towards the first control signal line, charging the first capacitor. As a result, the first connection point and the anode terminal reach a positive charging potential equal to the power supply potential minus the forward voltage of the first diode. At the same time, the cathode terminal is isolated from the ground potential by the off-state switching element, and no driving current flows to the LED. During the second period described above, the cathode terminal becomes ground potential due to the switched element being turned on, while the first connection point and the anode terminal temporarily reach a boosted potential, which is increased from the charging potential by the control potential amount via the first capacitor. As a result, the drive current flows from the first control signal line through the first capacitor, the first resistor, the LED, and the switched element in the turned-on state toward the ground potential.An LED driving circuit characterized in that the capacitance of the first capacitor is set such that the above-mentioned driving current continues to flow for more than half the length of the second period.

2. An LED driving circuit according to claim 1, characterized in that the resistance value of the first resistor element is set such that, at the start of the second period, the driving current becomes less than a predetermined upper limit of the forward current of the LED and less than a predetermined upper limit of the output current of the first control signal supply unit.

3. An LED driving circuit according to claim 1 or 2, characterized in that a second current-limiting resistor is interposed between the first diode and the first connection point.

4. An LED driving circuit according to claim 1 or 2, characterized in that the first resistive element is interposed in series with the first capacitor between the first connection point and the first control signal line, instead of between the first connection point and the switching element.

5. An LED driving circuit according to claim 1 or 2, characterized in that the period of the first control signal, when the first period and the second period are combined, is in the range of 100 microseconds to 10 milliseconds, and the second period has a length corresponding to a range of 1 / 2 to 1 / 20 of the period.

6. An LED driving circuit according to claim 1 or 2, wherein the control potential is equal to the power supply potential, the first diode is a Schottky barrier diode, the switching element is an NPN bipolar transistor or an N-channel field-effect transistor, and when the switching element is an NPN bipolar transistor, a base resistor is interposed between the first control signal line and the base terminal of the NPN bipolar transistor, while when the switching element is an N-channel field-effect transistor, a gate-source resistor is connected between the source terminal and the gate terminal of the N-channel field-effect transistor.

7. An LED driving circuit according to claim 1, wherein the load includes a plurality of LEDs, the first connection point is connected to the anode terminals of the plurality of LEDs, the first resistive element is interposed between the first connection point and the anode terminals of the plurality of LEDs, each switching element is interposed between the cathode terminals of the plurality of LEDs and the ground potential, each switching element is provided with a third control signal line corresponding to each switching element, each switching element is configured to turn off when the corresponding third control signal line is at the ground potential and to turn on when the corresponding third control signal line is at the control potential, and the LED driving circuit is further characterized by comprising a third control signal supply unit that performs potential control, keeping all of the third control signal lines at the ground potential during the first period, and keeping the third control signal line corresponding to the LED to be lit at the control potential and the third control signal line corresponding to the LED to be turned off at the ground potential during the second period.

8. An LED driving circuit configured to drive an LED by applying a voltage between the anode terminal and cathode terminal of an LED as a load, comprising: a first control signal supply unit that supplies a first control signal through a first control signal line, wherein the first control signal alternates between a first period when the first control signal line is at ground potential and a second period when the first control signal line is at a control potential higher than ground potential; a second control signal supply unit that supplies a second control signal through a second control signal line, wherein the second control signal alternates between a first period when the second control signal line is at the control potential and a second period when the second control signal line is at ground potential, in the opposite phase to the first control signal; and a first charge pump circuit including a first diode and a first capacitor connected in series between the power supply potential and the first control signal line, wherein the first diode allows current to flow in the forward direction from the power supply potential to the first capacitor, while blocking current in the reverse direction. The first connection point between the first diode and the first capacitor is connected to the anode terminal, and the second charge pump circuit includes a second diode and a second capacitor connected in series between the ground potential and the second control signal line, wherein the second diode allows current to flow in the forward direction from the second capacitor to the ground potential while blocking current in the reverse direction, the second connection point between the second diode and the second capacitor is connected to the cathode terminal, and the first current-limiting resistor element is interposed in series with respect to the LED between the first connection point and the second connection point.During the first period described above, a charging current flows from the power supply potential through the first diode and the first capacitor towards the first control signal line, charging the first capacitor. As a result, the first connection point and the anode terminal reach a positive charging potential equal to the power supply potential minus the forward voltage of the first diode. Simultaneously, a charging current flows from the second control signal line through the second capacitor and the second diode towards the ground potential, charging the second capacitor. As a result, the second connection point and the cathode terminal reach a voltage equal to the forward voltage of the second diode. During the second period described above, the first connection point and the anode terminal temporarily reach a boosted potential obtained by raising the charge potential by the control potential amount via the first capacitor, and the second connection point and the cathode terminal temporarily reach a step-down potential obtained by lowering the forward voltage of the second diode by the control potential amount via the second capacitor, thereby causing the drive current to flow from the first control signal line through the first capacitor, the first resistor element, the LED, and the second capacitor to the second control signal line, and the capacitances of the first capacitor and the second capacitor are set such that the drive current continues to flow for more than half the length of the second period described above.

9. An LED driving circuit according to claim 8, wherein the second control signal supply unit includes an inverter interposed between the power supply potential and the ground potential and controlled by the first control signal, and is configured to supply the output of this inverter as the second control signal through the second control signal line.

10. An LED driving circuit according to claim 8, wherein the load includes a plurality of LEDs, the first connection point is connected to the anode terminals of the plurality of LEDs, the first resistive element is interposed between the first connection point and the anode terminals of the plurality of LEDs, a separate switching element is interposed between the cathode terminals of the plurality of LEDs and the second connection point, a separate third control signal line is provided corresponding to each switching element, each switching element is turned off when the corresponding third control signal line is at ground potential and turned on when the corresponding third control signal line is at control potential, and the LED driving circuit is further characterized by comprising a third control signal supply unit that performs potential control, keeping all of the third control signal lines at ground potential during the first period, and keeping the third control signal line corresponding to the LED to be lit at control potential and the third control signal line corresponding to the LED to be turned off at ground potential during the second period.

11. An LED driving circuit according to claim 7 or 10, wherein the third control signal supply unit comprises a multiplexer including a plurality of input terminals and a number of output terminals greater than the number of input terminals, and the multiplexer is configured to receive the third control signals supplied to the third control signal line at the corresponding input terminal and to control the on / off state of a number of switching elements greater than the number of third control signal lines through the output terminals.

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