Driver circuits for light-emitting elements

Abstract

Circuit for operating a series of light-emitting diodes, wherein the circuit comprises: a first transistor (M1) comprising a gate, a source coupled to ground through a first resistive element (R2), and a drain; a circuit to provide a voltage of a predetermined value to the source of the first transistor (M1); a second transistor (M2) which has a gate, a source which is electrically coupled to ground and a drain for electrical connection with the series of light-emitting diodes; a second resistive element (R3) having a first end that is electrically coupled to the gate of the first transistor (M1) and a second end that is electrically coupled to the gate of the second transistor (M2); a first current source (16) which is electrically coupled between the second end of the second resistive element (R3) and ground; a third resistive element (R4) having one end electrically coupled to the drain of the first transistor (M1) and a second end electrically coupled to the drain of the second transistor (M2).

Description

[0001] The present disclosure relates to circuits for operating light-emitting elements, such as light-emitting diodes (LEDs).

[0002] LEDs are current-driven devices whose brightness is proportional to their forward current. The forward current can be controlled in several ways. For example, one technique is to use the LED IV (current-voltage IV) curve to determine what voltage to apply to the LED to produce a desired forward current. Another technique for regulating LED current is to drive the LED with a constant current source. The constant current source can help eliminate changes in current due to fluctuations in the forward voltage, resulting in a constant LED brightness. With this technique, the input power supply regulates the voltage across a current-sense resistor rather than regulating the output voltage. The power supply reference voltage and the value of the current-sense resistor determine the LED current.

[0003] One problem with some LED driver circuits is high power consumption.

[0004] US 2009 / 0 322 235 A1 describes a circuit with LEDs which are dimmed by switching between an operating current and a holding current or by voltage limiting a source / drain of a MOSFET which is coupled to the LED module.

[0005] US 2006 / 0 055 465 A1 describes a current mirror circuit comprising a current mirror unit, a feedback unit, and an amplifier unit. The current mirror unit has multiple terminal ends, one end of which is coupled to a current source and the other end of which is coupled to a load device.

[0006] The subject matter described in the present disclosure relates to LED driver circuits that, in some implementations, may help reduce power consumption.

[0007] In one aspect, a circuit for driving light-emitting elements includes, for example, a first transistor having a source coupled to ground through a first resistive element, and a second transistor having a gate electrically coupled to a gate of the first transistor, a source electrically coupled to ground, and a drain for electrically connecting to a first group of light-emitting elements. The circuit further includes a circuit for providing a predetermined voltage to the source of the first transistor, a circuit for compensating for a difference in respective gate-source voltages of the first and second transistors, and a circuit for compensating for a difference in respective drain-source voltages of the first and second transistors.

[0008] In a second aspect, a circuit for driving an array of light-emitting diodes includes a first transistor having a gate, a source coupled to ground through a first resistive element, and a drain. A circuit is provided for providing a voltage having a predetermined value to the source of the first transistor. A second transistor has a gate, a source electrically coupled to ground, and a drain for electrical connection to the array of light-emitting diodes. A second resistive element has a first end electrically coupled to a gate of the first transistor and a second end electrically coupled to the gate of the second transistor. A first current source is electrically coupled between the second end of the second resistive element and ground.A third resistive element has one end electrically coupled to the drain of the first transistor and a second end electrically coupled to the drain of the second transistor.

[0009] Various devices that may include the driver circuits, as well as methods of operating them, are described below.

[0010] Some implementations include one or more of the following advantages. For example, as discussed above, in some implementations, the circuits can achieve comparatively low power consumption. The second transistor generates a relatively controlled and stable drive current that, in some implementations, varies little, if at all, with changes in the LED string voltage.

[0011] Further aspects, features and advantages will become apparent from the following detailed description, the accompanying drawings and the claims. Brief description of the drawings Fig. Figure 1 is a simplified diagram illustrating the operation of multiple LED strings. Fig. Figure 2 shows details of an example circuit for driving a single row of LEDs. Fig. 3 is a flowchart of a method of operation of the driver circuit. Fig. 4 shows details of an example circuit for operating multiple LED rows. Fig. Figure 5 shows another example of a circuit for driving multiple LED rows. Fig. 6 shows an example of an LED driver circuit with a protection circuit. Detailed description

[0012] As in Fig. 1, outputs from the LED driver circuit 10 are each coupled to LED rows 11. In the example of Fig. 1, there are six LED rows 11 connected in parallel, each of which includes ten LEDs 11A connected in series. However, in some implementations, the driver circuit 10 may drive a different number of LED rows (e.g., 8 or 16), and in some cases, it may drive only a single LED row. Furthermore, in some implementations, the number of LEDs in each row 11 may differ from ten.

[0013] The number of LED rows, as well as the number of LEDs in each row, may depend on the specific type of device and application. The LED driver technology described herein can be used, for example, for backlighting and solid-state lighting applications. Examples of such applications include LCD televisions, PC monitors, specialty control panels (for example, in industrial, military, medical, or aerospace applications), and general lighting for commercial, residential, industrial, and administrative applications. The LED driver technology described herein can also be used in other applications, including backlighting for various handheld devices. The driver circuit 10 can be implemented as an integrated circuit, for example, fabricated from a substrate made of silicon or another semiconductor.

[0014] As in Fig. As shown in Figure 1, the driver circuit 10 includes connections to a power supply voltage (VCC) and to ground. The LED rows 11 are connected to an LED power supply voltage (V POWER-LED ) coupled.

[0015] As in the example of Fig. 2, the driver circuit 10 includes a plurality of current sources 12, 14, 16, an operational amplifier 18, resistors R1, R2, R3, R4, and transistors M1, M2. The reference current source 12 generates a current that flows through the resistor R1. The current flow creates a reference voltage V1 at the non-inverting input (in+) of the operational amplifier 18. Substantially the same voltage (V1) appears at the inverting input (in-) of the operational amplifier 18, and this voltage also appears at the resistor R2, which is coupled between the source of the transistor M1 and ground. Thus, the operational amplifier 18 regulates the voltage appearing at the source of the transistor M1 by maintaining the voltage at the inverting input (in) at the same level as the voltage appearing at the non-inverting input (in+).

[0016] As also in Fig. 2, the output of operational amplifier 18 is connected to the gate of transistor M1 and (through resistor R3) to the gate of transistor M2. Transistors M1, M2 may, for example, be implemented as MOS transistors. In the illustrated examples, the size (e.g., area) of transistor M2, which provides the current for a string of LEDs coupled to the drain of transistor M2, is X times larger than the size of transistor M1. The value of X can vary widely depending on the specific circuit design. In some implementations, the ratio of the sizes of transistors M1:M2 is on the order of about 1:1000. The relative sizes of transistors M1, M2 can be used to generate a larger current for the string of LEDs.For example, when the gate-source voltages (Vgs) of transistors M1, M2 are substantially equal, transistor M2 provides a controllable, substantially stable current that is substantially X times the current through transistor M1. In actual implementations, however, the gate-source voltages across the transistors differ from each other due to the fact that the source of transistor M2 is directly connected to ground, whereas the source of transistor M1 is connected to ground through resistor R2. Without additional circuit components, such as those described below (e.g., resistors R3, R4, and current source 16), the current produced by transistor M2 typically depends on the voltage of the LED string due to the difference in gate-source voltages.Therefore, in the absence of the additional circuit components (e.g., resistors R3, R4 and current source 16), the current generated by transistor M2 for the LED string varies and is therefore not well controlled or stable.

[0017] To help ensure that the current generated by the second transistor M2 remains at the desired level, additional circuit components (e.g., resistors R3, R4 and current source 16) are provided to compensate for differences in the gate-source voltages of the transistors M1, M2 and to compensate for differences in their drain-source voltages.

[0018] To compensate for the difference in the gate-source voltages of transistors M1, M2, resistor R3 is coupled between the gates of transistors M1, M2. Additionally, a current source 16 is coupled between the gate of transistor M2 and ground. The values ​​of resistor R3 and current source 16 should be selected such that the voltage V1 across resistor R2 is substantially equal to the value of resistor R3 multiplied by the current I3 generated by current source 16 (i.e., V1 = I3 × R3). The voltage generated by current I3 (from source 16) flowing through resistor R3 compensates for the difference in the gate-source voltages of transistors M1, M2. To compensate for the difference in the drain-source voltages (Vds) of the transistors M1, M2, the resistor R4 is coupled between the respective drains of the transistors.

[0019] As in Fig. 3, in operation, the circuit 10 provides a predetermined voltage at the source of the first transistor M1 (102), compensates for a difference in respective gate-source voltages of the first and second transistors M1, M2 (104), and compensates for a difference in respective drain-source voltages of the first and second transistors M1, M2 (106).

[0020] This example assumes the values ​​of resistors R2, R3, and R4 are the same. In this case, half the current from current source 14 flows through transistor M1 and resistor R2, and the same amount of current flows through resistor R4. Therefore, in this example, a current of I2 / 2 flows through transistor M1 (and resistor R2). Similarly, when the voltage of the LED string is lower than the power supply voltage (VCC), a current of I2 / 2 flows through resistor R4 to compensate for the difference in drain-source voltages between transistors M1 and M2.

[0021] From the above example, it can also be seen that the voltage V1 at the source of transistor 1 is equal to the product of the resistance R2 and the current flowing through that resistance (i.e., V1 = I2 / 2 × R2). The voltage V1 is equal to the product of the current from the current source 12 and the resistance R1 (i.e., V1 = I1 × R1). Values ​​for the current sources 12, 14 and the resistors R1, R2 can be selected using the above information.

[0022] As described above, the values ​​of resistor R3 and the third current source 16 are selected such that V1 = I3 × R3. Using the previously described example where R3 = R2, the value of the current source is set equal to I2 / 2 to compensate for the difference in gate-source voltages of transistors M1 and M2.

[0023] In some implementations, the values ​​of the resistors and current sources may differ from those in the example described above.

[0024] Using the driver circuit 10 of Fig. 2, the current generated by transistor M2 can be essentially independent of the voltage of the LED string. Circuit 10 can therefore provide a more stable drive current.

[0025] The extent of energy savings that can be achieved in some implementations can be estimated by considering a driver circuit without transistor M2, current sources 14, 16, and resistors R3, R4, but with the drain of transistor M1 coupled to the LED string. If V1 is equal to 250 mV and the current required for transistor M1 is equal to 60 mA, the power consumption is on the order of 0.015 W. If eight LED strings are present in the device, the power consumption in the string is on the order of 0.12 W. This voltage and current requirement across resistor R2 results in a considerable waste or loss of energy. In contrast, the driver circuit 10 can be Fig. 2 achieve a significant reduction in energy consumption, for example in the order of 99% in some implementations.

[0026] Furthermore, the driver circuit 10 can be Fig. 2 can lead to a significant reduction in the size of the chip area. For a driver circuit without transistor M2, current sources 14, 16 and resistors R3, R4, but with the drain of transistor M1 coupled to the LED string, the ratio R1:R2 can be in the order of 1000 in some implementations, which may require a large chip area for resistor R2. In contrast, the driver circuit of Fig. 2 does not require such a large ratio of resistance values ​​and can therefore significantly reduce the amount of required chip area (for example, by about 20% in some implementations).

[0027] Fig. 4 and Fig. 5 show examples of circuits for operating multiple LED strings. If the PWM control of the respective LED strings is to be essentially the same (e.g., the same phase and frequency), the circuit 20 can be Fig. 4. The circuit 20 is similar to the circuit 10 of Fig. 2 except that an additional transistor M3 is provided to provide the current for the second LED row. As in the example of Fig. As shown in Figure 4, the gate of transistor M3 is coupled to the gate of transistor M2, which in turn is coupled to the gate of transistor M1 through resistor R3, as described above. The drain of transistor M2 is coupled to the first LED row, whereas the drain of transistor M3 is coupled to the second LED row. The source of transistor M3, like the source of transistor M2, is directly coupled to ground. In this example, the size of transistor M3 can be substantially equal to the size of transistor M2.

[0028] If, on the other hand, the PWM control of the respective LED rows is to differ from each other, the circuit 30 can be Fig. 5. Different LED rows may, for example, require different currents if the rows contain different types of LEDs (for example, if the first row contains LEDs that emit light of a first color, and the second row contains LEDs that emit light of a second color that is different from the first color). The circuit 30 of Fig. 5 contains several (in this case two) copies of the circuit 10 of Fig. 2. Each circuit 10 is coupled to one of the LED rows.

[0029] Although Fig. 4 and Fig. 5 only depicts two LED rows, some implementations may include a larger number of LED rows. In that case, additional circuitry can be added as needed. For example, as shown in Fig. 4, additional transistors similar to M2 and M3 may be provided to generate the current required to drive the additional LED strings. Similarly, additional copies of circuit 10 may be provided to provide the current required to drive the additional LED strings.

[0030] Fig. 6 shows a driver circuit 40 which is similar to the circuit of Fig. 2, but including a protection diode 42 or other circuit components to protect the power source 14 in the event that the voltage across the LED array exceeds the power supply voltage VCC. Instead of the diode 42, other circuit components can be used, such as a clamp. Furthermore, the protection circuit can be separate from the power source 14 or be part of the power source 14.

[0031] Each resistive element R1, R2, R3, R4 can be implemented as a single resistive component or as a combination of resistive components connected in series or parallel.

[0032] Further implementations are also within the scope of the claims.

Claims

[1] A circuit for driving a series of light-emitting diodes, the circuit comprising: a first transistor (M1) having a gate, a source coupled to ground through a first resistive element (R2), and a drain; a circuit for providing a voltage having a predetermined value to the source of the first transistor (M1); a second transistor (M2) having a gate, a source electrically coupled to ground, and a drain for electrical connection to the series of light-emitting diodes; a second resistive element (R3) having a first end electrically coupled to the gate of the first transistor (M1) and a second end electrically coupled to the gate of the second transistor (M2); a first current source (16) electrically coupled between the second end of the second resistive element (R3) and ground; a third resistive element (R4) having one end electrically coupled to the drain of the first transistor (M1) and a second end electrically coupled to the drain of the second transistor (M2). [2] The circuit of claim 1, wherein the circuit for providing a regulated voltage to the source of the first transistor (M1) includes an operational amplifier (18) having a non-inverting input connected to a node in the circuit at the predetermined voltage, an inverting input electrically coupled to the source of the first transistor (M1), and an output electrically coupled to the gate of the first transistor (M1). [3] The circuit of claim 1, further comprising a third transistor (M3) having a gate electrically coupled to the gate of the second transistor (M2), a source electrically coupled to ground, and a drain for electrical connection to a second series of light emitting diodes. [4] The circuit of claim 3, wherein the second and third transistors (M2, M3) have substantially the same size and are larger than the first transistor (M1). [5] The circuit of claim 1, wherein the second transistor (M2) is larger than the first transistor (M1). [6] The circuit of claim 1, further comprising a second current source (14) coupled between the drain of the first transistor (M1) and a power supply voltage. [7] The circuit of claim 1, further comprising: a second current source (14) electrically coupled between the drain of the first transistor (M1) and a power supply voltage; and a circuit for protecting the second power source (14) when a voltage of the series of light-emitting diodes is greater than the power supply voltage. [8] The circuit of claim 7, wherein the circuit for protecting the second current source (14) includes a diode (42). [9] Device comprising: a first row of light-emitting elements; a first transistor (M1) having a gate, a source coupled to ground through a first resistive element (R2), and a drain; a circuit for providing a predetermined voltage to the source of the first transistor (M1); a second transistor (M2) having a gate, a source electrically connected to ground, and a drain electrically connected to the first row of light-emitting elements; a second resistive element (R3) having a first end electrically connected to the gate of the first transistor (M1) and a second end electrically connected to the gate of the second transistor (M2); a first current source (16) electrically connected between the second end of the second resistive element (R3) and ground; a third resistive element (R4) having one end electrically connected to the drain of the first transistor (M1) and a second end electrically connected to the drain of the second transistor (M2). [10] The apparatus of claim 9, further comprising: a second row of light-emitting elements; and a third transistor (M3) having a gate electrically connected to the gate of the second transistor (M2), a source electrically connected to ground, and a drain electrically connected to the second row of light-emitting elements. [11] The device of claim 10, wherein the second and third transistors (M2, M3) have substantially the same size and are larger than the first transistor (M1). [12] The device of claim 10, wherein the light-emitting elements in the first row are LEDs that emit light of a first color, and the light-emitting elements in the second row are LEDs that emit light of a second color. [13] The device of claim 9, wherein the light-emitting elements in the first row are LEDs. [14] The device of claim 9, wherein the second transistor (M2) occupies a larger area than the first transistor (M1). [15] The device of claim 9, further comprising a second current source (14) coupled between the drain of the first transistor (M1) and a power supply voltage. [16] The apparatus of claim 9, wherein the circuit for providing the predetermined voltage to the source of the first transistor (M1) includes an operational amplifier (18) having a non-inverting input connected to a circuit node at the predetermined voltage, an inverting input electrically connected to the source of the first transistor (M1), and an output electrically connected to the gate of the first transistor (M1). [17] The device according to claim 9, wherein the first and second transistors (M1, M2) are MOS type transistors. [18] The device of claim 9, wherein the first, second and third resistive elements (R3, R4) have substantially the same resistance value. [19] A method of generating a drive current for a series of LEDs in a circuit, the method comprising: Providing a predetermined voltage to a source of a first transistor (M1) in the circuit, the source of the first transistor (M1) being coupled to ground through a first resistive element (R2); and the circuit further comprises: a second transistor (M2), the second transistor (M2) having a gate, a source electrically coupled to ground, and a drain electrically coupled to the row of LEDs; a second resistive element (R3) having a first end electrically coupled to a gate of the first transistor (M1) and a second end electrically coupled to the gate of the second transistor (M2); a first current source (16) electrically coupled between the second end of the second resistive element (R3) and ground; and a third resistive element (R4) having one end electrically coupled to the drain of the first transistor (M1) and a second end electrically coupled to the drain of the second transistor (M2).

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