A light-emitting diode dimming drive circuit operating at a low power supply voltage

Abstract

The present invention relates to the field of drive circuits, and more particularly to a light-emitting diode dimming drive circuit operating at a low power supply voltage. The circuit comprises: a power supply, a capacitor, a constant current source generator, a first switch, a second switch, and a third switch. In a first mode, the second switch and the third switch are configured to maintain an on state in response to a received pulse, and the first switch is configured to maintain an off state. In a second mode, the second switch and the third switch are configured to maintain an off state, and the first switch is configured to maintain an on state in response to a received pulse. The first mode and the second mode are executed alternately. The present invention proposes a drive circuit that, compared to commonly used boost circuits, eliminates the need for an inductor. Compared to general voltage-doubling charge pump circuits, it eliminates the need for an output capacitor. Furthermore, the circuit boost is achieved with a minimum number of power switches, resulting in a smaller circuit area and lower cost.

Description

Technical Field

[0001] The present invention relates to the field of driving circuits, and in particular to a light emitting diode dimming driving circuit operating at a low power supply voltage. Background Art

[0002] Many portable electronic devices, such as mobile phones, chargers, laptops, tablets, and Bluetooth headsets, are equipped with WLEDs as operating indicators. By adjusting the brightness and flashing patterns of the WLEDs, different functions can be indicated. These portable electronic devices are generally powered by lithium batteries.

[0003] Light-emitting diodes (LEDs) require a bias voltage from the anode to the cathode to reach their forward turn-on voltage (hereinafter referred to as the turn-on voltage). For WLEDs, this voltage is typically around 3V. The brightness of an LED is determined by the current flowing through it; higher currents increase the brightness. If an LED switches on and off periodically, and its switching frequency is above the range where flicker is noticeable to the naked eye (typically above 50Hz), its brightness is determined by the average current flowing during the entire on-off period; higher average currents increase the brightness.

[0004] A simple LED driver circuit such as Figure 1 As shown in the figure, the battery symbol represents the power supply of the driver circuit, which consists of a switch S1 and a constant current generator I1. When S1 is on, the DC current I generated by I1 flows through the LED, causing it to emit light. When S1 is off, the current path is cut off, and the LED does not emit light. For such a driver circuit, I must be controlled at a desired constant value, and the voltage at the driver output node VLED must be raised above the turn-on voltage to ensure proper LED activation.

[0005] If the battery voltage is higher than the turn-on voltage, the LED can be driven directly. However, for some portable electronic devices that use low-voltage batteries (such as fully charged <3V) as power supply, and for applications where the battery voltage drops below the turn-on voltage during use, the drive circuit with the above structure cannot provide a drive voltage higher than the power supply voltage and cannot turn on the LED normally.

[0006] In order to use white light diodes under low power supply voltage conditions, the driver circuit often adopts a boost topology (called Boost circuit). A simplified LED driver circuit with boost function is shown as follows: Figure 2 When the power supply voltage is higher than the turn-on voltage, Figure 2 S1, I1 and Figure 1S1 and I1 use the same control method. When the power supply voltage is lower than the turn-on voltage, the boost circuit consisting of S2, S3, and the inductor operates, raising the equivalent voltage VLED across the LED to above the turn-on voltage, enabling normal LED operation. However, the inductor in the boost circuit is often large and expensive, making it unsuitable for drive circuits requiring a small footprint.

[0007] Existing technology also uses a voltage-doubling charge pump circuit topology, consisting of multiple power switches, charge pump capacitors, and output voltage-stabilizing capacitors. This circuit uses switching at a constant frequency to raise the output voltage to twice the supply voltage. This voltage is then driven by a constant current source, requiring only half the supply voltage. However, this architecture requires a relatively large output capacitor, especially for high drive currents. The resulting high component count and high cost of this topology are significant. Summary of the Invention

[0008] In view of the above technical problems, the present invention provides a light emitting diode dimming driving circuit operating at a low power supply voltage, so as to save component space and effectively drive the diode to emit light with a lower power supply voltage.

[0009] An object of the present invention is to provide a light-emitting diode dimming drive circuit operating at a low power supply voltage. The circuit includes: a power supply, a capacitor, a constant current source generator, a first switch, a second switch, and a third switch. One end of the first switch is connected to the power supply and one end of the third switch, and the other end is connected to the capacitor and one end of the second switch. The other end of the second switch is connected to the power supply, and the other end of the third switch is connected to the other end of the capacitor and one end of the constant current generator. The other end of the constant current generator is connected to a light-emitting diode. In a first mode, the second switch and the third switch are configured to maintain an on state in response to a received pulse, and the first switch is configured to maintain an off state; in a second mode, the second switch and the third switch are configured to maintain an off state, and the first switch is configured to maintain an on state in response to a received pulse; the first mode and the second mode are executed alternately.

[0010] Furthermore, the first switch and the second switch are further configured to maintain an off state in a third mode, and the third switch is configured to maintain an on state in response to a received pulse in the third mode.

[0011] Furthermore, the first switch, the second switch and the third switch are MOS transistors.

[0012] Furthermore, the first switch, the second switch and the third switch are N-type MOS transistors.

[0013] Furthermore, the capacity of the capacitor and the time constant formed by the constant current generator are greater than the duration of the second mode.

[0014] Furthermore, the circuit also includes a fourth switch, which is connected to the common end of the capacitor and the third switch, and to the input end of the constant current generator, wherein: the fourth switch is configured to maintain an off state when no pulse is received in the first mode, and to maintain an on state in response to a received pulse in the second mode.

[0015] By adopting the above scheme, the beneficial effects of the present invention are:

[0016] The present invention proposes a driving circuit that can save inductors compared with commonly used boost circuits, and save output capacitors compared with general voltage-doubling charge pump circuits, and achieve circuit boost with a minimum number of power switches, making the circuit area smaller and the cost lower. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 This is a circuit schematic diagram of an LED driving circuit of the present invention;

[0018] Figure 2 This is a circuit schematic diagram of an LED driving circuit with a boost function in the prior art of the present invention;

[0019] Figure 3 A circuit schematic diagram of a light emitting diode dimming drive circuit operating at a low power supply voltage according to an embodiment of the present invention;

[0020] Figure 4 1 is a waveform diagram of voltages Vc and Vout according to an embodiment of the present invention;

[0021] Figure 5 1 is a waveform diagram of pulse signals received by the first switch, the second switch, the third switch, and the fourth switch in one embodiment of the present invention;

[0022] Figure 6 This is a circuit schematic diagram of a light emitting diode dimming driving circuit operating at a low power supply voltage according to another embodiment of the present invention. DETAILED DESCRIPTION

[0023] Typical embodiments embodying the features and advantages of the present invention are described in detail in the following description. It should be understood that the present invention is capable of various variations in different embodiments without departing from the scope of the present invention, and that the descriptions and illustrations herein are intended to be illustrative rather than limiting.

[0024] In the description of the present invention, it should be understood that the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the quantity of the technical features indicated. Therefore, features specified as "first" or "second" may explicitly or implicitly include one or more of the aforementioned features.

[0025] In the description of the present invention, it should be noted that, unless otherwise expressly specified or limited, the term "connection" refers to the electrical connection of related elements according to their inherent characteristics and logical relationships in order to achieve the technical objectives of the present invention. This electrical connection may be a direct connection or an indirect electrical connection through an intermediary. Those skilled in the art will understand the specific meaning of each term in this application document based on specific circumstances.

[0026] like Figure 3 As shown, an embodiment of the present invention provides a light-emitting diode dimming drive circuit operating at a low power supply voltage. The circuit includes: a power supply, a capacitor Cfly, a constant current source generator I1, a first switch S1, a second switch S2, and a third switch S3. One end of the first switch S1 is connected to the power supply and one end of the third switch S3, and the other end is connected to the capacitor Cfly and one end of the second switch S2. The other end of the second switch S2 is connected to the power supply, the other end of the third switch S3 is connected to the other end of the capacitor Cfly and one end of the constant current generator I1, and the other end of the constant current generator I1 is connected to the light-emitting diode WLED. In a first mode, the second switch S2 and the third switch S3 are configured to maintain an on state in response to a received pulse, and the first switch S1 is configured to maintain an off state. In a second mode, the second switch S2 and the third switch S3 are configured to maintain an off state, and the first switch S1 is configured to maintain an on state in response to a received pulse. The first mode and the second mode are executed alternately.

[0027] When the power supply voltage Vs is lower than the turn-on voltage of the light-emitting diode WLED, the power supply cannot directly drive the light-emitting diode WLED. This embodiment utilizes a first switch S1, a second switch S2, a third switch S3, and a capacitor Cfly to drive the light-emitting diode WLED in two operating modes. In the first mode, the second and third switches S2 and S3 are simultaneously turned on, while the first switch S1 is turned off. If the on-state voltage drop of the third switch S3 is small and negligible, the output voltage Vout of the driver circuit equals the power supply voltage VS. Simultaneously, the voltage of the capacitor Cfly is charged to Vs. In this first mode, the operating voltage VLED of the light-emitting diode equals Vs, but is lower than the turn-on voltage, and the light-emitting diode WLED does not illuminate. In the second mode, the second and third switches S2 and S3 are turned off, while the first switch S1 is turned on. At this time, the capacitor Cfly supplies power to the output constant current source I1 generating circuit through the first switch S1, illuminating the light-emitting diode WLED.

[0028] The specific principle is as follows Figure 4 As shown, when the circuit operates in the first mode, the output voltage Vout is equal to the power supply voltage Vs. At this time, the capacitor Cfly is charged, so the voltage Vc rises. When the circuit operates in the second mode, the output voltage Vout is equal to Vs+Vc. Since the capacitor Cfly is discharging during this time period, the voltage drops. The charging and discharging of the capacitor Cfly in the first and second modes causes ripples in the voltage of the capacitor Cfly. If the time constant of the discharge of the capacitor Cfly is much longer than the time of the second mode, the ripple can be ignored. Therefore, the output voltage Vout of the second mode is approximately twice the power supply voltage Vs, that is, 2*Vs. As long as the power supply voltage is greater than half of the turn-on voltage of the light-emitting diode WLED, the present driving circuit can drive the light-emitting diode WLED.

[0029] In addition, it is worth explaining that the time constant of the discharge of the capacitor Cfly is determined by the capacity of the capacitor Cfly and the constant current generator I1.

[0030] In one embodiment, the first switch S1 and the second switch S2 are further configured to be maintained in an off state in the third mode, and the third switch S3 is configured to be maintained in an on state in response to a received pulse in the third mode.

[0031] Among them, when the power supply voltage Vs is higher than the turn-on voltage of the light-emitting diode WLED, there is no need to boost the power supply voltage, and there is no need to charge the capacitor Cfly. The power supply voltage Vs can directly drive the light-emitting diode through the third switch S3. Therefore, the driving circuit can adapt to a variable power supply circuit.

[0032] In one embodiment, the first switch S1, the second switch S2, and the third switch S3 are MOS transistors. Specifically, the first switch, the second switch, and the third switch are N-type MOS transistors.

[0033] Among them, a MOS transistor with a high-level gate voltage is used as a control switch, and the same or different high-level pulses act on the gates of the first switch S1, the second switch S2, and the third switch S3, respectively, thereby turning them on. After the pulse signal is removed, the corresponding first switch S1, the second switch S2, and the third switch S3 become disconnected, thereby making the switching states of the first switch S1, the second switch S2, and the third switch S3 can be switched arbitrarily.

[0034] In addition, the driving circuits of the gates of the first switch S1 , the second switch S2 , and the third switch S3 may be driven by CMOS inverters to drive and amplify in a push-pull stage.

[0035] like Figure 5 As shown, corresponding to the first mode and the second mode, waveforms of the gate voltages of the first switch S1, the second switch S2 and the third switch S3 varying with time are provided. Since the first mode and the second mode are performed alternately, the gate voltages of the first switch S1, the second switch S2 and the third switch S3 also vary alternately.

[0036] In one embodiment, if Figures 5 and 6 As shown, the circuit further includes a fourth switch S4, which is connected to a common end of the capacitor Cfly and the third switch S3, and to an input end of the constant current generator I1, wherein: the fourth switch S4 is configured to remain in an off state when no pulse is received in the first mode, and to remain in a on state in response to a received pulse in the second mode.

[0037] In one embodiment, a light-emitting diode (LED) dimming method is provided in conjunction with the aforementioned driving circuit. As known in the prior art, whether a light-emitting diode (WLED) can be turned on is determined by its turn-on voltage, while its operating brightness is determined by its average operating current (Iaverage). The peak current (Imax) in the circuit is determined by the constant current generator (I1). Therefore, the brightness of the light-emitting diode (WLED) can be varied by adjusting the peak current (Imax) of the constant current generator (I1) or controlling the time ratio (duty cycle) between the first and second modes.

[0038] Where, Iaverage=Imax*D1;

[0039] Where D1 is the duty cycle of the second mode. When the duty cycle is 50%, the average driving current Iaverage is Imax / 2. The larger the duty cycle D1 is, the closer the average current Iaverage is to the peak current Imax, and the higher the brightness of the light emitting diode WLED is.

[0040] At the same time, a lower frequency modulation signal can also be used to control the driving circuit (the first switch S1, the second switch S2 and the third switch S3) or the working enable switch of the constant current generator I1. The enable duty cycle of the modulation signal is D3, and the average value of the driving current is

[0041] Iaverage=Imax*D1*D3

[0042] The frequency of the adjustment signal should be lower than the switching frequency of the drive circuit, but higher than the frequency of light flicker visible to the naked eye, generally above 50Hz.

[0043] By using the above-mentioned various embodiments, the present invention proposes a driving circuit that can eliminate the inductor compared with the commonly used boost circuit, and eliminate the output capacitor compared with the general voltage-doubling charge pump circuit, and realizes circuit boost with a minimum number of power switches, so that the white light LED can be driven with arbitrary dimmable light under low battery voltage applications, and the circuit area is smaller and the cost is lower.

[0044] While the present disclosure has been described with reference to several exemplary embodiments, it should be understood that the terms used are illustrative and exemplary rather than restrictive. Since the present disclosure can be embodied in many forms without departing from the spirit or essence of the invention, it should be understood that the above-described embodiments are not limited to any of the foregoing details, but should be interpreted broadly within the spirit and scope of the appended claims. Therefore, all changes and modifications that fall within the scope of the claims or their equivalents are intended to be covered by the appended claims.

Claims

1. A light emitting diode dimming drive circuit operating at low power supply voltage, characterized in that: The circuit includes: a power supply, a capacitor, a constant current source generator, a first switch, a second switch, and a third switch, wherein one end of the first switch is connected to the power supply and one end of the third switch, and the other end of the first switch is connected to the capacitor and one end of the second switch, the other end of the second switch is connected to the power supply, the other end of the third switch is connected to the other end of the capacitor and one end of the constant current source generator, and the other end of the constant current source generator is connected to a light emitting diode, wherein: In a first mode, the second switch and the third switch are configured to maintain an on state in response to a received pulse, and the first switch is configured to maintain an off state; In a second mode, the second switch and the third switch are configured to be maintained in an off state, and the first switch is configured to be maintained in an on state in response to a received pulse; The first mode and the second mode are executed alternately; The time constant formed by the capacitance of the capacitor and the constant current source generator is greater than the duration of the second mode; The circuit further includes a fourth switch connected to a common terminal of the capacitor and the third switch, and to an input terminal of the constant current source generator, wherein: The fourth switch is configured to maintain an off state when no pulse is received in the first mode, and to maintain an on state in response to a received pulse in the second mode.

2. The light emitting diode dimming driving circuit operating at a low power supply voltage according to claim 1, characterized in that: The first switch and the second switch are further configured to maintain an off state in a third mode, and the third switch is configured to maintain an on state in response to a received pulse in the third mode.

3. The light emitting diode dimming driving circuit operating at a low power supply voltage according to claim 1, characterized in that: The first switch, the second switch and the third switch are MOS transistors.

4. The light emitting diode dimming driving circuit operating at a low power supply voltage according to claim 3, characterized in that: The first switch, the second switch and the third switch are N-type MOS transistors.

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