CN101583216A - Driving circuit and driving method of light emitting diode - Google Patents

Abstract

The invention provides a driving circuit and a driving method of a light emitting diode. The reference voltage generator generates a reference voltage according to the voltage of the cathode terminal of the light emitting diode, and the voltage conversion circuit dynamically adjusts the driving voltage value of the light emitting diode according to the reference voltage, thereby avoiding unnecessary power consumption caused by overhigh driving voltage.

Description

Light-emitting diode driving circuit and driving method

technical field

The invention relates to a driving circuit and a design method thereof, in particular to a driving circuit capable of dynamically adjusting output power and improving energy use efficiency and a design method thereof.

Background technique

Light emitting diode (Light Emitting Diode, LED) has the functions of power saving and fast switching speed. Car lights, traffic lights, billboard message marquees, large video walls, and even lighting inside projectors, etc. As high-end mobile phones adopt light-emitting diodes as backlight sources, light-emitting diodes have opened up new application fields. The most promising application of light-emitting diodes in the future is the flat-panel display market during 7-40. Once light-emitting diodes can be used as backlight sources for flat-panel displays, their market value will grow significantly.

When driving a circuit or backlight module with multiple LEDs in series, the conduction voltage of each LED will be slightly different due to process variation. In order to turn on all the LEDs, the output voltage is usually preset at a higher level. In order to prevent the phenomenon that a certain light-emitting diode cannot be lit due to a large conduction voltage, it also causes unnecessary power consumption.

1 is a drive circuit according to the conventional technology, which is composed of a boost circuit (boost circuit) 100 and a current source unit 140 to drive a plurality of LED strings 111-119. The boost circuit 100 adjusts the voltage level of the output voltage VOUT according to the duty cycle of the adjustment signal SRE. During driving, the voltage value of the output voltage VOUT can be determined only by determining the duty cycle of the input voltage VIN and the adjustment signal SRE. In order to make the light-emitting diode series 111-119 conduct current (light up), the output voltage VOUT will be adjusted to higher voltage levels. Therefore, the current source unit 140 needs to bear more voltage drop, resulting in redundant power consumption.

Contents of the invention

The invention provides a driving circuit capable of dynamically adjusting the output power, which can drive a light-emitting diode. The invention uses the cathode voltage of the light-emitting diode to automatically correct the size of the output voltage, so as to solve the problems of wasting energy and shortening the working life of the load in the prior art .

The invention also provides a driving circuit capable of dynamically adjusting output power, which can simultaneously drive multiple light emitting diodes, and adjust the driving voltage according to the cathode voltage of the light emitting diodes, so as to avoid unnecessary waste of power.

The present invention also provides a driving circuit capable of dynamically adjusting the output power, which can drive a plurality of light-emitting diodes in series, and adjust the driving voltage according to the cathode voltage of the light-emitting diodes in order to avoid unnecessary waste of power.

The present invention also provides a driving circuit design method that can dynamically adjust the output power, using the cathode terminal voltage of the light-emitting diode to judge whether the driving voltage is too high, and adjust it to an appropriate voltage level to avoid unnecessary power consumption .

The invention proposes a circuit for driving at least one light emitting diode, and the circuit includes a voltage conversion circuit and a reference voltage generator. Wherein, the voltage conversion circuit converts the input voltage into an output voltage, and provides the output voltage to the anode of the LED, wherein the output voltage corresponds to the reference voltage. The reference voltage generator is used to generate a reference voltage, and the reference voltage corresponds to the cathode voltage of the LED.

In an embodiment of the present invention, the reference voltage generator includes a detection unit and a pulse width adjustment unit. Wherein, the detection unit outputs a reference voltage according to the cathode voltage of the LED, and the pulse width adjustment unit adjusts a duty ratio of an adjustment signal according to the reference voltage and a feedback voltage corresponding to the output voltage. Wherein, if the cathode voltage of the light-emitting diode is greater than the threshold voltage, the voltage conversion circuit reduces the output voltage according to the duty cycle of the adjustment signal.

The invention proposes a circuit for driving multiple light emitting diodes, the circuit includes a voltage conversion circuit and a reference voltage. Wherein, the voltage conversion circuit converts an input voltage into an output voltage, and provides the output voltage to the anode of the LED, wherein the output voltage corresponds to the reference voltage. The reference voltage generator is used to generate a reference voltage corresponding to the cathode voltage of one of the LEDs.

The present invention proposes a circuit for driving a plurality of light-emitting diode series, each light-emitting diode series is composed of a plurality of light-emitting diodes connected in series, the circuit includes a voltage conversion circuit and a reference voltage generator, wherein the voltage conversion circuit will An input voltage is converted into an output voltage, and the output voltage is provided to the first terminal (anode terminal) of the LED series, wherein the output voltage corresponds to the reference voltage. The reference voltage generator is used to generate a reference voltage corresponding to the voltage of the second terminal (cathode terminal) of one of the LED series.

From another point of view, the present invention proposes a method for driving a plurality of light-emitting diode strings, each light-emitting diode string is formed by a plurality of light-emitting diodes connected in series, and the method includes the following steps: first, the An input voltage is converted into an output voltage, and the output voltage is provided to the first terminal (anode terminal) of the above-mentioned light-emitting diode series, wherein the above-mentioned output voltage corresponds to a reference voltage; next, a reference voltage is generated, and the reference voltage corresponds to The voltage of the second terminal (cathode terminal) of one of the LED series. Then, according to the reference voltage, adjust the output voltage.

The present invention utilizes the concept of feedback to dynamically adjust the voltage value of the output voltage through the cathode terminal voltage of the light-emitting diode, so as to avoid unnecessary power consumption caused by redundant voltage drop on the current source.

In order to make the above-mentioned features and advantages of the present invention more comprehensible, preferred embodiments are specifically cited below and described in detail with reference to the accompanying drawings.

Description of drawings

FIG. 1 is a diagram of a driving circuit according to a conventional technology.

FIG. 2A is a block diagram of a driving circuit according to a first embodiment of the present invention.

FIG. 2B is a diagram of a driving circuit according to the first embodiment of the present invention.

FIG. 3 is a diagram of a driving circuit according to a second embodiment of the present invention.

FIG. 4 is a diagram of a driving circuit according to a third embodiment of the present invention.

FIG. 5 is a diagram of a driving circuit according to a fourth embodiment of the present invention.

FIG. 6 is a flowchart of a driving method according to a fifth embodiment of the present invention.

Description of reference symbols

100, 220, 320, 243, 343, 443, 543: voltage conversion circuit

200, 300, 400, 500: drive circuit

230, 330, 430, 530: reference voltage generator

242, 252, 342, 352, 442, 452, 542, 552: Comparator

250, 350, 450, 550: Pulse width adjustment unit

251, 351, 451, 551: amplifier

210, 311-319: LEDs

111-119, 411-419, 511-519: LED serial

240, 340, 440, 540: detection unit

341, 441, 541: selection unit

253, 353, 453: drive unit

140, 262, 362, 462, 562: current source unit

420: Voltage conversion circuit of boost circuit

520: Voltage conversion circuit for step-down circuit

553: First drive unit

554: Second drive unit

555: Inverter

S61-S63: Steps

Detailed ways

first embodiment

Please refer to FIG. 2A , which is a block diagram of a driving circuit according to a first embodiment of the present invention. The driving circuit of this embodiment includes a voltage conversion circuit 220 and a reference voltage generator 230 . The reference voltage generator 230 is coupled between the voltage conversion circuit 220 and the LED 210 , the anode terminal of the LED 210 is coupled to the voltage conversion circuit 220 , and the cathode terminal is coupled to the current source unit 262 . The voltage conversion circuit 220 converts the input voltage VIN into an output voltage VOUT, and provides the output voltage VOUT to the anode of the LED 210 , wherein the output voltage VOUT corresponds to the reference voltage output by the reference voltage generator 230 .

The reference voltage generator 230 outputs a reference voltage according to the cathode voltage VD of the LED 210, and then converts the reference voltage into an adjustment signal SRE and outputs it to the voltage conversion circuit 220, and the voltage conversion circuit 220 adjusts the duty ratio of the signal SRE according to, Adjust the voltage value of the output voltage VOUT. When the output voltage VOUT is too large, an unnecessary voltage drop will be generated at the cathode terminal of the light-emitting diode 210 , and the reference voltage generator 230 will adjust the duty ratio of the signal SRE in real time to reduce the output voltage VOUT and avoid unnecessary power consumption.

Next, the circuit structure and operation details of this embodiment will be further described. Please refer to FIG. 2B , which is a diagram of a driving circuit according to this embodiment. The driving circuit 200 includes a voltage conversion circuit 220 and a reference voltage generator 230 . The voltage conversion circuit 220 is responsible for converting the input voltage VIN into an output voltage VOUT to drive the light-emitting diode 210. The reference voltage generator 230 is coupled between the cathode terminal of the light-emitting diode 210 and the voltage conversion circuit 220, and according to the cathode of the light-emitting diode 210 voltage, and output a reference voltage VRE to adjust the output voltage VOUT to avoid redundant power consumption.

The reference voltage generator 230 includes a detection unit 240 and a pulse width adjustment unit 250 . Wherein, the detection unit 240 further includes a comparator 242 and a voltage conversion circuit 243 , and the pulse width adjustment unit 250 includes an amplifier 251 , a comparator 252 and a driving unit 253 . The detection unit 240 is coupled to the cathode terminal of the LED 210 and adjusts the reference voltage VRE according to the cathode voltage VD between the LED 210 and the current source unit 212 . The pulse width adjustment unit 250 changes the duty cycle of the adjustment signal SRE according to the reference voltage VRE and the feedback voltage VFB corresponding to the output voltage VOUT. That is to say, when the cathode voltage VD of the light emitting diode 210 is greater than a threshold voltage value (that is, the preset voltage VSET in the detection unit 240 in FIG. 2 can be determined according to design requirements). The reference voltage generator 230 changes the duty cycle of the adjustment signal SRE to reduce the output voltage VOUT, and avoid excess power consumption caused by an excessively high output voltage VOUT.

In the detection unit 240 , the comparator 242 outputs a comparison voltage VCOM to the voltage conversion circuit 243 according to the cathode voltage VD and the preset voltage VSET. The voltage conversion circuit 243 then outputs the reference voltage VRE to the pulse width adjustment unit 250 according to the comparison voltage VCOM and the preset adjustment voltage VPR. The main function of the voltage conversion circuit 243 is to output the reference voltage VRE corresponding to the voltage value according to the adjustment mechanism of the pulse width adjustment unit 250 to adjust the duty cycle of the adjustment signal SRE.

In the pulse width adjustment unit 250 , the amplifier 251 outputs the adjustment voltage VTN according to the reference voltage VRE and the feedback voltage VFB, wherein the adjustment voltage VTN is a difference gain between the reference voltage VRE and the feedback voltage VFB. The comparator 252 then outputs the adjustment signal SRE according to the comparison result between the adjustment voltage VTN and the triangular wave signal VTRI. When the voltage level of the adjustment voltage VTN changes, the duty cycle of the adjustment signal SRE changes accordingly. The driving unit 253 is coupled between the comparator 252 and the voltage conversion circuit 220 to enhance the driving capability of the adjustment signal SRE.

In this embodiment, the voltage conversion circuit 220 can be a boost circuit (boost circuit) or a step-down circuit. The boost circuit is taken as an example in FIG. depends on the duty cycle. The boost circuit is mainly composed of a switch S1, an inductor L1, a first resistor R1, a second resistor R2, a capacitor C1 and a diode D1. The inductor L1 is coupled between the input voltage VIN and the switch S1, and the other end of the switch S1 is coupled to the ground terminal GND. The diode D1 is coupled between the inductor L1 and the output voltage VOUT, and the capacitor C1 is coupled between the output voltage VOUT and the ground terminal GND.

In addition, the first resistor R1 and the second resistor R2 are connected in series between the output voltage VOUT and the ground terminal GND, and the feedback voltage VFB generated on the shared node thereof is a divided voltage of the output voltage VOUT. The control terminal of the switch S1 is coupled to the adjustment signal SRE, and the boost circuit adjusts the output voltage VOUT to drive the LED 210 according to the duty ratio of the adjustment signal SRE. The current source 262 is coupled between the cathode terminal of the LED 210 and the ground GND, and is mainly used to limit the current passing through the LED 210 to protect the LED 210 and adjust the brightness of the LED 210 .

In this embodiment, the duty cycle of the adjustment signal SRE is determined according to the voltage VD between the LED 210 and the current source unit 212 . When the output voltage VOUT is too high, the shared node between the LED 210 and the current source unit 262 will produce a voltage drop, that is, the cathode voltage VD, and the driving circuit 200 uses the voltage change of the cathode voltage VD to determine whether the output voltage VOUT is too high. to adjust the voltage level of the output voltage VOUT. In other words, when the cathode voltage VD of the light-emitting diode 210 is greater than a threshold voltage (the preset voltage VSET), the voltage conversion circuit 220 reduces the output voltage VOUT according to the duty cycle of the adjustment signal SRE to reduce redundant power consumption.

second embodiment

Please refer to FIG. 3 , which is a structure diagram of a driving circuit according to a preferred embodiment of the present invention. The driving circuit 300 of this embodiment can be used to drive a plurality of LEDs 311 - 319 . The main difference between this driving circuit 300 and the aforementioned FIG. 2B lies in the selection unit 341 in the detection unit 330 . The selection unit 341 selects and outputs the lowest value among the cathode voltages VD1 - VD9 of the LEDs. The comparator 342 is used to compare the lowest value among the cathode voltages VD1 - VD9 of the LEDs 311 - 319 with the preset voltage VSET, and generate a comparison voltage VCOM. The voltage conversion circuit 343 outputs the reference voltage VRE according to the comparison voltage VCOM and the preset adjustment voltage VPR. Then, the output voltage VOUT will vary with the reference voltage VRE.

Due to process variations, the individual conduction voltages of the light emitting diodes 311 - 319 may be slightly different. Therefore, a light-emitting diode with a smaller turn-on voltage has a larger cathode voltage. When the output voltage VOUT is greater than the turn-on voltage required by the LEDs 311 - 319 , the current source unit 362 must bear the excess voltage drop of the output voltage VOUT, which causes unnecessary power consumption.

During the driving process, the output voltage VOUT only needs to be greater than the maximum conduction voltage of the light-emitting diodes 311-319, so the lowest value of the cathode voltages VD1-VD9 of the light-emitting diodes 311-319 can determine whether the output voltage VOUT is too large . In other words, as long as the lowest value of the cathode voltages VD1 - VD9 is maintained greater than a preset voltage VSET, all the LEDs 311 - 319 can be turned on.

Therefore, in this embodiment, the output voltage VOUT is adjusted according to the lowest value among the cathode voltages VD1-VD9 of the LEDs 311-319. When the lowest value among the cathode voltages VD1-VD9 of the LEDs 311-319 is greater than the preset When the voltage is VSET, the voltage converting circuit 320 reduces the output voltage VOUT until the lowest value among the cathode voltages VD1 - VD9 of the LEDs 311 - 319 is less than the preset voltage VSET. The rest of the operation details of this embodiment have been described in detail in the above-mentioned embodiment description of FIG. 2B , which can be easily deduced by those skilled in the art, and will not be repeated here.

third embodiment

Please refer to FIG. 4 , which is a diagram of a driving circuit according to another embodiment of the present invention. The driving circuit 400 of this embodiment can be used to drive a plurality of light emitting diode series 411-419, and each light emitting diode series 411-419 is composed of a plurality of light emitting diodes connected in series. The circuit structure of the driving circuit 400 is the same as that of the embodiment shown in FIG. Similarly, the voltage conversion circuit 420 converts the input voltage VIN into an output voltage VOUT, and provides the output voltage VOUT to the first terminals (anode terminals) of the LED series 411-419, wherein the output voltage VOUT corresponds to the reference voltage VRE. The reference voltage generator 440 outputs the reference voltage VPR according to one of the cathode voltages VD1 - VD9 of the second terminals of the LED series 411 - 419 .

Please refer to the description of FIG. 3 , by analogy, as long as the lowest value of the cathode voltages VD1 - VD9 is found, the light-emitting diode series 411 - 419 with the highest conduction voltage can be deduced. The voltage converting circuit 420 adjusts the voltage value of the output voltage VOUT according to the lowest value among the cathode voltages VD1 - VD9 of the LED series 411 - 419 . When the lowest value among the cathode voltages VD1 - VD9 of the LED series 411 - 419 is too large (greater than the preset voltage VSET), the output voltage VOUT is reduced immediately to avoid unnecessary power consumption. The rest of the operation details of this embodiment are the same as those of the above-mentioned embodiment in FIG. 3 , and will not be repeated here.

Fourth embodiment

In another embodiment of the present invention, the voltage conversion circuit may be a buck circuit. FIG. 5 is a diagram of a driving circuit according to another embodiment of the present invention. The voltage converting circuit 520 in FIG. 5 is a step-down circuit. The step-down circuit includes a first switch S1, a second switch S2, an inductor L1, a first resistor R1, a second resistor R2 and a capacitor C1. The first resistor R1 is connected in series with the second resistor R2, one end is coupled to the LED series 511-519, and the other end is connected to the ground voltage GND. Where the first resistor R1 is coupled to the second resistor R2, a feedback voltage VFB can be measured, and the feedback voltage VFB is a fixed divided voltage of the output voltage VOUT. One end of the inductor L1 is coupled to a common node of the first switch S1 and the second switch S2 , and the other end is coupled to a plurality of LED strings 511 - 519 . The other end of the first switch S1 is coupled to the input voltage VIN, and the other end of the second switch S2 is connected to the ground voltage GND. One end of the capacitor C1 is coupled to the plurality of LED series 511 - 519 , and the other end is coupled to the ground voltage GND.

In terms of the pulse width adjustment unit 550 , the difference between this embodiment and the aforementioned FIG. 4 mainly lies in the inverter 555 , the first driving unit 553 and the second driving unit 554 . Since the output voltage VOUT of the voltage converting circuit 520 is determined by the conduction time of the first switch S1 and the second switch S2, the voltage value of the output voltage VOUT can be adjusted by changing the duty ratio of the adjustment signal SRE. During the driving process, the conduction times of the first switch S1 and the second switch S2 need to be opposite to each other, so the adjustment signal SRE is output to the second switch S2 after passing through the inverter 555 .

In this embodiment, the reference voltage generator 530 also adjusts the reference voltage VRE according to the lowest value among the cathode voltages VD1-VD9 of the LED series 511-519, and then changes the duty ratio of the adjustment signal SRE to adjust the output The voltage value of voltage VOUT. When the output voltage VOUT is too high and an excessive voltage drop is formed between the LED series 511-519 and the current source unit 562, the reference voltage generator 530 will change the duty ratio of the adjustment signal SRE to reduce the output voltage VOUT. voltage value to avoid redundant power consumption. Those skilled in the art should be able to easily deduce the remaining operation details and application methods of this embodiment through the disclosure of this embodiment, which will not be repeated here.

fifth embodiment

From another point of view, this embodiment proposes a driving method for driving light emitting diodes, which can effectively reduce power consumption. FIG. 6 is a flowchart of a driving method according to this embodiment. The driving method of this embodiment can be used to drive a plurality of light-emitting diode strings, and each light-emitting diode string is formed by connecting a plurality of light-emitting diodes in series. The driving method includes the following steps: first, step S61 converts an input voltage into output voltage, and provide the output voltage to the first terminal (anode terminal) of the LED series, wherein the output voltage corresponds to a reference voltage. Next, step S62 generates (adjusts) a reference voltage according to the second terminal voltage (cathode terminal voltage) of one of the LED strings. Then, step S63 adjusts the output voltage according to the reference voltage. Because in this embodiment, there is a dynamic balance between the output voltage and the reference voltage, the change of the output voltage will affect the reference voltage, and the reference voltage will affect the voltage value of the output voltage, so as to avoid excess power caused by excessive output voltage consume. In this embodiment, the rest of the relevant details about the driving method are all described in detail in the above-mentioned description of the embodiment in FIG. 2 to FIG. 5 , and will not be repeated here.

Claims (26)

1. a circuit is used to drive at least one light-emitting diode, and foregoing circuit comprises:

One voltage conversion circuit is converted to an output voltage with an input voltage, and the anode of this output voltage to above-mentioned light-emitting diode is provided, and wherein above-mentioned output voltage is corresponding to a reference voltage; And

One reference voltage generator produces this reference voltage, and this reference voltage is corresponding to the cathode voltage of above-mentioned light-emitting diode.

2. circuit as claimed in claim 1, wherein this reference voltage generator comprises:

One detecting unit according to the cathode voltage of above-mentioned light-emitting diode, is exported this reference voltage; And

One pulse duration adjustment unit, according to reference voltage with corresponding to a feedback voltage of this output voltage, adjust the duty ratio of signal;

Wherein, if the cathode voltage of above-mentioned light-emitting diode is greater than a threshold voltage, this voltage conversion circuit reduces this output voltage according to this duty ratio of adjusting signal.

3. circuit as claimed in claim 2, wherein this detecting unit comprises:

One comparator in order to the cathode voltage and a predeterminated voltage of more above-mentioned light-emitting diode, and produces a comparative voltage; And

One voltage conversion circuit according to this comparative voltage and a default voltage VPR that adjusts, is exported this reference voltage.

4. circuit as claimed in claim 2, wherein this pulse duration adjustment unit comprises:

One amplifier, according to this reference voltage and this feedback voltage, voltage is adjusted in output one;

One comparator is adjusted a voltage and a triangular signal according to this, adjusts the duty ratio of this adjustment signal.

5. circuit as claimed in claim 4, wherein this pulse duration adjustment unit comprises a driver element, is coupled between this comparator and this voltage conversion circuit, in order to strengthen the driving force of this adjustment signal.

6. circuit as claimed in claim 2 also comprises a plurality of resistance, is coupled between this output voltage and the earth terminal, produces this feedback voltage in the dividing potential drop mode.

7. circuit as claimed in claim 1, wherein this voltage conversion circuit comprises a booster circuit or a reduction voltage circuit.

8. a circuit is used to drive a plurality of light-emitting diodes, and this circuit comprises:

One voltage conversion circuit is converted to an output voltage with an input voltage, and the anode of this output voltage to above-mentioned light-emitting diode is provided, and wherein above-mentioned output voltage is corresponding to a reference voltage; And

One reference voltage generator produces this reference voltage, and this reference voltage is corresponding to one of them cathode voltage of above-mentioned light-emitting diode.

9. circuit as claimed in claim 8, wherein this reference voltage generator comprises:

One detecting unit according to the minimum in the cathode voltage of above-mentioned light-emitting diode, is exported this reference voltage; And

One pulse duration adjustment unit, according to reference voltage with corresponding to a feedback voltage of this output voltage, adjust the duty ratio of signal;

Wherein, greater than a threshold voltage, this voltage conversion circuit reduces this output voltage according to this duty ratio of adjusting signal as if the minimum in the cathode voltage of above-mentioned light-emitting diode.

10. circuit as claimed in claim 9, wherein this detecting unit comprises:

Minimum in the cathode voltage of above-mentioned light-emitting diode is selected and exported to one selected cell;

One comparator in order to a minimum in the cathode voltage of more above-mentioned light-emitting diode and a predeterminated voltage, and produces a comparative voltage; And

One voltage conversion circuit according to this comparative voltage and the default voltage of adjusting, is exported this reference voltage.

11. circuit as claimed in claim 9, wherein this pulse duration adjustment unit comprises:

One amplifier, according to this reference voltage and this feedback voltage, voltage is adjusted in output one;

One comparator is adjusted a voltage and a triangular signal according to this, adjusts the duty ratio of this adjustment signal.

12. circuit as claimed in claim 11, wherein this pulse duration adjustment unit comprises driver element, is coupled between this comparator and this voltage conversion circuit, in order to strengthen the driving force of this adjustment signal.

13. circuit as claimed in claim 9 also comprises a plurality of resistance, is coupled between this output voltage and the earth terminal, produces this feedback voltage in the dividing potential drop mode.

14. circuit as claimed in claim 8, wherein voltage conversion circuit comprises a booster circuit or a reduction voltage circuit.

15. circuit as claimed in claim 8, wherein this reference voltage is corresponding to the minimum in the cathode voltage of above-mentioned light-emitting diode.

16. a circuit is used to drive a plurality of light-emitting diode serials, each described light-emitting diode serial is formed by a plurality of light-emitting diode serial connections, and this circuit comprises:

One voltage conversion circuit is converted to an output voltage with an input voltage, and one first end of this output voltage to above-mentioned light emitting diode string row is provided, and wherein above-mentioned output voltage is corresponding to a reference voltage; And

One reference voltage generator produces this reference voltage, and this reference voltage is corresponding to the voltage of one second end one of in the above-mentioned light emitting diode string Xingqi.

17. circuit as claimed in claim 16, wherein this reference voltage generator comprises:

One detecting unit according to the minimum in the cathode voltage of above-mentioned light-emitting diode serial, is exported this reference voltage; And

One pulse duration adjustment unit, according to reference voltage with corresponding to a feedback voltage of this output voltage, adjust the duty ratio of signal;

Wherein, greater than a threshold voltage, this voltage conversion circuit reduces this output voltage according to this duty ratio of adjusting signal as if the minimum in the cathode voltage of above-mentioned light-emitting diode serial.

18. circuit as claimed in claim 17, wherein this detecting unit comprises:

Minimum in the cathode voltage of above-mentioned light-emitting diode serial is selected and exported to one selected cell;

One comparator in order to a minimum in the cathode voltage of more above-mentioned light-emitting diode serial and a predeterminated voltage, and produces a comparative voltage; And

One voltage conversion circuit according to this comparative voltage and the default voltage of adjusting, is exported this reference voltage.

19. circuit as claimed in claim 17, wherein this pulse duration adjustment unit comprises:

One amplifier, according to this reference voltage and this feedback voltage, voltage is adjusted in output one;

One comparator is adjusted a voltage and a triangular signal according to this, adjusts the duty ratio of this adjustment signal.

20. circuit as claimed in claim 19, wherein this pulse duration adjustment unit comprises driver element, is coupled between this comparator and this voltage conversion circuit, in order to strengthen the driving force of this adjustment signal.

21. circuit as claimed in claim 17 also comprises a plurality of resistance, is coupled between this output voltage and the earth terminal, produces this feedback voltage in the dividing potential drop mode.

22. circuit as claimed in claim 16, wherein voltage conversion circuit comprises a booster circuit or a reduction voltage circuit.

23. circuit as claimed in claim 16, wherein this reference voltage is corresponding to the minimum in the voltage of second end of above-mentioned light-emitting diode serial.

24. a method is used to drive a plurality of light-emitting diode serials, each described light-emitting diode serial is all formed by a plurality of light-emitting diode serial connections, and this method comprises the following steps:

One input voltage is converted to an output voltage, and one first end of this output voltage to above-mentioned light emitting diode string row is provided, wherein above-mentioned output voltage is corresponding to a reference voltage;

Produce this reference voltage, this reference voltage is corresponding to the voltage of one second end one of in the above-mentioned light emitting diode string Xingqi; And

According to reference voltage, adjust this output voltage.

25. method as claimed in claim 24, wherein this reference voltage is corresponding to the minimum value in the voltage of second end of above-mentioned light-emitting diode serial.

26. method as claimed in claim 24, wherein according to reference voltage VRE, the step of adjusting this output voltage comprises:

If the minimum in the cathode voltage of above-mentioned light-emitting diode serial is greater than a threshold voltage, this voltage conversion circuit reduces this output voltage according to this duty ratio of adjusting signal.

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