CN101527989B - Circuit and method for driving light emitting diode - Google Patents

Abstract

A circuit and method for driving a light emitting diode, the method for driving the light emitting diode comprising the steps of: an input voltage is converted into an output voltage, and the output voltage is provided to the anode terminal of the LED string. When the output voltage is greater than the first threshold voltage, the output voltage is reduced, and when the minimum value of the cathode terminal voltages of the light emitting diode strings is greater than the second threshold voltage, the output voltage is also reduced. Therefore, the output voltage can be kept at a smaller voltage level to drive the LED string, so as to reduce the redundant power consumption.

Description

Drive circuit and the method for light-emitting diode

Technical field

The present invention relates to a kind of circuit that drives light-emitting diode, and particularly relate to a kind of circuit and method that drives LED serial.

Background technology

Light-emitting diode (Light Emitting Diode, LED) have the functions such as power saving, switching speed be fast, from being used in the status indicator lamp of electronic installation in the past, progressive to becoming the backlight of liquid crystal display, expanding to electrical lighting and the public shows again, as lamp for use in vehicle, traffic signal light, billboard information horse race lamp, large-scale video display wall, or even the illumination in projector etc.Along with the high-order mobile phone adopts light-emitting diode after backlight, light-emitting diode has been opened again new application.The application that following light-emitting diode is expected most is the flat-panel screens market at the 7-40 inch, in case light-emitting diode can become the backlight of flat-panel screens, its market output value will significantly be grown up.

The light-emitting diode that uses in above-mentioned every product is formed by many light-emitting diode connection in series-parallel.Due at present on the promotion circuit of many connection in series-parallel light-emitting diodes, can face because the difference of every light-emitting diode conducting voltage, and output voltage is preset in higher level, to prevent from running into the phenomenon that certain a string light-emitting diode conducting voltage can't be lighted greatly.

Fig. 1 is the circuit diagram of the promotion circuit of traditional many connection in series-parallel light-emitting diodes.As shown in Figure 1, the output voltage VO UT of this circuit is to be determined by feedback resistance R1, R2.And the conducting voltage that output voltage VO UT is greater than the required maximum of each LED serial SL1-SLN at least adds the minimum voltage that current source is required, just has enough voltage to make all LED serial SL1-SLN luminous (conducting).

The circuit of above-mentioned Fig. 1 is although available same group of voltage source (output voltage VO UT) promotes each LED serial SL1-SLN.Every light-emitting diode in LED serial SL1-SLN can produce different conducting voltage because of the difference of process conditions, therefore, in the prior art, for driving all light-emitting diodes, output voltage VO UT can be preset in higher level, with the phenomenon that prevents to light greatly because of certain a string light-emitting diode conducting voltage.Yet, output voltage VO UT is preset in higher level, can be with more voltage drop on current source 101 and reduce energy conversion efficiency and unnecessary power consumption.

Summary of the invention

One of purpose of the present invention is to provide circuit and the method for a driving light-emitting diode, can improve energy conversion efficiency and can adapt to the problem that every light-emitting diode conducting voltage difference causes, and also can reach the function of power saving.

One of purpose of the present invention is that a kind of circuit and method that drives light-emitting diode is being provided, and can adjust the magnitude of voltage of driving voltage according to the conducting voltage of light-emitting diode, and then reduce the power consumption of drive circuit.

The invention provides a kind of circuit, be used for driving a light-emitting diode, this circuit comprises: a change-over circuit, a first control circuit and a second control circuit.Change-over circuit is converted to an output voltage with an input voltage, and the anode of this output voltage to light-emitting diode is provided, and according to a control signal, reduces this output voltage.First control circuit greater than a first threshold voltage, enables control signal when output voltage.Second control circuit greater than a Second Threshold voltage, enables control signal when the cathode voltage of light-emitting diode.

The present invention reintroduces a kind of circuit, is used for driving a plurality of light-emitting diodes, and this circuit comprises: a change-over circuit, a first control circuit and a second control circuit.Change-over circuit is converted to an output voltage with an input voltage, and the anode of this output voltage to light-emitting diode is provided, and wherein, when enabling a control signal, reduces this output voltage.First control circuit higher than a first threshold voltage, enables control signal when output voltage.Second control circuit, the minimum value in the cathode voltage of light-emitting diode enables control signal greater than a Second Threshold voltage.

The present invention proposes again a kind of circuit, is used for driving a plurality of LED serials, and each LED serial is formed by a plurality of light-emitting diode serial connections, and this circuit comprises: a converting unit, one first controller and a second controller.Converting unit is converted to an output voltage with an input voltage, and the first end of this output voltage to LED serial is provided, and wherein ought enable a control signal, reduces this output voltage.The first controller greatly higher than a first threshold voltage, enables control signal when this output voltage.Second controller, the minimum value in the second terminal voltage of LED serial enables control signal greater than a Second Threshold voltage.

A kind of method of the another proposition of the present invention, be used for driving a plurality of LED serials, each LED serial is all formed by a plurality of light-emitting diode serial connections, the method comprises: an input voltage is converted to an output voltage, and provide the first end of this output voltage to LED serial, wherein, when enabling a control signal, reduce this output voltage; Greater than a first threshold voltage, enable this control signal when output voltage; And the minimum value in the second terminal voltage of LED serial enables this control signal greater than a Second Threshold voltage.

The present invention judges according to the cathode voltage of light-emitting diode whether output voltage is too high, and output voltage is adjusted to can promotes the required minimum voltage of each light-emitting diode according to this, to avoid unnecessary power consumption.By this, circuit of the present invention can adapt to the power dissipation problem that every light-emitting diode conducting voltage difference causes, and also can improve the power supply conversion efficiency of drive circuit.

For above and other objects of the present invention, feature and advantage can be become apparent, preferred embodiment of the present invention cited below particularly, and cooperation accompanying drawing are described in detail below.

Description of drawings

Fig. 1 is the circuit diagram that many traditional connection in series-parallel light-emitting diodes promote circuit.

Fig. 2 is the circuit block diagram of the driving light-emitting diode of the embodiment of the present invention.

Fig. 3 is the drive circuit figure according to Fig. 2 embodiment.

Fig. 4 is the oscillogram of the first predeterminated voltage of the embodiment of the present invention.

Fig. 5 A be the embodiment of the present invention the first clock signal with adjust signal concern oscillogram (when control signal CS anergy).

Fig. 5 B be the embodiment of the present invention the first clock signal with adjust signal concern oscillogram (when control signal CS enables).

Fig. 6 is another drive circuit figure according to Fig. 2 embodiment.

Fig. 7 is the figure of one drive circuit again according to Fig. 2 embodiment.

Fig. 8 is another drive circuit figure of Fig. 7.

Fig. 9 is the flow chart of a plurality of LED serials of driving of the embodiment of the present invention.

The reference numeral explanation

200: change-over circuit

210: first control circuit

220: second control circuit

230: current source cell

250: load

310: booster circuit

322: selected cell

324: the first comparators

326: the second comparators

332: or door

The 334:SR trigger

336,337: buffer

340: pwm unit

820: reduction voltage circuit

C1: electric capacity

CS: control signal

CLK1: the first clock signal

CLK2: second clock signal

D0: diode

D1-DN: light-emitting diode

GND: earth terminal

L1: inductance

R1, R2: resistance

S1: switch

SL1-SLN: LED serial

SRE: adjust signal

V1-VN: the cathode voltage of light-emitting diode

VSL1-VSLN: the second terminal voltage of LED serial

VIN: input voltage

VMIN: valley voltage

VOUT: output voltage

VFB: feedback voltage

VSET1: the first predeterminated voltage

VSET2: the second predeterminated voltage

S901-S903: the step of the embodiment of the present invention.

Embodiment

Fig. 2 is the circuit block diagram of the driving light-emitting diode of the embodiment of the present invention.Please refer to Fig. 2, this circuit comprises: change-over circuit 200, first control circuit 210, second control circuit 220, current source cell 230 and load 250.Change-over circuit 200 is converted to an output voltage VO UT to load 250 with an input voltage VIN, and according to the voltage drop between load 250 and current source cell 230, adjusts output voltage VO UT.Load 250 is coupled between output voltage VO UT and current source cell 230, therefore, when output voltage VO UT is too high, voltage drop can occur between current source cell 230 and load 250.In the present embodiment, load 250 can be the formed backlight of light-emitting diode, may be single light-emitting diode, a plurality of light-emitting diode or LED serial.

First control circuit 210 is coupled between change-over circuit 200 and output voltage VO UT, greater than first threshold voltage, enables a control signal as output voltage VO UT, makes change-over circuit 200 reduce output voltage VO UT.220 of second control circuits are coupled between the cathode terminal and change-over circuit 200 of load 250, when the voltage drop between load 250 and current source cell 230 during greater than Second Threshold voltage, enable above-mentioned control signal, make change-over circuit 200 reduce output voltage VO UT.Current source cell 230 is coupled to the cathode terminal of load 250, in order to bear unnecessary voltage drop and the magnitude of current that passes through of restriction load 250 with protection load 250.

Fig. 3 is the drive circuit figure according to Fig. 2 embodiment.Please refer to Fig. 3, drive circuit comprises change-over circuit 200, first control circuit 210, second control circuit 220, wherein change-over circuit 200 comprises booster circuit 310 and pwm unit 340, and pwm unit 340 still comprises or door 332, set-reset flip-floop 334 and buffer 336.First control circuit 210 is made of resistance R 1, R2 and comparator 324, and 220 of second control circuits are made of comparator 326.Booster circuit 310 is converted to output voltage VO UT with input voltage VIN, and the anode of this output voltage VO UT to light-emitting diode D1 is provided, in order to drive light-emitting diode D1.When output voltage VO UT is too high, or door 332 according to the output enable control signal CS of first control circuit 210 and second control circuit 220 to reduce output voltage VO UT.

Or door 332 outputs according to second clock signal CLK2 and the first comparator 324 and the second comparator 326, output control signal CS is to the replacement end R of set-reset flip-floop 334.The end S that arranges of set-reset flip-floop 334 is coupled to the first clock signal clk 1, and signal SRE is adjusted in its output Q output one.Buffer 336 is coupled between the control end (grid) of the output Q of set-reset flip-floop 334 and the switch S 1 in booster circuit 310, in order to strengthening the driving force of adjusting signal SRE, and increases the switching speed of switch S 1.In operating process, if output voltage VO UT is too high, just first control circuit 210 can enable control signal CS with second control circuit 220, makes and adjust the effect that signal SRE generation pulse is covered, allow the shut-in time of switch S1 with increase, and then output voltage VO UT is descended.

In other words, during greater than first threshold voltage, first control circuit namely enables control signal CS to reduce output voltage VO UT as output voltage VO UT.Greater than Second Threshold voltage, second control circuit just enables control signal CS to reduce output voltage VO UT when the cathode voltage of light-emitting diode D1.Wherein, first threshold voltage, Second Threshold voltage can be decided according to design requirement by the user.

Next, further illustrate circuit structure and the details of operation of the present embodiment.At first, first control circuit 210 comprises resistance R 1, R2 and the first comparator 324.Resistance R 1 is connected with resistance R 2 and is coupled between output voltage VO UT and earth terminal GND, with output voltage VO UT dividing potential drop to produce feedback voltage V FB.The positive input terminal of the first comparator 324 and negative input end couple respectively feedback voltage V FB and the first predeterminated voltage VSET1, and its output is coupled to or a door input of 332.Wherein, the setting of the first predeterminated voltage VSET1 as shown in Figure 4, when power supply opening, the first predeterminated voltage VSET1 can rise to definite value in time, therefore, the rate of climb of output voltage VO UT can be subject to the restriction of the first predeterminated voltage VSET1, avoids the excessive velocities of boosting and damages the circuit of load end.Wherein, the maximum voltage value of the first predeterminated voltage VSET1 and its rate of climb can be adjusted according to circumstances by the user.

During greater than the first predeterminated voltage VSET1, first control circuit 210 just enables control signal CS to reduce output voltage VO UT as feedback voltage V FB.Because feedback voltage V FB produces via the dividing potential drop of output voltage VO UT, therefore, as feedback voltage V FB greater than the first predeterminated voltage VSET1, represent that namely output voltage VO UT is greater than first threshold voltage, wherein, first threshold voltage can be got by the relation of resistance R 1, R2 corresponding to the first predeterminated voltage VSET1.In other words, the set point via feedback voltage V FB, resistance R 1, R2 can determine the first threshold magnitude of voltage.Have in the art and know that usually the knowledgeable should know by circuit theory the relation between first threshold voltage and the first predeterminated voltage VSET1 by inference, do not add tired stating at this.

Second control circuit 220 comprises the second comparator 326.The positive input terminal of the second comparator 326 and negative input end couple respectively cathode voltage V1 and the second predeterminated voltage VSET2 of light-emitting diode D1, and its output is coupled to or an input of door 332.During greater than the second predeterminated voltage VSET2, second control circuit 220 just enables control signal CS to reduce output voltage VO UT when the voltage drop between light-emitting diode D1 and current source cell 230 (that is cathode voltage V1 of light-emitting diode).Wherein, the second predeterminated voltage VSET2 namely is expressed as Second Threshold voltage.

Pwm unit 340 comprises or door 332, set-reset flip-floop 334 and buffer 336.Or the input of door 332 is respectively coupled to the output of the first comparator 324, output and the second clock signal CLK2 of the second comparator 326, and according to the output of second clock signal CLK2 and the first comparator 324 and the second comparator 326, export control signal CS to the replacement end R of set-reset flip-floop 334 at its output again.When the replacement end R reception control signal CS (high-voltage level) of set-reset flip-floop 334, set-reset flip-floop 334 can reset (Reset) action, make and adjust the effect that signal SRE generation pulse is covered, and export this in its output Q and adjust signal SRE.

Wherein, the relation of the first clock signal clk 1 of above-mentioned set-reset flip-floop 334 and adjustment signal SRE is as shown in Fig. 5 A and 5B.In Fig. 5 A, control signal CS is in disabled state (low voltage level), adjusts signal SRE similar to the clock of the first clock signal clk 1.In Fig. 5 B, control signal CS is in enabled state (high-voltage level), makes the waveform of adjusting signal SRE be covered at least one pulse.At this moment, adjust signal SRE exports the switch S 1 of booster circuit 310 to via buffer 336 control end (grid).Produce owing to adjusting signal SRE the effect (as shown in Fig. 5 B) that pulse is covered, and be the time increase of low voltage level, make the shut-in time of switch S 1 elongate, just output voltage VO UT decreases.

The second embodiment

Fig. 6 is another drive circuit figure according to Fig. 2 embodiment.Please refer to Fig. 6, drive circuit comprises change-over circuit 200, first control circuit 210, second control circuit 220, and wherein change-over circuit 200 comprises booster circuit 310 and pwm unit 340.First control circuit 210 is identical with Fig. 3, is not described at this.220 of second control circuits are made of selected cell 322 and the second comparator 326.Booster circuit 310 is converted to output voltage VO UT with input voltage VIN, and the anode of this output voltage VO UT to light-emitting diode D1-DN is provided, in order to drive light-emitting diode D1-DN.When output voltage VO UT is too high, or door 332 according to the output enable control signal CS of first control circuit 210 and second control circuit 220 to reduce output voltage VO UT.Downgrade the method for output voltage VO UT in the existing explanation of Fig. 3, do not described at this.

Next, further illustrate circuit structure and the details of operation of the present embodiment.At first, the structure of first control circuit 210 is identical with mode of operation and Fig. 3, does not add tired stating at this.

Second control circuit 220 comprises selected cell 322 and the second comparator 326.Selected cell 322 is coupled between the cathode terminal and the second comparator 326 of each light-emitting diode D1-DN, is used for the valley voltage VMIN of the voltage drop (that is cathode voltage V1-VN of light-emitting diode D1-DN) between selection and output light emitting diode D1-DN and current source cell 230.The positive input terminal of the second comparator 326 and negative input end couple respectively selected cell 322 and the second predeterminated voltage VSET2, and its output is coupled to or a door input of 332.During greater than the second predeterminated voltage VSET2, second control circuit 220 just enables control signal CS to reduce output voltage VO UT as the valley voltage VMIN that selects unit 322 outputs.Then, the motion flow that downgrades output voltage VO UT is identical with Fig. 3, is not described at this.

The 3rd embodiment

Fig. 7 is the figure of one drive circuit again according to Fig. 2 embodiment.Please refer to Fig. 7, drive circuit comprises change-over circuit 200, first control circuit 210, second control circuit 220, and wherein change-over circuit 200 comprises booster circuit 310 and pwm unit 340.First control circuit 210 is identical with Fig. 3 with second control circuit 220, is not described at this.Booster circuit 310 is converted to output voltage VO UT with input voltage VIN, and the first end of this output voltage VO UT to LED serial SL1-SLN (that is anode tap of each LED serial SL1-SLN) is provided, in order to drive LED serial SL1-SLN.When output voltage VO UT is too high, or door 332 according to the output enable control signal CS of first control circuit 210 and second control circuit 220 to reduce output voltage VO UT.Downgrade the method for output voltage VO UT in the existing explanation of Fig. 3, do not described at this.

Next, further illustrate circuit structure and the details of operation of the present embodiment.At first, the structure of first control circuit 210 is identical with mode of operation and Fig. 3, does not add tired stating at this.Second control circuit 220 comprises selected cell 322 and the second comparator 326.Selected cell 322 is coupled between the second end (that is cathode terminal of LED serial SL1-SLN) and the second comparator 326 of each LED serial SL1-SLN, is used for the valley voltage VMIN of the second terminal voltage VSL1-VSLN of selection and output light emitting diode tandem SL1-SLN.The positive input terminal of the second comparator 326 and negative input end couple respectively selected cell 322 and the second predeterminated voltage VSET2, and its output is coupled to or a door input of 332.During greater than the second predeterminated voltage VSET2, second control circuit 220 just enables control signal CS to reduce output voltage VO UT as the valley voltage VMIN that selects unit 322 outputs.Then, the motion flow that downgrades output voltage VO UT is identical with Fig. 3, is not described at this.

Have in the art and usually know the booster circuit 310 of the knowledgeable in should be known in above-described embodiment, also effective drop volt circuit 820 enforcements, as shown in Figure 8.

Fig. 8 is another drive circuit figure of Fig. 7.The circuit of Fig. 8 and Fig. 7, its main difference is that Fig. 8 is replaced by reduction voltage circuit 820 with booster circuit in Fig. 7 310, wherein, switch S 1 and S2 present complementary relation (switch S 1 are coupled to the inverse output terminal of set-reset flip-floop 334 via forward output Q, the switch S 2 that buffer 336 is coupled to set-reset flip-floop 334 via buffer 337

), namely when switch S 1 conducting, switch S 2 cut-offs, in other words, when switch S 1 cut-off, switch S 2 conductings.

When the time of switch S 1 conducting is longer than the time of switch S 2 conductings, can increase the magnitude of voltage of output voltage VO UT.When time of switch S 1 conducting than time of switch S 2 conductings in short-term, can reduce the magnitude of voltage of output voltage VO UT.Therefore when output voltage VO UT was excessive, first control circuit 210 or second control circuit 220 just enabled control signal CS to the replacement end R of set-reset flip-floop 334.At this moment, set-reset flip-floop 334 is carried out the function of resetting, and makes to adjust the effect that signal SRE generation pulse is covered, and then reduces the time of switch S 1 conducting, to reduce output voltage VO UT.

Have in the art and know that usually the knowledgeable is coupled to output voltage VO UT in should be known in above-described embodiment and is not limited to only have 2 resistance R 1, R2 to connect with resistance between earth terminal GND, visual demand increases to the resistance number of connecting more than 3 or 3.

Compare above-described embodiment and existing circuit, the circuit of above-described embodiment can by first control circuit 210 and second control circuit 220, be adjusted the size of output voltage VO UT.When output voltage VO UT is excessive, can enable control signal CS by first control circuit and second control circuit 220, with reduction output voltage VO UT, and make output voltage VO UT reach an optimum value (but just being the voltage of LED serial SL1-SLN conducting).In addition, existing circuit can only be set in output voltage VO UT higher level, so that each LED serial conducting.Significantly, the embodiment that is implemented out with spirit institute of the present invention can be than the consumption on the existing circuit minimizing energy, and can adapt to the problem that every light-emitting diode conducting voltage difference causes.

Fig. 9 is the flow chart of the driving LED serial method of the embodiment of the present invention.Please refer to Fig. 9, the method comprises: at first, and by in change-over circuit, input voltage being converted to output voltage (step S900).Output voltage is produced a feedback voltage and input in the first comparator after a plurality of electric resistance partial pressures, in order to judge that output voltage is whether greater than the first threshold voltage of this first comparator.Greater than first threshold voltage, enable a control signal when output voltage, to reduce output voltage (step S901).

On the other hand, provide first end to each LED serial with output voltage, then the second terminal voltage with each LED serial inputs to selected cell, and selects the valley voltage in the second terminal voltage of each diode tandem by this selected cell.Valley voltage is inputed in the second comparator, in order to judge that valley voltage is whether greater than the Second Threshold voltage of this second comparator.Greater than Second Threshold voltage, enable a control signal when valley voltage, to reduce output voltage (step S902).

It is worth mentioning that, although in above-described embodiment to the method that drives LED serial and use its circuit to depict a possible kenel, usually know that the knowledgeable should know but have in affiliated technical field, each manufacturer is all different for the design of change-over circuit 200, first control circuit 210 and second control circuit 220, and therefore application of the present invention is when being not restricted to the possible kenel of this kind.In other words, as long as can utilize first control circuit 210 and second control circuit 220 to detect the output voltage VO UT of change-over circuit 200, when output voltage is excessive, just enable control signal, make the adjustment signal SRE of pwm unit 340 produce the effect that pulse is covered, and then downgrade output voltage VO UT, be just to have met spiritual place of the present invention.

Indulge the above, the present invention detects the output voltage of change-over circuit by first control circuit and second control circuit, change again pulse frequency and the waveform of the clock signal in pwm unit according to the size of output voltage, and according to the change of this pulse frequency and waveform, and then the setting of reaching output voltage.Therefore, output voltage can be set in and can promote the required minimum voltage of each light-emitting diode, to avoid unnecessary power consumption, and the power dissipation problem that the difference of every light-emitting diode conducting voltage of adapting to circuit of the present invention causes also can improve the power supply conversion efficiency of drive circuit.

Although the present invention discloses as above with preferred embodiment; so it is not to limit the present invention; any affiliated technical field has knows the knowledgeable usually; without departing from the spirit and scope of the present invention; when can do a little change and retouching, so protection scope of the present invention is as the criterion when looking the claim person of defining of the present invention.

Claims (25)

1. A circuit for driving a light-emitting diode, used to drive a light-emitting diode, the circuit comprising: a conversion circuit, which converts an input voltage into an output voltage, and provides the output voltage to the anode of the light emitting diode, and reduces the output voltage according to a control signal; The first control circuit enables the control signal to reduce the output voltage when the output voltage is greater than a first threshold voltage; and The second control circuit enables the control signal to reduce the output voltage when the cathode voltage of the LED is greater than a second threshold voltage. 2. The circuit for driving light-emitting diodes as claimed in claim 1, wherein the first control circuit comprises: The first comparator, according to a first preset voltage corresponding to the first threshold voltage and a feedback voltage generated by voltage division through a plurality of resistors connected in series between the output voltage and a ground terminal, so as to enable the control signal. 3. The circuit for driving light emitting diodes as claimed in claim 1, wherein the second control circuit comprises: The second comparator enables the control signal according to a second preset voltage corresponding to the second threshold voltage and the cathode voltage of the LED. 4. The circuit for driving light-emitting diodes as claimed in claim 1, wherein the conversion circuit comprises: a pulse width modulation unit, outputting an adjustment signal according to a first clock signal and outputs of the first control circuit and the second control circuit; and The boost circuit adjusts the output voltage according to the adjustment signal. 5. The circuit for driving a light emitting diode as claimed in claim 4, wherein the pulse width modulation unit comprises: an OR gate, outputting the control signal according to a second clock signal and outputs of the first control circuit and the second control circuit; and An SR flip-flop, a set terminal S of the SR flip-flop is coupled to the first clock signal, a reset terminal R of the SR flip-flop is coupled to the control signal, and outputs the adjustment signal. 6. The circuit for driving a light emitting diode as claimed in claim 5, wherein the pulse width modulation unit comprises: The buffer is coupled between the boost circuit and the output of the SR flip-flop to enhance the driving capability of the adjustment signal. 7. The circuit for driving light-emitting diodes as claimed in claim 1, wherein the conversion circuit comprises: a pulse width modulation unit, outputting an adjustment signal according to a first clock signal and outputs of the first control circuit and the second control circuit; and The step-down circuit adjusts the output voltage according to the adjustment signal. 8. A circuit for driving light-emitting diodes, for driving a plurality of light-emitting diodes, the circuit comprising: a conversion circuit, converting an input voltage into an output voltage, and providing the output voltage to the anode of the light emitting diode, and reducing the output voltage according to a control signal; a first control circuit, enabling the control signal to reduce the output voltage when the output voltage is higher than a first threshold voltage; and The second control circuit enables the control signal to reduce the output voltage when the cathode voltage of the selected one of the LEDs is greater than a second threshold voltage. 9. The circuit for driving light emitting diodes as claimed in claim 8, wherein the first control circuit comprises: The first comparator, according to a first preset voltage corresponding to the first threshold voltage and a feedback voltage generated by voltage division through a plurality of resistors connected in series between the output voltage and a ground terminal, so as to enable the control signal. 10. The circuit for driving light emitting diodes as claimed in claim 8, wherein the second control circuit comprises: a selection unit for selecting and outputting a minimum value among the LED cathode voltages; and The second comparator enables the control signal according to a second preset voltage corresponding to the second threshold voltage and the output of the selection unit. 11. The circuit for driving light emitting diodes as claimed in claim 8, wherein the first control circuit comprises: a pulse width modulation unit, outputting an adjustment signal according to a first clock signal and outputs of the first control circuit and the second control circuit; and The boost circuit adjusts the output voltage according to the adjustment signal. 12. The circuit for driving a light emitting diode as claimed in claim 11, wherein the pulse width modulation unit comprises: an OR gate, outputting the control signal according to a second clock signal and outputs of the first control circuit and the second control circuit; An SR flip-flop, a set terminal S of the SR flip-flop is coupled to the first control signal, a reset terminal R of the SR flip-flop is coupled to the control signal, and outputs the adjustment signal. 13. The circuit for driving a light emitting diode as claimed in claim 12, wherein the pulse width modulation unit comprises: The buffer is coupled between the boost circuit and the output of the SR flip-flop to enhance the driving capability of the adjustment signal. 14. The circuit for driving a light emitting diode as claimed in claim 8, wherein the conversion circuit comprises: a pulse width adjustment unit, outputting an adjustment signal according to a first clock signal and outputs of the first control circuit and the second control circuit; and The step-down circuit adjusts the output voltage according to the adjustment signal. 15. The circuit for driving light emitting diodes according to claim 8, wherein the cathode voltage of the selected one of the light emitting diodes is a minimum value among the cathode voltages of the light emitting diodes. 16. A circuit for driving light-emitting diodes, used to drive a plurality of light-emitting diode strings, each of the light-emitting diode strings is formed by a plurality of light-emitting diodes connected in series, the circuit comprising: a conversion unit, which converts an input voltage into an output voltage, and provides the output voltage to a first end of the LED string, and reduces the output voltage according to a control signal; a first controller, enabling the control signal to reduce the output voltage when the output voltage is greater than a first threshold voltage; and The second controller enables the control signal to reduce the output voltage when the voltage at the second end of the selected one of the LED strings is greater than a second threshold voltage. 17. The circuit for driving a light emitting diode as claimed in claim 16, wherein the first control circuit comprises: The first comparator generates a feedback voltage according to a first preset voltage corresponding to the first threshold voltage and divides the voltage through a plurality of resistors connected in series between the output voltage and a ground terminal, so as to enable the control Signal. 18. The circuit for driving light emitting diodes as claimed in claim 16, wherein the second control circuit comprises: a selection unit for selecting and outputting the minimum value among the voltages at the second terminals of the light emitting diode strings; and The second comparator enables the control signal according to a second preset voltage corresponding to the second threshold voltage and the output of the selection unit. the 19. The circuit for driving a light emitting diode as claimed in claim 16, wherein the first control circuit comprises: a pulse width modulation unit, outputting an adjustment signal according to a first clock signal and outputs of the first control circuit and the second control circuit; and The boost circuit adjusts the output voltage according to the adjustment signal. 20. The circuit for driving a light emitting diode as claimed in claim 19, wherein the pulse width modulation unit comprises: an OR gate, outputting the control signal according to a second clock signal and outputs of the first control circuit and the second control circuit; An SR flip-flop, a set terminal S of the SR flip-flop is coupled to the first control signal, a reset terminal R of the SR flip-flop is coupled to the control signal, and outputs the adjustment signal. 21. The circuit for driving a light emitting diode as claimed in claim 20, wherein the pulse width modulation unit comprises: The buffer is coupled between the boost circuit and the output of the SR flip-flop to enhance the driving capability of the adjustment signal. 22. The circuit for driving a light emitting diode as claimed in claim 16, wherein the conversion circuit comprises: a pulse width modulation unit, outputting an adjustment signal according to a first clock signal and outputs of the first control circuit and the second control circuit; and The step-down circuit adjusts the output voltage according to the adjustment signal. 23. The circuit for driving LEDs as claimed in claim 16, wherein the voltage at the second terminal of the selected one of the LEDs is a minimum value among the voltages at the second terminals of the LED strings. 24. The circuit for driving LEDs as claimed in claim 16, wherein the second end of the LED string is a cathode of the LED string. 25. A method for driving a light emitting diode, for driving a plurality of light emitting diode strings, each of the light emitting diode strings is formed by a plurality of light emitting diodes connected in series, the method comprising: converting an input voltage into an output voltage, providing the output voltage to the first end of the light emitting diode string, and reducing the output voltage according to a control signal; When the output voltage is greater than a first threshold voltage, enabling the control signal to reduce the output voltage; and When the minimum value of the voltages at the second terminals of the LED series is greater than a second threshold voltage, the control signal is enabled to reduce the output voltage. the

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