CN101527989A - 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 a light emitting diode comprises the following steps: converting an input voltage into an output voltage, and providing the output voltage to the anode end of a light emitting diode string. When the output voltage is greater than a first threshold voltage, the output voltage is reduced, and when the minimum value of the cathode end voltage of the light emitting diode string is greater than a second threshold voltage, the output voltage is also reduced. Thus, the output voltage can be maintained at a relatively low voltage level to drive the light emitting diode string, so as to reduce unnecessary power consumption.

Description

The circuit of driven for emitting lights diode and method

Technical field

The present invention relates to a kind of circuit of driven for emitting lights diode, and particularly relate to a kind of circuit and method of driven for emitting lights diode tandem.

Background technology

Light-emitting diode (Light Emitting Diode, LED) have 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 the electrical lighting and the public again shows, as automobile-used lamp, traffic signal light, billboard information horse race lamp, large-scale video display wall, or even the illumination in the projector etc.Along with the high-order mobile phone adopts light-emitting diode behind backlight, light-emitting diode has been opened new application again.The application that following light-emitting diode is expected most is the flat-panel screens market when 7-40, in case light-emitting diode can become the backlight of flat-panel screens, its market output value will significantly be grown up.

Employed light-emitting diode in above-mentioned every product is formed in parallel by many light emitting diode string.Because 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 to run 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 though available same group of voltage source (output voltage VO UT) promotes each LED serial SL1-SLN.Every light-emitting diode among the 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 on current source 101 and reduce energy conversion efficiency and unnecessary power consumption with more voltage drop.

Summary of the invention

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

One of purpose of the present invention is in circuit that a kind of driven for emitting lights diode is provided and method, 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 to drive 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, when output voltage greater than a first threshold voltage, then enable control signal.Second control circuit, when the cathode voltage of light-emitting diode greater than one second threshold voltage, then enable control signal.

The present invention reintroduces a kind of circuit, is used to drive 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, then reduces this output voltage.First control circuit when output voltage is higher than a first threshold voltage, then enables control signal.Second control circuit, the minimum value in the cathode voltage of light-emitting diode then enables control signal greater than one second threshold voltage.

The present invention proposes a kind of circuit again, is used to drive 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 one second controller.Converting unit is converted to an output voltage with an input voltage, and one first end of this output voltage to LED serial is provided, and wherein ought enable a control signal, then reduces this output voltage.First controller when this output voltage is higher than a first threshold voltage greatly, then enables control signal.Second controller, the minimum value in second terminal voltage of LED serial then enables control signal greater than one second threshold voltage.

The present invention proposes a kind of method in addition, be used to drive 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 first end of this output voltage to LED serial, wherein, when enabling a control signal, then reduce this output voltage; When output voltage greater than a first threshold voltage, then enable this control signal; And the minimum value in second terminal voltage of LED serial then enables this control signal greater than one 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 redundant power consumption.By this, circuit of the present invention can adapt to the power dissipation problem that every light-emitting diode conducting voltage difference is caused, 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 conjunction with figs. 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 driven for emitting lights diode of the embodiment of the invention.

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

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

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

Fig. 5 B be the embodiment of the invention 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 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: 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: second terminal voltage of LED serial

VIN: input voltage

VMIN: valley voltage

VOUT: output voltage

VFB: feedback voltage

VSET1: first predeterminated voltage

VSET2: second predeterminated voltage

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

Embodiment

Fig. 2 is the circuit block diagram of the driven for emitting lights diode of the embodiment of the 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 the current source cell 230, adjusts output voltage VO UT.Load 250 is coupled between output voltage VO UT and the 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 the output voltage VO UT, when output voltage VO UT greater than first threshold voltage, then enable a control signal, make 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 the current source cell 230 during greater than second threshold voltage, then 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 the unnecessary voltage drop and the magnitude of current that passed 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 then 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 this output voltage VO UT is provided the anode to light-emitting diode D1, in order to driven for emitting lights 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 first comparator 324 and second comparator 326, output control signal CS is to the replacement end R of set-reset flip-floop 334.The end S that is provided with of set-reset flip-floop 334 is coupled to 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 the 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, first control circuit 210 just can enable control signal CS with second control circuit 220, makes to 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, as output voltage VO UT during greater than first threshold voltage, first control circuit promptly enables control signal CS to reduce output voltage VO UT.When the cathode voltage of light-emitting diode D1 greater than second threshold voltage, second control circuit just enables control signal CS to reduce output voltage VO UT.Wherein, first threshold voltage, second threshold voltage can be decided according to design requirement by the user.

Next, further specify the circuit structure and the details of operation of present embodiment.At first, first control circuit 210 comprises resistance R 1, R2 and first comparator 324.Resistance R 1 is connected with resistance R 2 and is coupled between output voltage VO UT and the earth terminal GND, with output voltage VO UT dividing potential drop to produce feedback voltage V FB.The positive input terminal of first comparator 324 and negative input end couple the feedback voltage V FB and the first predeterminated voltage VSET1 respectively, and its output then 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 subjected 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.

As feedback voltage V FB during greater than the first predeterminated voltage VSET1, first control circuit 210 just enables control signal CS to reduce output voltage VO UT.Because feedback voltage V FB is the dividing potential drop generation via output voltage VO UT, therefore, when feedback voltage V FB greater than the first predeterminated voltage VSET1, represent that promptly 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 then 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 present technique field and to know that usually the knowledgeable should know the relation between the first threshold voltage and the first predeterminated voltage VSET1 by circuit theory by inference, do not add tired stating at this.

Second control circuit 220 comprises second comparator 326.The positive input terminal of second comparator 326 and negative input end couple the cathode voltage V1 and the second predeterminated voltage VSET2 of light-emitting diode D1 respectively, and its output then is coupled to or an input of door 332.When the voltage drop between light-emitting diode D1 and the current source cell 230 (that is cathode voltage V1 of light-emitting diode) during greater than the second predeterminated voltage VSET2, second control circuit 220 just enables control signal CS to reduce output voltage VO UT.Wherein, the second predeterminated voltage VSET2 promptly 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 first comparator 324, the output and the second clock signal CLK2 of second comparator 326, and according to the output of second clock signal CLK2 and first comparator 324 and second comparator 326, export the replacement end R of control signal CS again to set-reset flip-floop 334 at its output.When the replacement end R of set-reset flip-floop 334 received control signal CS (high-voltage level), set-reset flip-floop 334 (Reset) action of can resetting made that adjusting signal SRE produces the effect that pulse is covered, and exports this in its output Q and adjust signal SRE.

Wherein, the relation of first clock signal clk 1 of above-mentioned set-reset flip-floop 334 and adjustment signal SRE is shown in Fig. 5 A and 5B.In Fig. 5 A, control signal CS is in disabled state (low voltage level), and it is similar to the clock of first clock signal clk 1 to adjust signal SRE.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 the effect (shown in Fig. 5 B) that pulse is covered owing to adjust signal SRE, 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.

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 the selected cell 322 and second comparator 326.Booster circuit 310 is converted to output voltage VO UT with input voltage VIN, and this output voltage VO UT is provided the anode to light-emitting diode D1-DN, in order to driven for emitting lights 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.The method that downgrades output voltage VO UT is not described at this in the existing explanation of Fig. 3.

Next, further specify the circuit structure and the details of operation of 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 the selected cell 322 and second comparator 326.Selected cell 322 is coupled between the cathode terminal and second comparator 326 of each light-emitting diode D1-DN, be used to select and output light emitting diode D1-DN and current source cell 230 between the valley voltage VMIN of voltage drop (that is cathode voltage V1-VN of light-emitting diode D1-DN).The positive input terminal of second comparator 326 and negative input end couple the selected cell 322 and the second predeterminated voltage VSET2 respectively, and its output then is coupled to or a door input of 332.As the valley voltage VMIN that selects unit 322 outputs during greater than the second predeterminated voltage VSET2, second control circuit 220 just enables control signal CS to reduce output voltage VO UT.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 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 driven for emitting lights diode tandem 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.The method that downgrades output voltage VO UT is not described at this in the existing explanation of Fig. 3.

Next, further specify the circuit structure and the details of operation of 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 the selected cell 322 and second comparator 326.Selected cell 322 is coupled between second end (that is cathode terminal of LED serial SL1-SLN) and second comparator 326 of each LED serial SL1-SLN, is used to select and the valley voltage VMIN of the second terminal voltage VSL1-VSLN of output light emitting diode tandem SL1-SLN.The positive input terminal of second comparator 326 and negative input end couple the selected cell 322 and the second predeterminated voltage VSET2 respectively, and its output then is coupled to or a door input of 332.As the valley voltage VMIN that selects unit 322 outputs during greater than the second predeterminated voltage VSET2, second control circuit 220 just enables control signal CS to reduce output voltage VO UT.Then, the motion flow that downgrades output voltage VO UT is identical with Fig. 3, is not described at this.

Have in the present technique field and to know that usually the knowledgeable should be known in the booster circuit 310 in the foregoing description, 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 among Fig. 7 310, wherein, switch S 1 presents complementary relationship (switch S 1 is coupled to the forward output Q of set-reset flip-floop 334, switch S 2 is coupled to set-reset flip-floop 334 via buffer 337 inverse output terminal Q via buffer 336) with S2, just end when switch S 1 conducting, switch S 2, in other words, when switch S 1 is ended, switch S 2 conductings.

When the time of time ratio switch S 2 conductings of switch S 1 conducting is long, can increase the magnitude of voltage of output voltage VO UT.When time of time ratio switch S 2 conductings of switch S 1 conducting 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 the replacement end R of control signal CS to set-reset flip-floop 334.At this moment, set-reset flip-floop 334 is carried out the function of resetting, and makes that adjusting signal SRE produces the effect that pulse is covered, and then reduces the time of switch S 1 conducting, to reduce output voltage VO UT.

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

Compare the foregoing description and existing circuit, the circuit of the foregoing description can be adjusted the size of output voltage VO UT by first control circuit 210 and second control circuit 220.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 being the voltage of LED serial SL1-SLN conducting just).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 is caused.

Fig. 9 is the flow chart of the driven for emitting lights diode tandem method of the embodiment of the invention.Please refer to Fig. 9, the method comprises: at first, and by in the change-over circuit input voltage being converted to output voltage (step S900).Output voltage is produced a feedback voltage and import in first comparator behind 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.When output voltage greater than first threshold voltage, then enable a control signal, to reduce output voltage (step S901).

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

What deserves to be mentioned is, though in the foregoing description to the method for driven for emitting lights diode tandem and use its circuit to depict a possible kenel, usually know that the knowledgeable should know but have in the 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 to have met spiritual place of the present invention just.

Indulge the above, the present invention detects the output voltage of change-over circuit by first control circuit and second control circuit, change the pulse frequency and the waveform of the clock signal in the pwm unit again 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 redundant power consumption, and circuit of the present invention can adapt to the power dissipation problem that difference caused of every light-emitting diode conducting voltage, also can improve the power supply conversion efficiency of drive circuit.

Though the present invention discloses as above with preferred embodiment; right its is not in order 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 doing 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 (26)

1. A circuit for driving 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 of 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 of 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 of 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 according to 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 of 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 of 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 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 of 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 of 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 of 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 of claim 11, wherein the pulse width modulation unit comprises: The OR gate outputs 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 of 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 of 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 of claim 8, wherein the cathode voltage of the selected one of the light emitting diodes is a minimum of the cathode voltages of the light emitting diodes. 16. A circuit for driving a plurality of light-emitting diode strings, each of which is composed of 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 substantially higher 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 of 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 of 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. 19. The circuit of 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 of claim 19, wherein the pulse width modulation unit comprises: The OR gate outputs 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 of 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 of 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 of claim 16, wherein the voltage across the second terminal of the selected one of the LEDs is the minimum of the voltages across the second terminals of the string of LEDs. 24. The circuit of claim 16, wherein the second end of the string of LEDs is a cathode of the string of LEDs. 25. A method for driving a plurality of light-emitting diode strings, each of said light-emitting diode strings being 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; enabling the control signal to reduce the output voltage when the output voltage is greater than a first threshold 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. 26. The method of claim 25, wherein the selection voltage is a minimum of cathode voltages of the diode strings.

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