CN101778503B - Light source device and light source driving circuit thereof - Google Patents

Abstract

A light source device and a light source driving circuit thereof. The light source driving circuit comprises an operational amplifier, a transistor and a switching unit. The first input terminal of the operational amplifier is coupled to a predetermined voltage. The first end of the transistor is coupled with the light source, and the control end of the transistor is coupled with the output end of the operational amplifier. The first signal end of the switching unit receives a first voltage larger than a preset voltage, the second signal end of the switching unit is coupled with the second input end of the operational amplifier, the third signal end of the switching unit is coupled with the second end of the transistor, and the enabling end of the switching unit receives a driving voltage. When the driving voltage is enabled, the voltage of the third signal end is transmitted to the second signal end. When the driving voltage is disabled, the voltage of the first signal terminal is transmitted to the second signal terminal.

Description

Light source device and light source driving circuit thereof

technical field

The present invention relates to a driving circuit, and in particular relates to a light source device and a light source driving circuit which can make the driving current of the light source increase and decrease at the same speed.

Background technique

In recent years, due to continuous breakthroughs in optoelectronic technology, light emitting diodes (light emitting diodes, LEDs), which have the advantages of low cost and simple operation, have been widely used. Among them, monochrome applications include traffic signs, hand-held lighting, and instrument indicators, while color applications include advertising billboards and super-large displays.

In the case of a display, in order to switch the driving current, the driving circuit of the general light-emitting diode often needs to change the input reference voltage of the operational amplifier or turn off the operational amplifier, resulting in a large change in the operating current of the internal chip.

FIG. 1 is a schematic circuit diagram of a known light source device. Referring to FIG. 1, in the light source device 100, the LED series D1-Dn are coupled between the system voltage Vdd and the drain of the transistor M1, and the current I is the driving current of the LED series _D1 - _Dn . According to the electrical characteristics of the operational amplifier 101 (that is, when only one input voltage is input, the voltage at the positive input terminal will be equal to the voltage at the negative input terminal), the current value of the driving current I is Vin/R (that is, the driving voltage Vin voltage divided by the resistance of resistor R). Therefore, the magnitude of the driving current I can be determined by the voltage of the driving voltage Vin, that is, the driving current I can be switched by adjusting the voltage of the driving voltage Vin. In addition, if the driving current I is to be turned off, the operational amplifier 101 can also be turned off to reduce the voltage at the gate terminal of the transistor M1 to a low voltage level. The above-mentioned operation methods of the switch will cause a large change in the operating current of the internal chip. Moreover, in terms of switching the driving current I by adjusting the voltage of the driving voltage Vin, the voltage control of the previous stage (that is, the light source controller 102) must be very precise, otherwise, the speed of the increase and decrease of the driving current I will be slow. no the same.

Contents of the invention

The invention provides a light source device, which can reduce the variation range of the operating current of the internal chip.

The present invention also provides a light source driving circuit, which can make the driving current of the light source increase and decrease at the same speed.

The invention provides a light source driving circuit, which includes an operational amplifier, a transistor, a switching unit and a first resistor. The operational amplifier has a first input terminal, a second input terminal and an output terminal, and its first terminal is coupled to a preset voltage. The transistor has a first terminal, a second terminal and a control terminal, the first terminal is coupled to the light source, and the control terminal is coupled to the output terminal of the operational amplifier. The switching unit has a first signal terminal, a second signal terminal, a third signal terminal and an enabling terminal, the first signal terminal receives a first voltage greater than a preset voltage, and the second signal terminal is coupled to the second input of the operational amplifier end, its third signal end is coupled to the second end of the transistor, and its enabling end receives a driving voltage. When the driving voltage is enabled, the switching unit transmits the voltage of the third signal terminal to the second signal terminal. When the driving voltage is disabled, the switching unit transmits the voltage of the first signal terminal to the second signal terminal. The first resistor is coupled between the second terminal of the transistor and the second voltage.

The present invention further provides a light source device, which includes a light source driving circuit. The first end of the light source is coupled to the third voltage. The light source driving circuit includes an operational amplifier, a transistor, a switching unit and a first resistor. The operational amplifier has a first input terminal, a second input terminal and an output terminal, and its first terminal is coupled to a preset voltage. The transistor has a first terminal, a second terminal and a control terminal, the first terminal is coupled to the second terminal of the light source, and the control terminal is coupled to the output terminal of the operational amplifier. The switching unit has a first signal terminal, a second signal terminal, a third signal terminal and an enabling terminal, the first signal terminal receives a first voltage greater than a preset voltage, and the second signal terminal is coupled to the second input of the operational amplifier end, its third signal end is coupled to the second end of the transistor, and its enabling end receives a driving voltage. When the driving voltage is enabled, the switching unit transmits the voltage of the third signal terminal to the second signal terminal. When the driving voltage is disabled, the switching unit transmits the voltage of the first signal terminal to the second signal terminal. The first resistor is coupled between the second terminal of the transistor and the second voltage.

Based on the above, the light source device and its light source driving circuit of the present invention use the switching unit to switch the voltage transmission between the signal terminals according to the driving voltage, so that the circuit operation function of the operational amplifier can be switched between the comparator and the voltage follower. Moreover, when the operational amplifier is regarded as a comparator, the output voltage thereof makes the transistor non-conductive; when the operational amplifier is regarded as a voltage follower, the transistor is conductive to drive the light source. In this way, the fluctuation range of the operating current of the internal chip can be reduced, and at the same time, because the electrical characteristics of the operational amplifier are stable, the increase and decrease speeds of the driving current of the light source will be the same.

In order to make the above-mentioned features and advantages of the present invention more comprehensible, the following specific embodiments are described in detail together with the accompanying drawings.

Description of drawings

FIG. 1 is a schematic circuit diagram of a known light source device.

FIG. 2 is a schematic circuit diagram of a light source device according to a first embodiment of the present invention.

FIG. 3 is a schematic circuit diagram of a light source device according to a second embodiment of the present invention.

FIG. 4 is a schematic circuit diagram of a light source device according to a third embodiment of the present invention.

FIG. 5 is a schematic circuit diagram of a light source device according to a fourth embodiment of the present invention.

FIG. 6 is a schematic circuit diagram of a light source device according to a fifth embodiment of the present invention.

7 is a schematic circuit diagram of a light source device according to a sixth embodiment of the present invention.

[Description of main component symbols]

100, 200, 300, 400, 500, 600, 700: light source device

101, 211: Operational amplifier

210, 310, 410, 510, 610, 710: drive circuit

212, 311, 411, 511, 611, 711: switching unit

220: light source

D ₁ ~D _n , L ₁ ~L _n : light emitting diodes

M1, N1, P1: Transistors

S1, S2: switch

R, R1, R2: Resistors

I, I ₁ , I ₂ : Current

Vdd, Vcc: system voltage

V1: first voltage

Vpre: preset voltage

Vin, VDR: driving voltage

Detailed ways

[first embodiment]

FIG. 2 is a schematic circuit diagram of a light source device according to a first embodiment of the present invention. Please refer to FIG. 2 , in this embodiment, the light source device 200 includes a driving circuit 210 , a light source 220 and a light source controller 230 , wherein the light source 220 is an example of light emitting diode series L ₁ -L _n here. The light source 220 is coupled between the system voltage Vcc (ie, the third voltage) and the driving circuit 210 . The driving circuit 210 is coupled to the light source controller 230 to receive the driving voltage V _DR . The driving circuit 210 includes an operational amplifier 211 , a switching unit 212 , a transistor N1 , and a first resistor R1 , wherein the transistor N1 is an NMOS transistor as an example. The switching unit 212 includes a first switch S1 and a second switch S2 .

The positive input terminal (namely the first input terminal) of the operational amplifier 211 receives the preset voltage Vpre, the negative input terminal (ie the second input terminal) is coupled to the second terminal of the first switch S1, and the output terminal is coupled to the transistor The gate terminal of N1 (that is, the control terminal). A first end of the first switch S1 is coupled to a first voltage V1, wherein the first voltage V1 is greater than a predetermined voltage Vpre, and both the first voltage V1 and the predetermined voltage Vpre are fixed values. A first terminal of the second switch S2 is coupled to the second terminal of the first switch S1 , and a second terminal of the second switch S2 is coupled to the source terminal (ie, the second terminal) of the transistor N1 . The drain terminal (ie, the first terminal) of the transistor N1 is coupled to the light source 210 . The first resistor R1 is coupled between the source terminal of the transistor N1 and the ground voltage (ie, the second voltage).

In this embodiment, both the switches S1 and S2 operate according to the driving voltage V _DR . When the driving voltage V _DR is enabled (for example, a logic high voltage level), the switch S1 is not conducting, and the switch S2 is conducting. At this time, the operational amplifier 211 operates according to the circuit and can be regarded as a voltage follower. Moreover, the voltage at the gate terminal of the transistor N1 will be pulled up by the voltage at the output terminal of the operational amplifier 211, so that the transistor N1 is turned on, and the voltage drop of the resistor R1 will be equal to the voltage value of the preset voltage Vpre after the operation of the circuit, and will be transmitted to negative input. At the same time, since the transistor N1 is turned on, the driving circuit 210 can provide the driving current _I1 to the light source 220 . The driving current _I1 is roughly equal to the voltage value of the preset voltage Vpre divided by the resistance value of the resistor R1.

When the driving voltage V _DR is disabled (for example, a logic low voltage level), the switch S1 is turned on, and the switch S2 is turned off, so that the first voltage V1 is transmitted to the negative input terminal. At this time, the operational amplifier 211 operates according to its circuit and can be regarded as a comparator. Since the first voltage V1 is greater than the preset voltage Vpre, that is, the voltage at the negative input terminal is higher than the voltage at the positive input terminal, that is, an open-loop state is formed due to an excessive voltage difference between the positive and negative input terminals of the operational amplifier 211, and the positive and negative input terminals The resulting negative voltage difference (that is, the voltage at the negative input terminal is higher than the voltage at the positive input terminal) will be amplified, so the output terminal of the operational amplifier 211 will output a low voltage level (such as ground voltage), making the transistor N1 non-conductive , to turn off the driving current I ₁ .

As far as the control technology of the light source brightness is concerned, the brightness of the light source 220 can be adjusted through the technique of pulse width modulation, that is, the brightness of the light source 220 can be controlled by modulating the pulse width of the driving voltage V _DR (that is, the driving current I ₁ size). Since the above-mentioned control method is realized by using the circuit operation of the operational amplifier 211, since the output slew rate of the operational amplifier 211 is related to the tail current (tail) of the input terminal of the operational amplifier 211 itself when the operational amplifier 211 is in an open loop, no matter whether it is increased or Decreasing the slew rate is also the same, so the driving current _I1 increases or decreases at the same speed. Moreover, the voltage level requirements of the driving voltage V _DR in the above manner can be lower than known, that is, the chip level used in the previous stage (that is, the light source controller 230) can be lower, thereby reducing the level of the previous stage. The cost of building.

[Second embodiment]

FIG. 3 is a schematic circuit diagram of a light source device according to a second embodiment of the present invention. Please refer to FIG. 2 and FIG. 3 , the biggest difference lies in the switching unit 311 of the driving circuit 310 . In the switch unit 312 , a second resistor R2 is used to replace the switch S2 , wherein the resistance of the first resistor R2 is greater than the resistance of the switch S1 when the switch S1 is turned on. When the driving voltage V _DR is enabled, the switch S1 is not turned on. At this time, the operational amplifier 211 makes the voltage drop of the resistor R1 equal to the voltage value of the preset voltage Vpre after the operation of the circuit, and since the current of the negative input terminal is almost zero, the voltage drop of the resistor R2 is also almost zero (that is, the resistor R2 can be considered as a short circuit). Therefore, the voltage drop on the resistor R1 (approximately equal to the preset voltage Vpre) will be transferred to the negative input terminal of the operational amplifier 211, so that the circuit of the operational amplifier 211 operates as a voltage follower.

Moreover, the voltage at the gate terminal of the transistor N1 is pulled up by the voltage at the output terminal of the operational amplifier 211 , so that the transistor N1 is turned on to provide the driving current _I1 to the light source 220 . The driving current _I1 is roughly equal to the voltage value of the preset voltage Vpre divided by the resistance value of the resistor R1. When the driving voltage V _DR is disabled, the switch S1 is turned on. Since the resistance value of the resistor R2 is much greater than the resistance value when the switch S1 is turned on, the turn-on of the switch S1 will not cause a voltage drop, so that the first voltage V1 will be transmitted to the negative input terminal (that is, the resistor R2 can be regarded as an open circuit. ). At this time, the voltage of the negative input terminal of the operational amplifier 211 will be approximately equal to the first voltage V1, so that the operational amplifier 211 forms an open loop state, and the negative voltage difference between the positive and negative input terminals will also be amplified, so the output terminal of the operational amplifier 211 will output The low voltage level makes the transistor N1 non-conductive to turn off the driving current I ₁ .

[Third embodiment]

FIG. 4 is a schematic circuit diagram of a light source device according to a third embodiment of the present invention. Please refer to FIG. 2 and FIG. 4 , the biggest difference lies in the switching unit 411 of the driving circuit 410 . In the switching unit 411, a second resistor R2 is used to replace the switch S1, wherein the resistance of the first resistor R2 is greater than the resistance of the switch S2 when it is turned on. When the driving voltage V _DR is enabled, the switch S2 is turned on. At this moment, after the operation of the operational amplifier 211 , the voltage drop of the resistor R1 is equal to the voltage value of the preset voltage Vpre, that is, the voltage of the negative input terminal is equal to the voltage value of the preset voltage Vpre. Moreover, the resistance value of the resistor R2 is much larger than the resistance value when the switch S2 is turned on, and the voltage difference between the voltage V1 and the preset voltage Vpre will be absorbed by the resistor R2 (that is, the resistor R2 can be regarded as an open circuit), so that the operational amplifier 211 The circuit operates like a voltage follower. The driving current _I1 is roughly equal to the voltage value of the preset voltage Vpre divided by the resistance value of the resistor R1.

When the driving voltage V _DR is disabled, the switch S2 is non-conductive. At this time, since the negative input end does not generate current, the resistor R2 does not generate a voltage drop (that is, the resistor R2 can be regarded as a short circuit), so that the first voltage V1 is transmitted to the negative input end of the operational amplifier 211 . At this time, the operation of the operational amplifier 211 will be the same as that of a comparator, and the voltage at the negative input terminal is higher than the positive input terminal, so the output terminal of the operational amplifier 211 will output a low voltage level, making the transistor N1 non-conductive to turn off the drive current I ₁ .

[Fourth Embodiment]

FIG. 5 is a schematic circuit diagram of a light source device according to a fourth embodiment of the present invention. Referring to FIG. 5 , in this embodiment, a light source device 500 includes a driving circuit 510 , a light source 520 and a light source controller 230 , wherein components with similar functions to those in the first embodiment use similar reference numerals. The light source 520 is coupled between the ground voltage (that is, the third voltage) and the driving circuit 510, and the light source 520 is also exemplified by the light emitting diode series L ₁ ˜L _n . The driving circuit 510 is coupled to the light source controller 230 to receive the driving voltage V _DR . The driving circuit 510 includes an operational amplifier 211 , a switching unit 511 , a transistor P1 , and a resistor R1 , where the transistor P1 is a PMOS transistor as an example. The switching unit 511 includes switches S1 and S2 .

The negative input terminal (ie, the first input terminal) of the operational amplifier 211 receives the preset voltage Vpre, its positive input terminal (ie, the second input terminal) is coupled to the second terminal of the switch S1, and its output terminal is coupled to the transistor P1. Gate terminal (that is, the control terminal). A first end of the switch S1 is coupled to a first voltage V1. A first terminal of the switch S2 is coupled to a second terminal of the switch S1 , and a second terminal of the switch S2 is coupled to a source terminal (ie, the second terminal) of the transistor P1 . The drain terminal (ie, the first terminal) of the transistor P1 is coupled to the light source 210 . The resistor R1 is coupled between the source terminal of the transistor N1 and the system voltage Vcc (ie, the second voltage).

The operation of the switching unit 511 is similar to that of the switching unit 211 in the embodiment shown in FIG. 2 , so it will not be repeated here. When the driving voltage V _DR is enabled, the transistor P1 is turned on to provide the driving current I ₂ to the light source 520, wherein the driving current I ₂ is approximately equal to the voltage of the system voltage Vcc minus the preset voltage Vpre and divided by the resistor R1 resistance value. When the driving voltage V _DR is disabled, since the positive and negative input terminals of the operational amplifier 211 will present a positive voltage difference (that is, the voltage at the positive input terminal is greater than the voltage at the negative input terminal), the output terminal of the operational amplifier 211 will output a high voltage. Level (for example, system voltage Vcc). Therefore, the transistor P1 is rendered non-conductive, and the driving current I ₂ is turned off.

[Fifth Embodiment]

FIG. 6 is a schematic circuit diagram of a light source device according to a fifth embodiment of the present invention. Please refer to FIG. 5 and FIG. 6 , the biggest difference is the switching unit 611 of the driving circuit 610 . In the switching unit 611, a second resistor R2 is used to replace the switch S2. The operation of the switching unit 611 is similar to that of the switching unit 311 in the embodiment shown in FIG. 3 , so it will not be repeated here. When the driving voltage V _DR is enabled, the transistor P1 is turned on to provide the driving current I ₂ to the light source 520, wherein the driving current I ₂ is approximately equal to the voltage value of the system voltage Vcc minus the voltage value of the preset voltage Vpre and then Divide by the resistance value of resistor R1. When the driving voltage V _DR is disabled, the output terminal of the operational amplifier 211 will output a high voltage level due to a positive voltage difference between the positive and negative input terminals of the operational amplifier 211 . Therefore, the transistor P1 is rendered non-conductive, and the driving current I ₂ is turned off.

[Sixth embodiment]

7 is a schematic circuit diagram of a light source device according to a sixth embodiment of the present invention. Please refer to FIG. 5 and FIG. 7 , the biggest difference is the switching unit 711 of the driving circuit 710 . In the switching unit 711, a second resistor R2 is used to replace the switch S1. The operation of the switching unit 711 is the same as that of the switching unit 411 in the embodiment shown in FIG. 4 , so it will not be repeated here. When the driving voltage V _DR is enabled, the transistor P1 is turned on to provide the driving current I ₂ to the light source 520, wherein the driving current I ₂ is approximately equal to the voltage value of the system voltage Vcc minus the voltage value of the preset voltage Vpre and then Divide by the resistance value of resistor R1. When the driving voltage V _DR is disabled, the output terminal of the operational amplifier 211 will output a high voltage level due to a positive voltage difference between the positive and negative input terminals of the operational amplifier 211 . Therefore, the transistor P1 is rendered non-conductive, and the driving current I ₂ is turned off.

It is worth mentioning that the transistor N1 in the above first, second and third embodiments can be replaced by an NPN junction transistor, its base is coupled to the output terminal of the operational amplifier, its collector is coupled to the light source, and its emitter The pole coupling resistor and the switching unit can also realize the circuit operation of the first, second and third embodiments. Moreover, the transistor P1 in the above fourth, fifth and sixth embodiments can be replaced by a PNP junction transistor, its base is coupled to the output terminal of the operational amplifier, its collector is coupled to the light source, and its emitter is coupled to the resistor and the switching unit, the circuit operations of the fourth, fifth and sixth embodiments can also be realized.

To sum up, the light source device and the light source driving circuit of the various embodiments of the present invention use the switching unit to switch the voltage transmission between the signal terminals according to the driving voltage, so that the operation function of the operational amplifier can be controlled between the comparator and the voltage follower. to switch. Moreover, when the operational amplifier is regarded as a comparator, the output voltage thereof makes the transistor non-conductive; when the operational amplifier is regarded as a voltage follower, the transistor is conductive to drive the light source. In this way, the fluctuation range of the operating current of the internal chip can be reduced, and at the same time, because the electrical characteristics of the operational amplifier are stable, the increase and decrease speeds of the driving current of the light source will be the same.

Although the present invention has been disclosed above with the embodiments, it is not intended to limit the present invention. Anyone with ordinary knowledge in the technical field can make some changes and modifications without departing from the spirit and scope of the present invention. Therefore, the scope of protection of the present invention should be defined by the appended claims.

Claims (34)

1. A light source driving circuit, comprising: An operational amplifier having a first input terminal, a second input terminal and an output terminal, the first input terminal is coupled to a preset voltage; A transistor has a first terminal, a second terminal and a control terminal, the first terminal is coupled to a light source, and the control terminal is coupled to the output terminal of the operational amplifier; A switch unit has a first signal terminal, a second signal terminal, a third signal terminal and an enabling terminal, the first signal terminal receives a first voltage greater than the preset voltage, the second signal terminal is coupled connected to the second input terminal of the operational amplifier, the third signal terminal is coupled to the second terminal of the transistor, and the enabling terminal receives a driving voltage, wherein when the driving voltage is enabled, the switching unit The voltage of the signal terminal is transmitted to the second signal terminal, and when the driving voltage is disabled, the switching unit transmits the voltage of the first signal terminal to the second signal terminal; and A first resistor is coupled between the second terminal of the transistor and a second voltage. 2. The light source driving circuit according to claim 1, wherein the switching unit comprises: A first switch has a first end and a second end, the first end of the first switch receives the first voltage, the second end of the first switch is coupled to the second input end of the operational amplifier, wherein When the driving voltage is enabled, the first switch is non-conductive, and when the driving voltage is disabled, the first switch is conductive; and A second switch has a first terminal and a second terminal, the first terminal of the second switch is coupled to the second input terminal of the operational amplifier, the second terminal of the second switch is coupled to the second terminal of the transistor terminal, wherein when the driving voltage is enabled, the second switch is turned on, and when the driving voltage is disabled, the second switch is not turned on. 3. The light source driving circuit according to claim 1, wherein the switching unit comprises: A first switch has a first terminal and a second terminal, the first terminal of the first switch is coupled to the first voltage, the second terminal of the first switch is coupled to the second input terminal of the operational amplifier, Wherein, when the driving voltage is enabled, the first switch is non-conductive, and when the driving voltage is disabled, the first switch is conductive; and A second resistor is coupled between the second input terminal of the operational amplifier and the second terminal of the transistor. 4. The light source driving circuit according to claim 1, wherein the switching unit comprises: a second resistor coupled between the first voltage and the second input terminal of the operational amplifier; A second switch has a first terminal and a second terminal, the first terminal of the second switch is coupled to the second input terminal of the operational amplifier, the second terminal of the second switch is coupled to the second terminal of the transistor terminal, wherein when the driving voltage is enabled, the second switch is turned on, and when the driving voltage is disabled, the second switch is not turned on. 5. The light source driving circuit as claimed in claim 1, wherein the first input terminal and the second input terminal of the operational amplifier are respectively a positive input terminal and a negative input terminal. 6. The light source driving circuit as claimed in claim 1, wherein the second voltage is a ground voltage. 7. The light source driving circuit as claimed in claim 6, wherein the transistor is an NMOS transistor. 8. The light source driving circuit as claimed in claim 7, wherein the first terminal of the transistor is a drain terminal, the second terminal is a source terminal, and the control terminal is a gate terminal. 9. The light source driving circuit as claimed in claim 6, wherein the transistor is an NPN junction transistor. 10. The light source driving circuit as claimed in claim 9, wherein the first terminal of the transistor is a collector terminal, the second terminal is an emitter terminal, and the control terminal is a base terminal. 11. The light source driving circuit as claimed in claim 1, wherein the second voltage is a system voltage. 12. The light source driving circuit as claimed in claim 11, wherein the transistor is a PMOS transistor. 13. The light source driving circuit as claimed in claim 12, wherein the first terminal of the transistor is a drain terminal, the second terminal is a source terminal, and the control terminal is a gate terminal. 14. The light source driving circuit as claimed in claim 11, wherein the transistor is a PNP junction transistor. 15. The light source driving circuit as claimed in claim 14, wherein the first terminal of the transistor is a collector terminal, the second terminal is an emitter terminal, and the control terminal is a base terminal. 16. The light source driving circuit as claimed in claim 1, wherein the preset voltage is a fixed voltage. 17. A light source device comprising: a light source, the first end of which is coupled to a third voltage; and A light source driving circuit, including: An operational amplifier having a first input terminal, a second input terminal and an output terminal, the first terminal is coupled to a preset voltage; A transistor has a first terminal, a second terminal and a control terminal, the first terminal is coupled to the second terminal of the light source, and the control terminal is coupled to the output terminal of the operational amplifier; A switch unit has a first signal terminal, a second signal terminal, a third signal terminal and an enabling terminal, the first signal terminal receives a first voltage greater than the preset voltage, the second signal terminal is coupled connected to the second input terminal of the operational amplifier, the third signal terminal is coupled to the second terminal of the transistor, and the enabling terminal receives a driving voltage, wherein when the driving voltage is enabled, the switching unit The voltage of the signal terminal is transmitted to the second signal terminal, and when the driving voltage is disabled, the switching unit transmits the voltage of the first signal terminal to the second signal terminal; and A first resistor is coupled between the second terminal of the transistor and a second voltage. 18. The light source device according to claim 17, wherein the switching unit comprises: A first switch has a first end and a second end, the first end of the first switch receives the first voltage, the second end of the first switch is coupled to the second input end of the operational amplifier, wherein When the driving voltage is enabled, the first switch is non-conductive, and when the driving voltage is disabled, the first switch is conductive; and A second switch has a first terminal and a second terminal, the first terminal of the second switch is coupled to the second input terminal of the operational amplifier, the second terminal of the second switch is coupled to the second terminal of the transistor terminal, wherein when the driving voltage is enabled, the second switch is turned on, and when the driving voltage is disabled, the second switch is not turned on. 19. The light source device according to claim 17, wherein the switching unit comprises: A first switch has a first terminal and a second terminal, the first terminal of the first switch is coupled to the first voltage, the second terminal of the first switch is coupled to the second input terminal of the operational amplifier, Wherein, when the driving voltage is enabled, the first switch is non-conductive, and when the driving voltage is disabled, the first switch is conductive; and A second resistor is coupled between the second input terminal of the operational amplifier and the second terminal of the transistor. 20. The light source device according to claim 17, wherein the switching unit comprises: a second resistor coupled between the first voltage and the second input terminal of the operational amplifier; A second switch has a first terminal and a second terminal, the first terminal of the second switch is coupled to the second input terminal of the operational amplifier, the second terminal of the second switch is coupled to the second terminal of the transistor terminal, wherein when the driving voltage is enabled, the second switch is turned on, and when the driving voltage is disabled, the second switch is not turned on. 21. The light source device as claimed in claim 17, wherein the first input terminal and the second input terminal of the operational amplifier are respectively a positive input terminal and a negative input terminal. 22. The light source device as claimed in claim 17, wherein the second voltage is a ground voltage. 23. The light source device as claimed in claim 22, wherein the transistor is an NMOS transistor. 24. The light source device as claimed in claim 23, wherein the first terminal of the transistor is a drain terminal, the second terminal is a source terminal, and the control terminal is a gate terminal. 25. The light source device as claimed in claim 22, wherein the transistor is an NPN junction transistor. 26. The light source device as claimed in claim 24, wherein the first terminal of the transistor is a collector terminal, the second terminal is an emitter terminal, and the control terminal is a base terminal. 27. The light source device as claimed in claim 17, wherein the second voltage is a system voltage. 28. The light source device as claimed in claim 27, wherein the transistor is a PMOS transistor. 29. The light source device as claimed in claim 28, wherein the first terminal of the transistor is a drain terminal, the second terminal is a source terminal, and the control terminal is a gate terminal. 30. The light source device as claimed in claim 27, wherein the transistor is a PNP junction transistor. 31. The light source device as claimed in claim 30, wherein the first terminal of the transistor is a collector terminal, the second terminal is an emitter terminal, and the control terminal is a base terminal. 32. The light source device of claim 17, wherein the light source comprises at least one light emitting diode. 33. The light source device as claimed in claim 17, wherein the preset voltage is a fixed voltage. 34. The light source device as claimed in claim 17, wherein the third voltage is a ground voltage or a system voltage.

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