TWI457892B - Light-emitting diode driving circuit and operating method thereof - Google Patents

Description

Light-emitting diode driving circuit and operating method thereof

The invention relates to a driving circuit, and more particularly to a light emitting diode driving circuit and a method of operating the same.

In general, if the LED backlight device with constant current control is in a fast start, the external power supply and the control timing are disordered, the LED string is likely to cause secondary flicker.

As shown in FIG. 1 , a common method for solving the secondary flicker phenomenon of the above-mentioned LED string is to add a discharge circuit 1 to the circuit. The discharge circuit 1 includes a power transistor element NMOS and a discharge resistor R, and is controlled by the discharge control unit 10 to force the output capacitor C to discharge when the LED is turned off. Although this method can improve the secondary flicker phenomenon of the above-mentioned light-emitting diode, it also causes additional power consumption during standby.

Therefore, the present invention provides a light emitting diode driving circuit and a method of operating the same to solve the above problems encountered in the prior art.

One aspect of the present invention is to provide a light emitting diode driving circuit. In a preferred embodiment, the LED driving circuit is coupled to the LED string and the power switch, and the LED string is connected in series with the power switch. The LED driving circuit includes a current detecting unit, a reference voltage generating unit and an amplifier. The current detecting unit is coupled to the power switch for detecting a switching current of the power switch to generate a current detecting signal. The reference voltage generating unit is coupled to the current detecting unit for generating a reference voltage according to the linear control signal. The amplifier is coupled to the power switch, the current detecting unit and the reference voltage generating unit for controlling the operation of the power switch, so that the waveform of the switch current has a soft rising edge at startup and a slow falling edge when closed ( Soft falling edge).

In one embodiment, the LED driving circuit further includes a dimming control unit coupled between the amplifier and the power switch for providing a dimming control signal.

In one embodiment, the LED driving circuit further includes a linear control unit coupled to the current detecting unit and the reference voltage generating unit for receiving the current detecting signal and outputting the linear control signal.

In an embodiment, the LED driving circuit further includes a starting unit, a coupling wiring control unit and a current detecting unit for receiving the startup signal and selectively starting the linearity according to the activation signal at a high level or a low level. Control unit or current detection unit.

In one embodiment, the linear control signal is an analog control signal, and the reference voltage generating unit assumes an arc shape between the first voltage value and the second voltage value according to the reference voltage generated by the linear control signal.

In one embodiment, the linear control signal is a digital control signal, and the reference voltage generating unit exhibits a non-arc shape in a waveform between the first voltage value and the second voltage value according to the reference voltage generated by the linear control signal.

In another preferred embodiment, the LED driving circuit is coupled to the LED string and the power switch, and the LED string is connected in series with the power switch. The light emitting diode driving circuit includes a delay control unit, a reference voltage generating unit, and an amplifier. The delay control unit is configured to provide a delay control signal. The reference voltage generating unit is coupled to the delay control unit for generating a reference voltage according to the delay control signal. Amplifier coupled to power supply The switch and the reference voltage generating unit are configured to receive the switching current, the reference voltage and the dimming control signal of the power switch to control the operation of the power switch, so that the switch current is delayed after being delayed for a period of time.

Another aspect of the present invention is to provide a method of operating a light emitting diode driving circuit. In a preferred embodiment, the LED driving circuit is coupled to the at least one LED string and the at least one power switch, and the LED string is connected in series with the power switch. The operation method of the LED driving circuit comprises the following steps: (a) detecting a switching current of the power switch to generate a current detecting signal; (b) generating a reference voltage according to the linear control signal; (c) receiving the current detecting signal, The reference voltage and the dimming control signal are used to control the operation of the power switch such that the waveform of the switching current has a slowly rising edge at startup and a slow falling edge when closed.

In another preferred embodiment, the LED driving circuit is coupled to the at least one LED string and the at least one power switch, and the LED string is connected in series with the power switch. The operation method of the LED driving circuit comprises the following steps: (a) providing a delay control signal; (b) generating a reference voltage according to the delay control signal; (c) receiving a switching current of the power switch, a reference voltage, and a dimming control signal In order to control the operation of the power switch, the switch current is turned off after a delay, so as to prevent secondary flashing of the LED string.

Compared with the prior art, the LED driving circuit and the operating method thereof according to the present invention can have the following advantages: (1) reducing the board area required for the overall system; (2) eliminating the need for external power transistor components and discharge resistors. , can reduce the power consumption during standby; (3) slow start and slow turn off the switch current so that the waveform of the switch current has a slow rising edge at the start and a slow falling edge when closed can effectively improve the traditional light-emitting diode The second flicker phenomenon of the string, and can be applied to a variety of different LED driving circuit architectures; (4) the method of turning off the switching current after delaying for a period of time can also effectively improve the secondary flicker of the conventional LED string phenomenon.

The advantages and spirit of the present invention will be further understood from the following detailed description of the invention.

A preferred embodiment of the present invention is a light emitting diode driving circuit. In fact, the LED driving circuit in this embodiment is applied to a light-emitting diode backlight module or other LED device for driving and controlling the operation of the LED. In the embodiment, the LED driving circuit adopts a soft start and a soft shut down switching current, so that the waveform of the switching current I _SW has a soft rising edge at the startup. And when it is closed, it has a soft falling edge to effectively improve the secondary flicker phenomenon of the conventional LED string.

Please refer to FIG. 2. FIG. 2 is a schematic diagram of a light emitting diode driving circuit according to this embodiment. As shown in FIG. 2 , the LED driving circuit 2 is coupled to at least one LED string 3 and at least one power switch 4 . At least one LED string 3 is coupled to the power source VS. The LED string 3 is connected in series with the power switch 4. The resistor R is coupled between the power switch 4 and the ground. The output capacitor C is coupled between the power source VS and the ground, and is connected in parallel with the LED string 3. The LED driving circuit 2 includes a starting unit 20, a linear control unit 22, a reference voltage generating unit 24, a current detecting unit 26, an amplifier 28, and a dimming control unit 29.

The starting unit 20 is coupled to the wiring control unit 22 and the current detecting unit 26; the linear control unit 22 is coupled to the reference voltage generating unit 24 and the current detecting unit 26; the reference voltage generating unit 24 is coupled to the amplifier 28; and the current detecting unit 26 is coupled The first input terminal + and the second input terminal of the amplifier 28 are respectively coupled to the reference voltage generating unit 24 and the current detecting unit 26, and the output terminal K is coupled to the amplifier 28 and the power detecting switch 4 and the detecting resistor Rs. The power switch 4 is connected; the dimming control unit 29 is coupled between the amplifier 28 and the power switch 4.

When the starting unit 20 receives the external start signal S _EN , the starting unit 20 selectively activates the linear control unit 22 or current according to the start signal S _EN at a high level or a low level. Detection unit 26. When the start signal S _{EN is} at the high level, the start unit 20 activates the linear control unit 22; when the start signal S _{EN is} at the low level, the start unit 20 activates the current detecting unit 26.

When the current detecting unit 26 is activated, the current detecting unit 26 detects the switching current I _{SW of the} power switch 4 to generate the current detecting signal S _Det to the linear control unit 22. When the linear control unit 22 is activated, the linear control unit 22 will receive the current detection signal S _Det and output the linear control signal S _LC to the reference voltage generating unit 24. When the reference voltage generating unit 24 receives the linear control signal S _LC , the reference voltage generating unit 24 generates the reference voltage V _ref to the first input terminal + of the amplifier 28 according to the linear control signal S _LC . As for the second input of the amplifier 28, the current detecting signal S _{Det is} received from the current detecting unit 26.

When the amplifier 28 receives the current detection signal S _Det , the reference voltage V _ref and the dimming control signal S _Dim , the amplifier 28 controls the operation of the power switch 4 so that the waveform of the switching current I _SW has a slow rise at startup. The edge has a gently falling edge when closed to prevent secondary blinking of each of the light-emitting diodes 30 in the light-emitting diode string 3.

It should be noted that the linear control unit 22 of the present invention may have different types, such as an analog linear control unit and a digital linear control unit, which are respectively described below.

First, please refer to FIG. 3A to FIG. 3C. FIG. 3A illustrates an embodiment of the linear control unit 22 of FIG. 2 as an analog linear control unit. FIG. 3B illustrates the reference voltage V when the start signal S _{EN is} at a high level. _Ref presents a schematic diagram of a circular arc-shaped linear increase; FIG. 3C shows a schematic diagram in which the reference voltage V _ref exhibits a circular arc-shaped linear decrease when the start signal S _{EN is} at a low level.

As shown in the figure, if the linear control unit 22 is an analog linear control unit, the linear control signal S _LC outputted is an analog control signal. When the start signal S _{EN is} at a high level, the transistor switch M1 is activated and the transistor is activated. The switch M2 is turned off, and the capacitor C1 performs a charging process, so that the reference voltage V _ref exhibits a circular arc linear increase; when the start signal S _{EN is} at the low level, the transistor switch M2 is activated and the transistor switch M1 is turned off, and the capacitor C1 is discharged. The program causes the reference voltage V _{ref to} exhibit a circular arc-shaped linear decrease. It should be noted that the reference voltage V _ref generated by the reference voltage generating unit 24 according to the analog linear control signal S _LC can exhibit other smooth shapes in addition to the linear increase or decrease of the circular arc shape, and there is no specific limitation. .

4A to 4C, FIG. 4A illustrates an embodiment of the linear control unit 22 of FIG. 2 as a digital linear control unit; and FIG. 4B illustrates the reference voltage V when the start signal S _{EN is} at a high level. _Ref presents a schematic diagram of a stepwise linear increase; FIG. 4C shows a schematic diagram of the stepped linear decrease of the reference voltage V _ref when the start signal S _{EN is} at a low level.

As shown, when the start signal S _{EN is} at the high level, the digital start unit N1 is activated and the digital close unit N2 is turned off, and the capacitor C1 performs a charging procedure such that the reference voltage V _ref exhibits a stepwise linear increase; when the start signal S is activated _{When EN is} at the low level, the digital off unit N2 is activated and the digital start unit N1 is turned off, and the capacitor C1 performs a discharge procedure such that the reference voltage V _ref exhibits a stepwise linear decrease. It should be noted that the reference voltage V _ref generated by the reference voltage generating unit 24 according to the digital linear control signal S _LC can exhibit other stepwise linear increase or decrease, and can also exhibit other non-arc shapes. limit.

Please refer to FIG. 5. FIG. 5 is a timing diagram showing that the switching current I _SW exhibits a circular arc linear increase at the time of starting and a circular arc linear decrease when closed when the analog linear control unit is used. As shown in FIG. 5, when the start signal S _EN changes from the low level to the high level at time t1, the switch current I _SW begins to exhibit a circular arc linear increase until the time t2, the switch current I _SW increases to the maximum value. . The period ΔT1 from time t1 to time t2 is the time required to slowly start the switch current I _SW .

When the start signal S _EN changes from the high level to the low level at time t3, the switch current I _SW begins to exhibit a circular arc linear decrease until the time t4, the switch current I _SW drops to a minimum value. The period ΔT2 from time t3 to time t4 is the time required to slowly turn off the switch current I _SW . Since the switch current I _SW has been turned off at time t4, at this time, the reference voltage generating unit 24 releases the control of the external start signal S _EN .

In summary, the slow start and slow turn-off current I _SW used in this embodiment can effectively avoid the secondary flicker phenomenon of the LED string 3 caused by the quick start in the prior art, and can be applied to various Light-emitting diode drive circuit architecture, such as AC AC/DC DC plus current balance architecture, Boost plus current balance architecture or Buck/Boost plus current balance architecture.

Another preferred embodiment of the present invention is also a light emitting diode driving circuit. The LED driving circuit in this embodiment adopts a slow start and a delay to turn off the switching current to improve the secondary flicker phenomenon of the conventional LED string. Please refer to FIG. 6. FIG. 6 is a schematic diagram of the LED driving circuit in this embodiment.

As shown in FIG. 6 , the LED driving circuit 6 is coupled to at least one LED string 3 and at least one power switch 4 . At least one LED string 3 is coupled to the power source VS. The LED string 3 is connected in series with the power switch 4. The detecting resistor Rs is coupled between the power switch 4 and the ground. The output capacitor C is coupled between the power source VS and the ground, and is connected in parallel with the LED string 3. The light emitting diode driving circuit 6 includes a starting unit 60, a delay control unit 62, a linear control unit 64, a reference voltage generating unit 66, an amplifier 68, and a dimming control unit 69.

The activation unit 60 is coupled to the delay control unit 62 and the linear control unit 64; The delay control unit 62 is coupled to the reference voltage generating unit 66; the linear control unit 64 is coupled to the reference voltage generating unit 66; the reference voltage generating unit 66 is coupled to the amplifier 68; the first input terminal + and the second input terminal of the amplifier 68 are respectively coupled The reference voltage generating unit 66 is connected between the power switch 4 and the detecting resistor Rs, and the output terminal K is coupled to the power switch 4; the dimming control unit 69 is coupled between the amplifier 68 and the power switch 4.

When the start unit 60 receives the external start signal S _EN , the start unit 60 selectively activates the delay control unit 62 or the linear control unit 64 according to the start signal S _EN at a high level or a low level. When the start signal S _{EN is} at the high level, the start unit 20 activates the linear control unit 64; when the start signal S _{EN is} at the low level, the start unit 20 activates the delay control unit 62.

When the start signal S _{EN is} at a high level, the linear control unit 64 is activated and the linear control unit 64 will output a linear control signal S _LC to the reference voltage generating unit 66. When the start signal S _{EN is} at the low level, the delay control unit 62 is activated, and the delay control unit 62 will provide the delay control signal S _delay to the reference voltage generating unit 66.

When the start signal S _{EN is} at the high level, the reference voltage generating unit 66 receives the linear control signal S _LC , and the reference voltage generating unit 66 generates the reference voltage V _ref according to the linear control signal S _LC to the first input of the amplifier 68. +. As for the second input of the amplifier 68, the voltage V _{R is} received from the junction between the power switch 4 and the sense resistor Rs. After the amplifier 68 receives the voltage V _R , the reference voltage V _ref and the dimming control signal S _Dim , the amplifier 68 controls the operation of the power switch 4 such that the waveform of the switching current I _SW has a slowly rising edge at the start. In order to prevent the secondary blinking phenomenon of each of the light-emitting diodes 30 in the light-emitting diode string 3.

When the start signal S _{EN is} at the low level, the reference voltage generating unit 66 receives the delay control signal S _delay , and the reference voltage generating unit 66 generates the reference voltage V _ref to the first input of the amplifier 68 according to the delay control signal S _delay . +. As for the second input of the amplifier 68, the voltage V _{R is} received from the junction between the power switch 4 and the sense resistor Rs. After the amplifier 68 receives the voltage V _R , the reference voltage V _ref and the dimming control signal S _Dim , the amplifier 68 controls the operation of the power switch 4 to cause the switching current I _{SW to be} delayed for a period of time to be turned off to prevent the light emission. Each of the light-emitting diodes 30 in the polar body string 3 generates a secondary flicker phenomenon.

As shown in FIG. 7, when the start signal S _EN is changed from the low level to the high level at time t1, since the reference voltage V _ref is generated by the reference voltage generating unit 66 according to the linear control signal S _LC , The switching current I _SW begins to increase linearly in a circular arc until the time t2, the switching current I _SW increases to a maximum value. The period ΔT1 from time t1 to time t2 is the time required to slowly start the switch current I _SW .

When the start signal S _EN changes from the high level to the low level at time t3, since the reference voltage V _ref is generated by the reference voltage generating unit 66 according to the delay control signal S _delay , the switching current I _{SW is} in time. T3 does not start to decrease, but after a delay time ΔT3, it is not directly reduced to the minimum value until time t5. At this time, the reference voltage generating unit 66 releases the control of the external start signal S _EN .

In summary, the linear increase of the switch current I _SW and the delay of turning off the switch current I _SW in this embodiment can effectively avoid the failure of the output capacitor C to be completed in the prior art, resulting in the next start signal S. _{When the EN} becomes a high level, the LED string 3 generates a secondary flicker phenomenon.

Another embodiment of the present invention is a method of operating a light emitting diode driving circuit. In this embodiment, the LED driving circuit operation method is used to operate the LED driving circuit. The LED driving circuit is coupled to the at least one LED string and the at least one power switch, and the LED string is connected in series with the power switch.

Please refer to FIG. 8. FIG. 8 illustrates a light emitting diode driving circuit according to this embodiment. Flow chart of the method of operation. As shown in FIG. 8, first, the method performs step S10, receives an activation signal, and determines that the activation signal is at a high level or a low level. If the result of the determination in the step S10 is that the start signal is at the low level, the method performs step S12 to detect the switching current of the power switch to generate the current detecting signal. If the result of the determination in step S10 is that the start signal is at the high level, the method proceeds to step S14 to generate a linear control signal according to the current detection signal.

Next, the method performs step S16 to generate a reference voltage according to the linear control signal. In fact, the linear control signal can be a digital linear control signal or an analog linear control signal. The reference voltage has a first voltage value and a second voltage value. If the linear control signal is a digital linear control signal, the waveform of the reference voltage between the first voltage value and the second voltage value may be stepped or other non-arc shape. However, if the linear control signal is an analog linear control signal, the waveform between the first voltage value and the second voltage value of the reference voltage may have an arc shape or other smooth shape, but is not limited thereto.

Then, the method performs step S18, and receives the current detection signal, the reference voltage, and the dimming control signal to control the operation of the power switch, so that the switch current has a slowly rising edge at the start and a falling edge when closed, to prevent The light-emitting diode string produces a secondary flicker phenomenon.

Another embodiment of the present invention is also a method of operating a light emitting diode driving circuit. In this embodiment, the LED driving circuit operation method is used to operate the LED driving circuit. The LED driving circuit is coupled to the at least one LED string and the at least one power switch, and the LED string is connected in series with the power switch.

Please refer to FIG. 9. FIG. 9 is a flow chart showing a method for operating the LED driving circuit according to this embodiment. As shown in FIG. 9, first, the method performs step S20, receives an activation signal, and determines that the activation signal is at a high level or a low level. If the result of the determination in step S20 is that the start signal is at the low level, the method proceeds to step S22 to provide a delay control signal. If the result of the determination in step S20 is that the start signal is at a high level, The method performs step S24 to provide a linear control signal.

Next, the method performs step S26 to generate a reference voltage according to the delay control signal or the linear control signal. In fact, the linear control signal can be a digital linear control signal or an analog linear control signal. The reference voltage has a first voltage value and a second voltage value. If the linear control signal is a digital linear control signal, the waveform of the reference voltage between the first voltage value and the second voltage value may be stepped or other non-arc shape. However, if the linear control signal is an analog linear control signal, the waveform between the first voltage value and the second voltage value of the reference voltage may have an arc shape or other smooth shape, but is not limited thereto. Then, the method performs step S28, and receives the switch current, the reference voltage, and the dimming control signal to control the operation of the power switch, so that the switch current is turned off after being delayed for a period of time to prevent the secondary blinking phenomenon of the LED string.

Compared with the prior art, the LED driving circuit and the operating method thereof according to the present invention can have the following advantages: (1) reducing the board area required for the overall system; (2) eliminating the need for external power transistor components and discharge resistors. , can reduce the power consumption during standby; (3) slow start and slow turn off the switch current so that the waveform of the switch current has a slow rising edge at the start and a slow falling edge when closed, which can effectively improve the LED string The second flicker phenomenon, and can be applied to a variety of different LED driving circuit architecture; (4) the method of turning off the switching current after delay for a period of time can also effectively improve the secondary flicker phenomenon of the LED string.

The features and spirits of the present invention are more apparent from the detailed description of the preferred embodiments. On the contrary, the intention is to cover various modifications and equivalents within the scope of the invention as claimed.

S10~S28‧‧‧ Process steps

1‧‧‧discharge circuit

12‧‧‧Chip switch

14‧‧‧Lighting diode current controller

NMOS‧‧‧ power transistor components

R‧‧‧Discharge resistor

10‧‧‧Discharge Control Unit

C‧‧‧ output capacitor

LED‧‧‧Light Emitting Diode

2, 6‧‧‧Lighting diode driving circuit

3‧‧‧Lighting diode strings

4‧‧‧Power switch

VS‧‧‧ power supply

20, 60‧‧‧ starter unit

22, 64‧‧‧linear control unit

24‧‧‧reference voltage generating unit

26, 66‧‧‧ Current detection unit

28, 68‧‧ ‧Amplifier

29, 69‧‧‧ dimming control unit

K‧‧‧ output

+‧‧‧first input

-‧‧‧second input

S _EN ‧‧‧Start signal

I _SW ‧‧‧Switch Current

S _Det ‧‧‧current detection signal

S _LC ‧‧‧linear control signal

V _ref ‧‧‧reference voltage

S _Dim ‧‧‧ dimming control signal

30‧‧‧Lighting diode

M1~M2‧‧‧Chip Switch

N1‧‧‧ digital starter unit

N2‧‧‧ digital shutdown unit

T1~t5‧‧‧Time

62‧‧‧Delay control unit

△T1‧‧‧Time required to slow start switch current

△T2‧‧‧Time required to slowly turn off the switch current

S _delay ‧‧‧delay control signal

△T3‧‧‧delay time

I _B ‧‧‧current source

R1~R2‧‧‧ resistor

Vb‧‧‧ voltage

Rs‧‧‧Detection resistance

C1‧‧‧ capacitor

FIG. 1 is a schematic diagram showing an external discharge circuit of a light-emitting diode driving circuit in the prior art.

2 is a schematic diagram of a light emitting diode driving circuit according to an embodiment of the present invention.

3A illustrates an embodiment of the linear control unit of FIG. 2 as an analog linear control unit; FIG. 3B illustrates a schematic diagram in which the reference voltage exhibits a circular arc linear increase when the start signal is at a high level; FIG. 3C illustrates When the start signal is at a low level, the reference voltage exhibits a circular arc-shaped linear reduction.

4A illustrates an embodiment of the linear control unit of FIG. 2 as a digital linear control unit; FIG. 4B illustrates a schematic diagram in which the reference voltage exhibits a stepwise linear increase when the enable signal is at a high level; FIG. 4C illustrates when the start is initiated. When the signal is at a low level, the reference voltage exhibits a stepped linear reduction.

FIG. 5 is a timing diagram showing a linear increase and a linear decrease in the arc current when the analog current control unit is used.

6 is a schematic diagram of a light emitting diode driving circuit according to another embodiment of the present invention.

FIG. 7 is a timing diagram showing that the switch current is turned off after a delay period when the start signal is changed from the high level to the low level.

FIG. 8 is a flow chart showing a method of operating a light emitting diode driving circuit according to another embodiment of the present invention.

FIG. 9 is a flow chart showing a method of operating a light emitting diode driving circuit according to another embodiment of the present invention.

2‧‧‧Lighting diode driving circuit

C‧‧‧ output capacitor

3‧‧‧Lighting diode strings

4‧‧‧Power switch

VS‧‧‧ power supply

20‧‧‧Starting unit

22‧‧‧Linear Control Unit

24‧‧‧reference voltage generating unit

26‧‧‧current detection unit

28‧‧‧Amplifier

29‧‧‧ dimming control unit

K‧‧‧ output

+‧‧‧first input

-‧‧‧second input

S _EN ‧‧‧Start signal

I _SW ‧‧‧Switch Current

S _Det ‧‧‧current detection signal

S _LC ‧‧‧linear control signal

V _ref ‧‧‧reference voltage

S _Dim ‧‧‧ dimming control signal

30‧‧‧Lighting diode

Rs‧‧‧Detection resistance

Claims (16)

An LED driving circuit is coupled to at least one LED string and at least one power switch. The LED string is connected in series with the power switch. The LED driving circuit comprises: a current detecting unit. The power switch is coupled to detect a switching current of the power switch to generate a current detecting signal; a reference voltage generating unit coupled to the current detecting unit for generating a reference according to a linear control signal And an amplifier coupled to the power switch, the current detecting unit and the reference voltage generating unit for controlling the operation of the power switch, so that the waveform of the switch current has a slowly rising edge at startup and is turned off. It has a gently falling edge. The illuminating diode driving circuit of claim 1, further comprising: a dimming control unit coupled between the amplifier and the power switch for providing a dimming control signal. The illuminating diode driving circuit of claim 1, further comprising: a linear control unit coupled to the current detecting unit and the reference voltage generating unit for receiving the current detecting signal and outputting the current detecting signal Linear control signal. The illuminating diode driving circuit of claim 3, further comprising: a starting unit coupled to the linear control unit and the current detecting unit for receiving an activation signal and being high according to the activation signal Level or low level selection The linear control unit or the current detecting unit is selectively activated. The illuminating diode driving circuit of claim 1, wherein the linear control signal is an analog control signal, and the reference voltage generating unit generates the reference voltage according to the linear control signal at a first voltage value. The waveform between the second voltage value and the second voltage value assumes an arc shape. The illuminating diode driving circuit of claim 1, wherein the linear control signal is a digital control signal, and the reference voltage generating unit generates the reference voltage according to the linear control signal at a first voltage value. The waveform between the second voltage value and the second voltage value exhibits a non-arc shape. An LED driving circuit is coupled to at least one LED string and at least one power switch. The LED string is connected in series with the power switch. The LED driving circuit comprises: a delay control unit. A reference voltage generating unit is coupled to the delay control unit for generating a reference voltage according to the delay control signal; and an amplifier coupled to the power switch and the reference voltage generating unit to Controlling the operation of the power switch causes the switch current to be turned off after a delay. The illuminating diode driving circuit of claim 7, further comprising: a dimming control unit coupled between the amplifier and the power switch for providing a dimming control signal. The light-emitting diode driving circuit described in claim 7 is further packaged The start-up unit is coupled to the delay control unit and the reference voltage generating unit for receiving an activation signal and selectively enabling the reference voltage generating unit or the delay according to whether the activation signal is at a high level or a low level control unit. The illuminating diode driving circuit of claim 9, wherein the starting unit starts the delay control unit to provide the delay control signal to the reference voltage generating unit when the starting signal is at a low level. A method for operating a light-emitting diode driving circuit, the LED driving circuit is coupled to at least one LED string and at least one power switch, and the LED string is connected in series with the power switch, and the method includes The following steps: (a) detecting a switching current of the power switch to generate a current detecting signal; (b) generating a reference voltage according to a linear control signal; and (c) receiving the current detecting signal, the reference voltage And a dimming control signal to control the operation of the power switch such that the waveform of the switch current has a gently rising edge at startup and a slow falling edge when closed. The method of claim 11, further comprising the steps of: receiving an activation signal and selectively performing step (a) or (b) according to whether the activation signal is at a high level or a low level. The method of claim 11, wherein the linear control signal is an analog control signal, and the waveform generated by the step (b) is represented by a waveform between a first voltage value and a second voltage value. An arc shape. The method of claim 11, wherein the linear control signal is a digital control signal, and the reference voltage generated in the step (b) is at a first voltage value. The waveform between the second voltage value and the second voltage value exhibits a non-arc shape. A method for operating a light-emitting diode driving circuit, the LED driving circuit is coupled to at least one LED string and at least one power switch, and the LED string is connected in series with the power switch, and the method includes The following steps: (a) providing a delay control signal; (b) generating a reference voltage according to the delay control signal; and (c) receiving a switching current of the power switch, the reference voltage, and a dimming control signal to control The operation of the power switch causes the switch current to be turned off after a delay. The method of claim 15, further comprising the steps of: receiving an activation signal and selectively performing step (b) or (a) according to whether the activation signal is at a high level or a low level.