TWI461097B - Light-emitting diode driving device - Google Patents

Description

Light-emitting diode driving circuit

The present invention relates to a Light-Emitting Diode (LED) driving circuit, and more particularly to an LED driving circuit suitable for a high-power LED light source.

1 is a block diagram of an existing LED driving circuit commonly used in large-size liquid crystal display products. Referring to FIG. 1, a large-sized liquid crystal display product usually needs to use a high-power LED light source, which is usually composed of LED light strings LB1 LB LBn, and each LED light string includes a plurality of LEDs coupled in series. , where n is a positive integer. When n is greater than 1, these LED strings LB1 LB LBn are driven in parallel. The LED driving circuit 1 is for driving the LED light strings LB1 LBLBn, and includes a main power supply circuit 11, a standby power supply circuit 12, a boost conversion circuit 13, constant current source circuits 141 to 14n, an LED feedback control circuit 15, and an LED. The short circuit protection circuit 16 and the dimming control circuit 17.

The LED driving circuit 1 receives the stable DC bus voltage Vbus generated by the rectification, power factor correction and filtering of the AC mains, and is generally about 400 volts (VV). The bus voltage Vbus is stepped down by the main power supply circuit 11 to generate main power voltages Vm1 and Vm2, for example, 24V and 12V, respectively. The main power supply voltage Vm1 is used to generate a string input voltage Vin, which is slightly larger than the required operating voltage of the LED string, after being boosted by the boost converter circuit 13, for example, 86V, to be output to the LED string LB1~LBn. Input. Main power supply voltage Vm2 is used to provide liquid crystal display It shows the required power supply inside the product such as audio circuit and panel drive circuit. The bus voltage Vbus is further stepped down by the standby power supply circuit 12 to generate a standby power voltage Vsb, for example, 5V, for providing power required for components in the liquid crystal display product, such as a motherboard microcontroller, which still needs to operate in the standby mode.

The boost converter circuit 13 includes a boost converter, an output filter circuit, and an output feedback control circuit. Among them, the boost converter includes an inductor L1 as an energy storage element and a transistor Q1 and a diode D1 as switching elements for performing boost conversion. The output filter circuit includes a capacitor C1 for reducing the output voltage ripple of the boost converter. The output feedback control circuit includes a Pulse-Width Modulation (PWM) controller U11 for controlling the switching duty cycle of the transistor Q1 according to the output of the LED feedback control circuit 15 to adjust the output voltage of the boost converter ( In this example, the string input voltage Vin). The constant current source circuits 141~14n are respectively coupled to the output ends of the LED light strings LB1~LBn for controlling the currents of the LED light strings LB1~LBn to be constant and equal in size. Please refer to FIG. 2 , which is a circuit diagram of a constant current source circuit commonly used in a three-terminal adjustable shunt regulator. Taking the constant current source circuit 141 as an example, it includes a transistor Q2, resistors R1 R R3, and a shunt regulator U21 having an anode terminal A, a cathode terminal K, and a reference terminal R. When the shunt regulator U21 uses the AZ432, it has a 1.25V reference voltage source inside, and the reference terminal R voltage will be stabilized at 1.25V, so that the operating current Ilb1 of the LED string LB1 is constant and its value is Ilb1=1.25V/ R1.

Please refer to FIG. 1 together with FIG. 2, if the output voltage of the LED string is too low, the 汲 extreme and the source of the transistor Q2 falling in the constant current source circuit The voltage drop between the terminals (hereinafter referred to as the 汲 source voltage drop) will be low and will be changed from the normal operation in the constant current region to the variable resistance region, and the LED light string cannot be controlled by constant current. Vgs>Vth and Vds>Vgs-Vth in the constant current region, Vgs>Vth and Vds<Vgs-Vth when operating in the variable resistance region, where Vgs is the voltage drop between the gate terminal and the source terminal of the transistor Q2, Vds is the drain voltage drop of transistor Q2, and Vth is the threshold voltage of transistor Q2. Therefore, the LED feedback control circuit 15 detects the output voltage of the LED string LB1~LBn through the diodes D11~D1n, and controls the PWM controller U11 to increase the transistor according to the lowest of the detected output voltages. The switching duty cycle of Q1 is to increase the input voltage Vin of the string, so that the voltage drop falling in all the constant current source circuits 141~14n is in a reasonable state and can work normally to perform constant current control on the LED string LB1~LBn. In addition, if the number of LED shorts in the LED string is too large, the voltage drop of the cathode of the transistor Q2 falling in the constant current source circuit may be high to increase the power consumption or even burn the transistor Q2. Therefore, the LED short circuit protection circuit 16 detects the output voltage of the LED lamp string LB1~LBn through the diodes D21~D2n, and turns off the PWM controller when the highest of the detected output voltages is greater than the protection point voltage. U11 causes the boost converter circuit 13 to no longer perform boost conversion to protect internal components such as transistors of the constant current source circuit from burning.

The motherboard micro controller can output the switching signal On/Off and the dimming signal Dim. Among them, the switch signal On/Off is used to turn off the PWM controller U11 in the standby mode to make it more energy-saving in the standby mode. The dimming signal Dim is used to control the constant current source circuits 141~14n through the dimming control circuit 17. Working and non-working state, and then controlling the working and non-operating state of the LED light string LB1~LBn, so as to control the average brightness of the light emitted by the LED light string LB1~LBn, which is the burst mode dimming.

The existing LED driving circuit 1 uses the bus voltage Vbus to be stepped down by the main power circuit 11 and then boosted by the boost converter circuit 13 to generate the string input voltage Vin to provide the required operating voltage of the LED string LB1~LBn. In this way, the power conversion efficiency after two energy conversions will be reduced. Secondly, the constant current source circuits 141~14n usually adopt a shunt regulator U21 such as AZ432, and the reference terminal R voltage is stabilized at 1.25V. When the operating current of the LED string is large, it is coupled to the reference terminal R and the ground. The resistor R1 will have a large power consumption, which is equivalent to reducing the power conversion efficiency.

The object of the present invention is to provide an LED driving circuit that is further optimized for existing LED driving circuits to achieve lower design cost and improved power conversion efficiency.

To achieve the above object or other objects, the present invention provides an LED driving circuit including a main power supply circuit, an LED feedback control circuit, n constant current source circuits, and a dimming control circuit, where n is a positive integer. The main power circuit includes a main power converter and an output feedback control circuit. The main power converter receives the bus voltage generated by the rectification, power factor correction and filtering of the AC mains and generates a main power supply voltage from the output of the main power circuit to the input of the n LED strings. The output feedback control circuit includes a first sampling resistor and a second sampling resistor. The first sampling resistor is coupled between the output end of the main power circuit and the feedback end, and the second sampling resistor is coupled to the second sampling resistor. Between the feedback end and the ground, the output feedback control circuit controls the main power converter to adjust the main power supply voltage according to the feedback terminal voltage. The LED feedback control circuit is coupled to the output end of the LED string, and the switch and the third sampling resistor are coupled in series between the feedback end and the ground, and the voltage at the output end of any LED string is less than compensation. When the voltage is applied, the control switch is turned on, otherwise the control switch is turned off. Each constant current source circuit includes a transistor, an operational amplifier and a resistor. The transistor and the resistor are coupled in series between the output end of the corresponding LED string and the ground, and the operational amplifier detects the voltage across the resistor and The reference voltage is compared and the transistor is controlled to make the operating current of the corresponding LED string constant. The dimming control circuit is coupled to the constant current source circuit, receives the dimming signal, and accordingly controls the working and non-operating states of the constant current source circuit to implement burst mode dimming.

In an embodiment of the invention, the dimming control circuit is further coupled to the LED feedback control circuit, and controls whether the LED feedback control circuit operates according to the dimming signal.

In an embodiment of the invention, the LED driving circuit further includes a reference voltage source circuit. A reference voltage source circuit is coupled to the constant current source circuit to generate a reference voltage of less than 1.25 volts.

In an embodiment of the invention, the LED driving circuit further includes an LED short circuit protection circuit. The LED short circuit protection circuit is coupled to the output end of the LED light string. When the LED light string is working normally, the protection signal is output when the output voltage of any LED light string is detected to be greater than the protection point voltage, and vice versa. The protection signal, wherein the protection signal causes the dimming control circuit to control the constant current source circuit to stop working by the dimming signal.

In an embodiment of the invention, the LED driving circuit is suitable for a liquid crystal display product such as a liquid crystal display or a liquid crystal television. LCD products include motherboard microcontrollers. The main board micro controller is coupled to the dimming control circuit and the LED short circuit protection circuit, and outputs a dimming signal and causes the dimming control circuit to stop the constant current source circuit from being stopped by the dimming signal when receiving the protection signal.

In an embodiment of the invention, the LED driving circuit further includes a standby power circuit. The standby power circuit receives the bus voltage and generates a standby power voltage to supply power to the motherboard microcontroller.

The LED driving circuit of the present invention directly outputs the main power supply voltage generated by the voltage conversion of the bus bar voltage through the main power supply circuit to the input end of the LED light string to provide the required operating voltage of the LED light string, so that only one energy conversion is performed. The power conversion efficiency will be improved. Secondly, the constant current source circuit uses an operational amplifier to receive the lower reference voltage generated by the reference voltage source circuit to compare with the voltage corresponding to the operating current of the LED string, thereby reducing the constant current source circuit. Power consumption. In addition, the LED driving circuit of the present invention further adopts an LED feedback control circuit to adjust the main power supply voltage accordingly, so that the voltage drop falling in the constant current source circuit is in a reasonable state and can work normally to perform constant current control on the LED light string; And the LED short circuit protection circuit uses too many LED short circuits in the LED light string to cause the output terminal voltage to be too large, and the main power supply circuit no longer performs voltage conversion through the output feedback control circuit to protect the internal components from burning.

The above and other objects, features and advantages of the present invention will become more <RTIgt;

3 is a block diagram of an LED driving circuit suitable for a large-sized liquid crystal display product according to an embodiment of the present invention, and FIG. 4 is a circuit diagram of the LED driving circuit shown in FIG. Please refer to FIG. 3 and FIG. 4 at the same time. Large-size liquid crystal display products usually need to use a high-power LED light source, which is usually composed of LED light strings LB1~LBn, and each LED light string includes multiple serials. A coupled LED, where n is a positive integer. When n is greater than 1, these LED strings LB1 LB LBn are driven in parallel. The LED driving circuit 2 is used for driving the LED light strings LB1 LBLBn, which includes a main power supply circuit 21, a standby power supply circuit 22, a reference voltage source circuit 23, n constant current source circuits 241~24n, an LED feedback control circuit 25, and an LED short circuit. Protection circuit 26 and dimming control circuit 27.

The LED driving circuit 2 receives the stable DC bus voltage Vbus generated by the rectification, power factor correction and filtering of the AC mains, and is generally about 400V. The bus voltage Vbus is stepped down by the main power supply circuit 21 to generate main power voltages Vm1 and Vm2, for example, 86V and 12V, respectively. The main power supply voltage Vm1 is used for directly outputting to the input end of the LED light string LB1~LBn as the input voltage of the light string, so that the power conversion efficiency of only one energy conversion will be improved. The main power supply voltage Vm2 is used to provide power required for liquid crystal display products such as audio circuits and panel drive circuits. The bus voltage Vbus is further stepped down by the standby power supply circuit 22 to generate a standby power voltage Vsb, for example, 5V, for providing power required for components in the liquid crystal display product, such as a motherboard micro controller, which still need to work in the standby mode. And provide reference voltage source circuit 23 and dimming control Circuit 27 requires power.

The main power supply circuit 21 includes a main power converter 211, an output filter circuit 212, and an output feedback control circuit 213. The main power converter 211 is, for example, a buck converter for receiving the bus line voltage Vbus generated by the rectification, power factor correction and filtering of the AC mains, and accordingly generating the main power voltage Vm1 from the main power circuit. The output of 21 is output to the input terminals of the LED strings LB1 to LBn. The output filter circuit 212 includes capacitors C1 and C2 and an inductor L1 for reducing the main power supply voltage Vm1 chopped by the main power converter 211. The output feedback control circuit 213 includes a first sampling resistor Rf1, a second sampling resistor Rf2, resistors R1 and R2, a capacitor C3, a shunt regulator U22, an optocoupler U3 and a PWM controller U12, wherein the first sampling The resistor Rf1 is coupled between the output terminal of the main power supply circuit 21 and the feedback terminal FB, and the second sampling resistor Rf2 is coupled between the feedback terminal FB and the ground. The output feedback control circuit 213 detects the main power supply voltage Vm1 through the first sampling resistor Rf1 and the second sampling resistor Rf2, and the feedback terminal FB voltage corresponds to the detected main power supply voltage Vm1, and the feedback terminal FB voltage is connected in parallel. The voltage regulator U22 amplifies the error and then transmits it to the PWM controller U12 via the photocoupler U3 to control the switching duty cycle in the main power converter 211 to adjust the main power supply voltage Vm1 to be stable. When the shunt regulator U22 uses TL431, it has a 2.5V reference voltage source inside, and the reference terminal R voltage will be stable at 2.5V, so the main power supply voltage Vm1=2.5V×(Rf1+Rf2)/Rf2.

The reference voltage source circuit 23 includes a transistor Q1, a shunt regulator U23, resistors R3 R R5 and a capacitor C4 for receiving standby power The power supply of Vsb is pressed and a precise reference voltage Vr1 is generated accordingly. When the shunt regulator U23 uses TL431, Vr1 = 2.5V × (R4 + R5) / R5. The reference voltage source circuit 23 further includes resistors R6 and R7 for further generating a reference voltage Vr of less than 1.25 V, Vr = Vr1 × R7 / (R6 + R7).

The constant current source circuits 241~24n are respectively coupled to the output ends of the LED light strings LB1~LBn for controlling the currents of the LED light strings LB1~LBn to be constant and equal in size. Each of the constant current source circuits 24i includes a transistor Mi, an operational amplifier OPi, and a resistor Rsi, where i is any positive integer from 1 to n. The transistor Mi has a first end, a second end and a control end. In this example, the transistor Mi is an N-channel field effect transistor, and the first end, the second end and the control end are respectively a 汲 extreme, a source terminal and a The gate is extreme. The 汲 terminal and the source terminal of the transistor Mi are respectively coupled to the output end of the LED lamp string LBi and the first end of the resistor Rsi, and the second end of the resistor Rsi is coupled to the ground, so the transistor Mi and the resistor Rsi The series is coupled between the output end of the corresponding LED string LBi and the ground. The operational amplifier OPi has a positive power terminal, a negative power terminal, a positive input terminal, a negative input terminal and an output terminal, and the positive power terminal receives the power supply of the internal power supply voltage Vcc and the negative power terminal is coupled to the ground, and the positive input terminal receives the reference voltage source. The reference voltage Vr generated by the circuit 23 is less than 1.25V and the negative input terminal is coupled to the first end of the resistor Rsi to detect the voltage across the resistor Rsi. The cross-voltage corresponds to the operating current of the LED string LBi, so the operation The amplifier OPi compares the voltage across the resistor Rsi with the reference voltage Vr to output a signal from its output to the gate terminal of the transistor Mi to adjust the source voltage drop, so that the operating current of the LED string LBi is constant and its value is Ilbi=Vr/Rsi. When Rr=0.3V, Ilbi=150mA, resistor Rsi The resistance value is taken as 2Ω. The constant current source circuits 241~24n can reduce the power consumption of the resistors Rs1~Rsn coupled between the transistor and the ground by using a lower reference voltage Vr, which is equivalent to improving the power conversion efficiency.

The LED feedback control circuit 25 includes a transistor Q2 as a switch, a third sampling resistor Rf3, an operational amplifier OP, resistors R8 to R14, and a capacitor C5. The transistor Q2 has a first end, a second end and a control end, and the first end and the second end are respectively coupled to the feedback end FB and the first end of the third sampling resistor Rf3, and the third sampling resistor Rf3 The second terminal is coupled to the ground, so that the transistor Q2 as a switch and the third sampling resistor Rf3 are coupled in series between the feedback terminal FB and the ground. The positive power terminal of the operational amplifier OP receives the power supply of the internal power supply voltage Vcc and the negative power supply terminal is coupled to the ground, the positive input terminal receives the compensation point voltage and the negative input terminal is coupled to the connection terminal FB1 to be respectively coupled through the diodes D11~D1n Connect to the output of the LED string LB1~LBn to detect the output voltage of the LED string LB1~LBn. Wherein, the value of the compensation point voltage is VR9×(R12+R13+R14)/(R11+R12+R13+R14), and the voltage across the resistor R9 is VR9=Vr1×[R9//(R11+R12+R13+ R14)]/[R8+R9//(R11+R12+R13+R14)].

The operational amplifier OP compares the compensation point voltage received by the lowest and positive input terminals of the LED lamp strings LB1 LB LBn detected by the negative input terminal to output a signal from the output terminal to the control of the transistor Q2. The terminal controls its conduction or disconnection. When it is detected that the lowest of the output voltages of the LED string LB1~LBn is less than the compensation point voltage, that is, when the output voltage of any LED string is detected to be less than the compensation point voltage, it indicates that it falls to a constant current source. The voltage drop of the transistor in the circuit is low and there will be a Normal operation in the constant current region becomes the risk of operating in the variable resistance region and cannot perform constant current control on the LED light string. At this time, the operational amplifier OP controls the transistor Q2 to be turned on, so that the second sampling resistor in the output feedback control circuit 213 is turned on. Rf2 is equivalent to being coupled in parallel with a resistor R, which is R=Rds2+Rf3, wherein Rds2 is the on-resistance value of the 汲 terminal and the source terminal of the transistor Q2, so the main power supply voltage Vm1=2.5V× (Rf1+Rf2//R)/(Rf2//R), the main power supply voltage Vm1 rises, so that the voltage drop falling in all the constant current source circuits 241~24n is in a reasonable state and can work normally on the LED light string LB1 ~LBn does constant current control. Conversely, when it is detected that the output voltages of the LED strings LB1~LBn are greater than the compensation point voltage, it means that the voltage drop falling in all the constant current source circuits 241~24n is in a reasonable state and can work normally for the LED string. LB1~LBn do constant current control, and the operational amplifier OP controls the transistor Q2 to be turned off.

The LED short circuit protection circuit 26 includes Zener diodes ZD1 and ZD2, resistors R15 and R16, and a capacitor C6. The cathode end of the Zener diode ZD1 is coupled to the connection terminal DET1 to be coupled to the output ends of the LED strings LB1 LB LBn through the diodes D21 D D2 n to detect the output voltage of the LED string LB1 LB LBn. Resistors R15 and R16 are coupled in series between the anode terminal of Zener diode ZD1 and ground. The two ends of the capacitor C6 are respectively coupled to the two ends of the resistor R16, and the capacitor C6 is used to filter the high frequency noise. The cathode end and the anode end of the Zener diode ZD2 are respectively coupled to the two ends of the resistor R16, and the cathode end of the Zener diode ZD2 is also coupled to the protection end DET to be coupled to the main board micro controller. The nanodiode ZD2 is used as a voltage clamp for the protection terminal DET to ensure that the dimming signal Dim is at a low level. When the transistors M1~Mn in the constant current source circuits 241~24n are turned off, the voltage of the protection terminal DET does not exceed the maximum withstand voltage of the corresponding pin of the motherboard microcontroller.

When it is detected that the highest of the output voltages of the LED string LB1~LBn is greater than the protection point voltage, that is, when the output voltage of any LED string is detected to be greater than the protection point voltage, it indicates that the LED string is in the LED string. Too many LED short-circuits cause the voltage drop of the transistor in the corresponding constant current source circuit to be high, which increases the power consumption or the risk of burning the transistor. At this time, the Zener diode ZD1 works in the collapse zone. When turned on, the value of the protection terminal DET voltage is (Vdet1-Vz1)×R16/(R15+R16) or Vz2, Vz1 and Vz2 are the breakdown voltages of the Zener diodes ZD1 and ZD2, respectively, and Vdet1 is the voltage of the connection terminal DET1. And the value is the highest of the output voltage of the LED string LB1~LBn minus the diode forward voltage of the diode, so the protection terminal DET voltage is high level, which is equivalent to output protection signal to the motherboard micro Controller. Conversely, when it is detected that the output voltages of the LED strings LB1~LBn are less than the protection point voltage, it means that the number of LED shorts in all the LED strings LB1~LBn are within the preset range without increasing too much. The power consumption may have the risk of burning the transistor. At this time, the Zener diode ZD1 operates in the reverse bias region and only a small reverse saturation current passes, so the protection terminal DET voltage is at a low level, which is equivalent to no output protection. Signal to the motherboard microcontroller.

The dimming control circuit 27 includes transistors Q3 and Q4 and resistors R17-R19 for receiving the dimming signal Dim and controlling the operation and non-operation states of the constant current source circuits 241-24n to realize burst mode dimming. When dimming letter When the Dim is at a high level, the transistors Q3 and Q4 are both turned on, and the standby power supply voltage Vsb received by the emitter end of the transistor Q4 can be transmitted to the collector terminal of the transistor Q4 to output the internal supply voltage Vcc, Vcc=Vsb-Vec4. Wherein, Vec4 is the voltage drop between the emitter end and the collector terminal when the transistor Q4 is turned on, so the internal supply voltage Vcc is supplied to the constant current source circuits 241~24n and the LED feedback control circuit 25 to operate, thereby enabling the LED string LB1~LBn work. Conversely, when the dimming signal Dim is at a low level, the transistors Q3 and Q4 are both turned off, and the standby power supply voltage Vsb received by the emitter end of the transistor Q4 cannot be transmitted to the collector terminal of the transistor Q4 and no internal power supply is output. Since the voltage Vcc is constant, the constant current source circuits 241 to 24n and the LED feedback control circuit 25 stop supplying power, and the LED lamp strings LB1 to LBn are stopped. Therefore, by the time ratio of the high level and the low level of the dimming signal Dim, the time ratio of the constant current source circuits 241~24n working and not working can be controlled, thereby controlling the operation and non-operation of the LED string LB1~LBn. The time ratio is used to control the average brightness of the light emitted by the LED string LB1~LBn, which is the burst mode dimming.

The motherboard micro controller can output the switching signal On/Off and the dimming signal Dim. Among them, the switch signal On/Off is used to turn off the PWM controller U12 in the standby mode to make it more energy-saving in the standby mode. The dimming signal Dim is used to control the operation and non-operation states of the constant current source circuits 241 to 24n by the dimming control circuit 27 to realize burst mode dimming. Moreover, the motherboard micro controller can be set to determine whether the protection terminal DET is at a high level or receives a protection signal only when the dimming signal Dim is at a high level (ie, when the LED string LB1~LBn is working normally), once the protection end DET is at a high level or receives a protection letter When the number is on, the main board microcontroller immediately controls the constant current source circuits 241~24n and the LED feedback control circuit 25 to stop the operation by the dimming control circuit 27 through the dimming signal Dim, thereby stopping the operation of the LED string LB1~LBn to protect the internal The components will not burn out.

While the present invention has been described above in terms of the preferred embodiments thereof, it is not intended to limit the invention, and the present invention may be modified and modified without departing from the spirit and scope of the invention. The scope of protection is subject to the definition of the scope of the patent application.

1‧‧‧LED drive circuit

11‧‧‧Main power circuit

12‧‧‧Standby power circuit

13‧‧‧Boost conversion circuit

141~14n‧‧‧ constant current source circuit

15‧‧‧LED feedback control circuit

16‧‧‧LED short circuit protection circuit

17‧‧‧ dimming control circuit

2‧‧‧LED drive circuit

21‧‧‧Main power circuit

211‧‧‧Main power converter

212‧‧‧Output filter circuit

213‧‧‧Output feedback control circuit

22‧‧‧Standby power circuit

23‧‧‧Reference voltage source circuit

241~24n‧‧‧ constant current source circuit

25‧‧‧LED feedback control circuit

26‧‧‧LED short circuit protection circuit

27‧‧‧ dimming control circuit

C1~C6‧‧‧ capacitor

D1, D11~D1n‧‧‧ diode

D21~D2n‧‧‧ Diode

L1‧‧‧Inductors

LB1~LBn‧‧‧LED light string

M1~Mn, Q1~Q4‧‧‧O crystal

OP‧‧‧Operational Amplifier

OP1~OPn‧‧‧Operational Amplifier

R1~R19‧‧‧Resistors

Rf1‧‧‧first sampling resistor

Rf2‧‧‧Second sampling resistor

Rf3‧‧‧ third sampling resistor

Rs1~Rsn‧‧‧Resistors

U11, U12‧‧‧ PWM controller

U21~U23‧‧‧Shutdown Regulator

U3‧‧‧Optocoupler

ZD1, ZD2‧‧‧ Zener diode

A‧‧‧Anode end of shunt regulator

K‧‧‧ cathode terminal of shunt regulator

Reference end of R‧‧‧ shunt regulator

DET1, FB1‧‧‧ connection

DET‧‧‧ protected end

FB‧‧‧Feedback

Vbus‧‧‧ busbar voltage

Vm1, Vm2‧‧‧ main power supply voltage

Vsb‧‧‧Standby power supply voltage

Vin‧‧‧ string input voltage

Vr, Vr1‧‧‧ reference voltage

Vcc‧‧‧ internal supply voltage

Ilb1~Ilbn‧‧‧ Current

On/Off‧‧‧ switch signal

Dim‧‧‧ dimming signal

1 is a block diagram of an existing LED driving circuit commonly used in large-size liquid crystal display products.

2 is a circuit diagram of a constant current source circuit of the conventional three-terminal adjustable shunt regulator shown in FIG.

3 is a block diagram of an LED driving circuit suitable for a large-sized liquid crystal display product in accordance with an embodiment of the present invention.

4 is a circuit diagram of the LED driving circuit shown in FIG.

2‧‧‧LED drive circuit

21‧‧‧Main power circuit

211‧‧‧Main power converter

212‧‧‧Output filter circuit

213‧‧‧Output feedback control circuit

22‧‧‧Standby power circuit

23‧‧‧Reference voltage source circuit

241~24n‧‧‧ constant current source circuit

25‧‧‧LED feedback control circuit

26‧‧‧LED short circuit protection circuit

27‧‧‧ dimming control circuit

D11~D1n‧‧‧ Diode

D21~D2n‧‧‧ Diode

LB1~LBn‧‧‧LED light string

DET1, FB1‧‧‧ connection

DET‧‧‧ protected end

FB‧‧‧Feedback

Vbus‧‧‧ busbar voltage

Vm1, Vm2‧‧‧ main power supply voltage

Vsb‧‧‧Standby power supply voltage

Vr, Vr1‧‧‧ reference voltage

Vcc‧‧‧ internal supply voltage

On/Off‧‧‧ switch signal

Dim‧‧‧ dimming signal

Claims (7)

An LED driving circuit comprises: a main power circuit comprising a main power converter and an output feedback control circuit, the main power converter receiving a bus generated by rectification, power factor correction and filtering of the AC mains And generating a main power voltage from the output end of the main power circuit to the input end of the n LED strings, the output feedback control circuit including a first sampling resistor and a second sampling resistor The first sampling resistor is coupled between the output end of the main power supply circuit and a feedback end, the second sampling resistor is coupled between the feedback end and the ground, and the output feedback control circuit is configured according to the feedback end Voltage control, the main power converter adjusts the main power voltage, wherein n is a positive integer; a light emitting diode feedback control circuit is coupled to the output end of the light emitting diode string, including a switch and a third The sampling resistor is coupled in series between the feedback end and the ground, and controls the switch to be turned on when detecting that the output voltage of any of the LED strings is less than a compensation point voltage, and vice versa. Controlling the switch to be disconnected; n constant current source circuits, wherein each constant current source circuit comprises a transistor, an operational amplifier and a resistor, and the transistor is coupled in series with the resistor to a corresponding light emitting diode Between the output end of the body light string and the ground, the operational amplifier detects the voltage across the resistor and compares it with a reference voltage and controls the transistor to make the operating current of the corresponding light-emitting diode string constant; A dimming control circuit is coupled to the constant current source circuits, receives a dimming signal and controls the working and non-operating states of the constant current source circuits to implement burst mode dimming. The illuminating diode driving circuit according to claim 1, wherein The dimming control circuit is further coupled to the LED feedback control circuit, and controls the operation of the LED feedback control circuit according to the dimming signal. The illuminating diode driving circuit of claim 1, further comprising a reference voltage source circuit coupled to the constant current source circuits to generate the reference voltage of less than 1.25 volts. The illuminating diode driving circuit of claim 1, further comprising a light emitting diode short circuit protection circuit coupled to the output end of the light emitting diode string During the normal operation of the light-emitting diode strings, when a voltage of the output terminal of any of the LED strings is detected to be greater than a protection point voltage, a protection signal is output, and if not, the protection signal is not output. The protection signal causes the dimming control circuit to control the constant current source circuits to stop working through the dimming signal. The illuminating diode driving circuit of the fourth aspect of the invention is applicable to a liquid crystal display product, the liquid crystal display product comprising a motherboard micro controller, the motherboard micro controller being coupled to the dimming control circuit and the The light-emitting diode short-circuit protection circuit outputs the dimming signal and, when receiving the protection signal, causes the dimming control circuit to control the constant current source circuits to stop working by the dimming signal. The illuminating diode driving circuit of claim 5, further comprising a standby power circuit, wherein the standby power circuit receives the bus voltage and generates a standby power voltage to supply the motherboard microcontroller. The illuminating diode driving circuit of claim 5, wherein the liquid crystal display product comprises a liquid crystal display or a liquid crystal television.