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Manufacturing method of backlight drive circuit

2020-02-07

【 Background technology

With the continuous progress of technology, the backlight technology of liquid crystal display device has been continuously developed.The backlight of the traditional liquid crystal display device is a cold cathode fluorescent lamp (CCFL).However, due to the disadvantages of CCFL backlight, such as poor color reduction ability, low luminous efficiency, high discharge voltage, poor low-temperature discharge characteristics, and long heating time to reach stable gray level, the backlight technology using LED backlight has been developed.


[0003] in LED backlight, a special backlight drive circuit is required to provide the LED string with its normal luminous driving voltage.Figure 1 is a schematic diagram of a backlit drive circuit with prior art.As shown in figure 1, the backlight driver circuit includes boost circuit 110, LED string 120 and constant current driver chip 130.


The boost circuit 110 is controlled by the constant current driver chip 130 to boost the input voltage Vin to meet the needs of driving the LED string 120.The input voltage Vin also supplies the constant current driver chip 130 to make the constant current driver chip 130 work normally.The constant-current driver chip 130 receives the external Pulse Width Modulation (PWM) Modulation signal to control the current flowing through the LED string 120, so that the LED string 120 will glow normally.


Specifically, the constant-current driver chip 130 includes control module 131 and operational amplifier 132.Control module 131 receives the enabling signal ENA, enabling the entire constant current driver chip 130 to start working.Control module 131 outputs drive signal to boost circuit 110's MOS transistor Ql, wherein, when the MOS transistor Ql conducts, inductance L stores energy;When the MOS transistor Ql is turned off, the inductor L releases energy, providing the LED string 120 with a voltage that meets its normal luminescence.


[0006] 132 positive phase of the operational amplifier input to receive a constant voltage VI, operational amplifier 132 negative phase of the input voltage at the ends of the feedback resistor of RT, operational amplifier output coupling received 132 MOS transistor Q2 grid, 132 will be a constant voltage operational amplifier Vl and the ends of the resistor RT, after comparing the voltage output control signal to control the MOS transistor Q2 voltage difference between the gate and the source of value, and then to determine the current through the LED string of 120.The duty cycle of the current flowing through the LED string 120 is determined by the duty cycle of the PWM dimming signal.When the PWM dimming signal is high voltage, the op 132 can effectively control the MOS transistor Q2; when the PWM dimming signal is low voltage, op 132 can no longer control the MOS transistor Q2, the MOS transistor Q2 is in the off state, and no current flows through the LED string 120.


[0007] however, since there is parasitic capacitor C between gate and source of MOS transistor Q2, when applied voltage is applied between gate and source of MOS transistor Q2, parasitic capacitor C will be charged first. When parasitic capacitor C is fully charged and applied voltage still exists, MOS transistor Q2 will be conducted.Such problems is that the frequency of the PWM dimming signal after fixed, when the duty cycle is very small, mean 132 operational amplifier output control signals for the duration of the MOS transistor Q2 is very short, lead to MOS transistor Q2 parasitic capacitance between the gate and the source of C charging time is very short, not to the parasitic capacitance C is full of electricity, and MOS transistor Q2 cannot in a fully conducting state, the current flowing through the LED string of 120, constitute no preset current (the default is to make the LED string of 120 normal luminous current);Especially during the startup process, if the duty cycle of PWM dimming signal is too small, the MOS transistor Q2 cannot be in the on-off state in time, and the constant current driver chip 130 May misjudge the LED string 120 in the open state, which will affect the normal operation of the whole backlight driver circuit.


[invention contents]

[0008] in order to solve the problems existing in the above prior art, the purpose of the invention is to provide a backlight driver circuit that will not produce abnormal operation even if the duty cycle of PWM dimming signal is very small.


[0009] according to one aspect of the invention, a backlight driver circuit is provided, including a boost circuit, a constant-current driver chip and an LED string coupled to the boost circuit. The backlight driver circuit also includes a detection module, wherein the boost circuit provides the input voltage boost to the LED string.Receiving external PWM dimming signal detection module and arithmetic for the outside of the duty ratio of PWM dimming signal detection module based on the outside of the duty cycle of PWM dimming signal with the preset threshold value of the comparison results, determine whether to produce control signal to the constant current driver chips, to make the constant current driver chip control electric current flows through the LED string.


[0010] in addition, when the duty cycle of the external PWM dimming signal is less than the preset threshold, the detection module truncates the external PWM dimming signal and generates a PWM dimming signal with a duty cycle of the preset threshold to the constant-current driver chip;Or, when the duty cycle of the external PWM dimming signal is less than the preset threshold, the detection module truncates the external PWM dimming signal and outputs a low-level signal to the enabling signal input end of the constant-current driver chip.


[0011] in addition, the booster circuit including inductors, described the first MOS transistor, rectifier diode, capacitor and the inductor used to receive at the end of the input voltage, the other end of the inductor coupling received positive, rectifier diode rectifier diode cathode coupling received the LED string is end, the end coupling capacitor received rectifier diode is between the end of the cathode and LED string, the first of the MOS transistor drain decoupling from the other end of the inductor and rectifier diode, the first MOS transistor source electrical grounding, first MOS transistor grid coupling constant current driver chip.


[0012] in addition, the LED string including multiple leds, the second MOS transistor and resistor connected in series, among them, the second MOS transistor drain coupling received a series of multiple LED anode, the second source of the MOS transistor coupled to one end of the resistor and the other end of the resistor electrical grounding, the second MOS transistor grid coupling constant current driver chip.


[0013] in addition, the constant current driver chip, including control module and the operational amplifier, described in the control module Settings can make the signal input, among them, the control module receives the input voltage and by enabling signal input can make the signal input, and control modules separately decoupled received booster circuit and series multiple LED anode, the positive phase input terminal of the operational amplifier receives a constant voltage operational amplifier negative phase of the input coupling from one end of the second source of the MOS transistor and resistor between the operational amplifier output coupling of receiving the second MOS transistor gate.


[0014] in addition, one end of the detection module receives an external PWM dimming signal, and the other end of the detection module is coupled to the output end of the operational amplifier;Or one end of the detection module receives an external PWM dimming signal and the detection module is coupled to the output end of the operational amplifier and the input end of the enabling signal respectively.


[0015] of the present invention backlight driving circuit, the PWM dimming signal's duty cycle is very small, can own produced duty ratio as described in the preset threshold of PWM dimming signal or produce low level signal to the constant current driver chip, so that the normal LED string light or constant current driver chip stop working, and then makes a backlight driving circuit work normally.

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