LED backlight drive circuit

In this circuit, Vd is the TFT-LCD backlight operating DC voltage, and its size is designed according to the specifications provided by the LCD manufacturer. Transistors VT1 and VT2 form a constant current source circuit. VT1 and VT2 work in the amplification state. When VT2 works, adjust the base resistor R22 to make the emitter current of VT2 reach the rated value. Because the internal resistance will become smaller after the LED is turned on, the current will gradually increase. The purpose of increasing VT1 is to shunt the base current of VT2. By adjusting the feedback resistor R20, the current flowing through the LED can be controlled and the current can be stabilized through VT1.