An LED driver circuit and its control circuit for controlling its power switch are provided. The LED driver includes a switch, a PWM controller, a current source and a signal controller. The switch has a first end outputting a driving voltage to at least one LED. The PWM controller provides a PWM signal. The current source provides a driving current flowing through the LED when a dimming control signal is asserted. The signal controller turns off the switch when the dimming control signal is de-asserted and relays the PWM signal from the PWM controller to the switch so that the switch is controlled by the PWM signal when the dimming control signal is asserted.