A control circuit for a power converting circuit includes a multifunctional pin, a pulse-width-modulation (PWM) signal generating circuit, a voltage detecting circuit and a zero current detecting circuit. The voltage detecting circuit detects a signal on the multifunctional pin. When the signal on the multifunctional pin is greater than a predetermined value, the voltage detecting circuit configures the PWM signal generating circuit to intermittently conduct a current switch of the power converting circuit. The zero current detecting circuit detects the signal of the multifunctional pin to determine the conduction status of the current switch. When the signal of the multifunctional pin is less than the predetermined value, the voltage detecting circuit configures the PWM signal generating circuit to turn off the current switch.