A switching mode power supply, having: a power switch; an energy storage component coupled to the power switch; a current sense resistor configured to generate a current sense signal; a mode select resistor configured to generate a mode select resistor; a ZCD (Zero Cross Detecting) circuit configured to generate a ZCD signal; and a control circuit configured to provide a switch control signal to control the on and off of the power switch, the control circuit having a multi-function pin configured to receive the mode select signal, the current sense signal and the ZCD signal.