A circuit for controlling a switching power supply includes a disable signal generator generating a disable signal in response to an input clock signal, a timer circuit generating a timeout signal in response to the disable signal, a comparison signal generator generating a comparison signal in response to an output signal of the power supply, a first threshold signal generator generating a first threshold signal in response to the comparison signal, the first threshold signal having a value greater than that of the comparison signal, and a first comparator comparing the first threshold signal and a sense signal to de-assert the modulation signal when the sense signal is equal to or greater than the first threshold signal and the timeout signal has a first logic value.