An apparatus includes a regulator, a modulation circuit, a first delay generator, and a second delay generator. The regulator generates a regulator control output signal to control a current of a power converter to regulate an output voltage of the power converter, the modulation circuit modulates a switching frequency of the power converter, the first delay generator controls a first delay time to turn on a switch of the power converter in a switching cycle, based on a change in the switching frequency of the power converter, and the second delay generator controls a second delay time to turn the switch of the power converter off in the switching cycle, based on the first delay time and a duty cycle of the power converter.