An apparatus and method for reducing an output voltage ripple of a converter are provided. The apparatus may include a controller for controlling a converter, wherein the controller may include a clock generating circuit that generates a periodic clock signal containing periodic clock pulses, and a control circuit that causes the clock generating circuit to asynchronously initiate a clock pulse based on a difference between a feedback voltage of the converter and a reference voltage. The apparatus may also include an on-time modulation circuit which modulates the on-time based on the difference between the reference voltage and the sampled output voltage.