A method and apparatus for controlling a step-down converter, including setting an on-state time (TON) to a default TON; switching, when an output voltage of the step-down converter falls below a reference output voltage, the step-down converter to an on-state for a duration of the TON; and switching, at the end of the TON, the step-down converter to an off-state for a minimum off-state time (TOFF).