PFM mode operation allows the switching frequency to be reduced and for a control method that prevents the inductor current from dropping below zero during light loads. Rather than applying square pulses of varying widths to the inductor, square pulse trains with a fixed 50% duty cycle are used to charge the inductor to a predefined current limit then discharge the inductor current to, but not below, zero. The frequency of these pulse trains is then varied to produce the desired output voltage with the aid of the output filter capacitor.