A circuit and method for operating a switching mode power supply. A clock is driven by a current source to generate pulses at a fixed frequency using pulse width modulation for normal load demands. For light load demands, the current to the clock is reduced, and therefore the clock generates pulses at a lower, variable frequency and fixed duration using pulse frequency modulation. Thus, depending on the load condition, either fixed frequency pulses or fixed duration pulses are automatically provided to a power stage for conversion to an output voltage.