A switch-mode power supply and control circuitry that reduces variation in the switching frequency of the power supply with changes in loading. In one example, a switch-mode power supply includes an inductor, a transistor, and control circuitry. The transistor is coupled to the inductor, and configured to charge the inductor. The control circuitry is coupled to the transistor. The control circuitry is configured to turn off the transistor for a first time period. The first time period is a function of voltage across the inductor during the first time period. The control circuitry is also configured to determine whether the switch-mode power supply is operating in continuous conduction mode or discontinuous conduction mode. The control circuitry is further configured to add a predetermined fixed interval to the first time based on the switch-mode power supply operating in discontinuous conduction mode.