This paper introduces a novel low-power digital controller for high frequency dc-dc switch-mode power supplies (SMPS) that is well-suited for integration in power management systems of small handheld devices. The controller operates at programmable constant switching frequencies up to 20 MHz and can change mode of operation to improve the overall SMPS efficiency. The key elements of the controller are a high frequency digital pulse-width modulator (DPWM), based on a segmented ring oscillator, and a signal-race based digital pulse-frequency modulator (DPFM). The controller is implemented in a standard CMOS 0.18 mum process and its operation verified both through simulations and experiments