In this paper, a robust relay control of a buck converter is proposed. The controller is synthesized based on the design of a continuous controller for the average model of a buck converter usually used when Pulse Width Modulation (PWM) is considered. To deal with parameter uncertainties, the controller implements an integral action. In this paper, local stability is proven for the hybrid system, thereby ensuring zero steady state error on the voltage output. A subset of the robust basin of attraction is explicitly constructed. The effectiveness of this control is illustrated experimentally.