This work proposes a control law for the boost converter with unknown constant resistive load. It focuses on Hybrid Dynamical System (HDS) theory, which considers the voltage and current signals as continuous-time variables and the switching signal as discrete-time variable. In several applications the voltage has to be constant. To ensure that the voltage value is robust with respect to any reference, an adaptive scheme is proposed. This adaptation is accomplished using a state observer and assuming that all states are accessible. Then, the full system stability can be established by using a singular perturbation analysis. The hybrid adaptive controller is tested in simulations.