This paper presents an observer-based fixed-frequency sliding-mode controller for DC-DC boost converters in continuous conduction mode. Here, the general-purpose slidingmode controller, where the inductor current error and the output voltage error are chosen as the state variables, is modified to achieve superior dynamic performance. In the proposed method, instead of measuring the inductor current, the input voltage and output current are monitored where a state observer is used to provide feedback and reference signals to the controller. Simulation studies are carried out to verify the improved dynamic response of the proposed observer-based against the generalpurpose fixed-frequency sliding-mode controller for voltage and load variations.