The present paper deals with the design of a discrete-time integral sliding mode controller (DISMC) using only input-output measurement. The proposed strategy of control is developed and considered as a new method for voltage controller of electronic power systems, such as DC-DC buck converter. An input-output model of DC-DC buck converter is given. Simulations results are presented to demonstrate the good effectiveness and the efficiency of the proposed DISMC in terms of reduction of chattering phenomenon and robustness against external disturbances and parameters variations.