DC-DC converters are an important class of power converters. Output voltage is subject of change due to various factors such as converter input voltage change, load change and losses in circuit elements. In order to obtain a constant output voltage, a controller must be added to the converter. In this paper, problem of designing controller for Buck converter in presence of circuit none idealities such as: switch resistance, inductor and capacitor series resistance, diode voltage drop and load current variations is solved. To do this, using State Space Averaging (SSA), small signal model of converter is obtained. In the next step, a controller is designed based on Integral of Time multiplied by Absolute Error (ITAE) performance index. Controller performance is tested with the aid of simulation for different scenarios. (C) 2017 The Authors. Published by IASE. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).