I. INTRODUCTION
Step-down DC to DC converter (buck converter) is a power electronic circuit which produces a lower output voltage than input voltage. This converter is an important circuit in several portable electronic devices, and it can be used in many industrial applications, such as computer hardware, dc-motor drives, telecommunication equipment, etc. The controller of this converter is usually designed by an analog circuit which is simple and cheap. In addition, analog circuit is suitable for the circuit with high switching frequency which is normally adopted in DC to DC converter. Designing of a high performance DC to DC converter controller requires the technique which can incorporate both robustness and performance in the controller design. In the previous research works, many techniques can fulfill the above mentioned requirements, e.g. , mixed sensitivity, loop shaping control, 2DOF loop shaping control [1] [2] [3], etc. In these techniques, the designing of weighting function and controller is based on the concept of optimal control theory.