Control design considerations for voltage regulator modules

Fast response to load current switching is the most important requirement for voltage regulator modules (VRM). This paper addresses several aspects of VRM control design in order to meet this requirement. A comparison between voltage-mode control and peak-current control is presented first to highlight the advantages of voltage-mode control in terms of transient response to load variations. A simple mathematical model is then presented to facilitate sequential design of the power stage and the controller. The concept of critical inductance is examined and the proposed model is shown to provide a more reliable basis for the inductor design. The analyses are supported by numerical simulation as well as experimental results from a 100 W, 3.3 V-output half-bridge converters.