Effect of target impedance and control loop design on VRM stability

Stability analysis of a VRM used in a typical microprocessor power supply application is presented. With every generation of microprocessors, the decreasing voltages, increasing currents and di/dts are demanding faster VRM transient responses. This analysis focuses on the influence of constantly dropping target impedances and increasing crossover frequencies on the output of multi-phase, high-frequency synchronous rectifier type DC-DC converters. Attempts are made to relate the switching frequency of the VRM to its stability regime. Results are summarized with guidelines on feedback loop stabilization under diminishing target impedances.