A special requirement of the voltage regulator module (VRM) is fast transient response with tight output voltage tolerance. This paper investigates the relationship between the control bandwidth and the transient response in buck converters in both voltage and current mode controls. A critical bandwidth value is discovered, beyond which pushing the bandwidth can no longer reduce the output voltage spike during the transient. This critical bandwidth is determined by the characteristics of the output filter bulk capacitor. In order to reduce the size and improve the transient response of the VRM, better capacitors with low equivalent series resistance (ESR) show that high control bandwidth and high switching frequency are the trends for new designs. Simulation and experimental results prove the theoretical analysis, and a design guideline is proposed for the VRM.
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