Paralleling Electrolytic and Ceramic Capacitors Perks Up POL Transient Response

by Timothy Hegarty, National Semiconductor, Tucson, Ariz., How2Power Today, Mar 30 2011

Focus:
This article offers perspective on the step-load transient behavior of point-of-load (POL) dc-dc converters (i.e. synchronous buck converters) and the dependence of their transient response on output filter capacitance. The author explores the rationale of paralleling capacitors of dissimilar chemistries yet complementary performance using buck converter and capacitor models to analyze transient performance. Specifically, the author examines the combination of a solid polymer electrolytic capacitor with a multilayer ceramic. Using SIMPLIS, the author simulates load transient performance of a POL converter based on the LM27402 PWM controller, allowing examination of peak deviation and settling time under two output capacitor configurations. One consists of a 270-uF SANYO POSCAP in parallel with a 100-uF TDK ceramic; and the other consists of two 100-uF TDK ceramics in parallel. Following the simulation, the author measures transient performance of the LM27402-based POL module on the bench with the two output capacitor configurations.

What you’ll learn:

• How to understand why the parallel combination of polymer electrolytic and ceramic capacitors improves buck converter transient response versus use of ceramic capacitors alone

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