A Simple Method to Determine ESR Requirements for Stable Regulators

Focus:

When applying linear voltage regulators, there are three external paramenters (load current, output capacitance, and output capacitor ESR) that affect regulator stability. Yet voltage regulator datasheets don't say much about this relationship. In addition, previous articles on the subject of regulator stability have centered on the impact of phase margin, overall stability, and how to measure it. In contrast, this article describes how, for a given value of output capacitance and a given load current, you can calculate the value of capacitor ESR that will be needed to obtain a desired phase margin. Determining the ESR needed at the lowest operating current ensures the regulator will be stable across its load current range. The ESR calculation requires a measurement of the regulator's wideband output impedance as demonstrated here on an LM317 using the OMICRON-Lab Bode 100 vector network analyzer and the Picotest J2111A current injector, which are also used to non-invasively measure the circuit's phase margin and bandwidth. Article includes discussion of output impedance characteristics of linear regulators.

What you’ll learn:

• How to determine the output capacitor ESR required to ensure the stability of a linear voltage regulator
• How to achieve a desired phase margin in linear regulator circuits given a specified output capacitance and load current range

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Author & Publication:

Steven Sandler, AEi Systems, Los Angeles, Calif., Power Electronics Technology, Aug 04 2011

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