Measuring Loop Response Of Regulators With Inaccessible Feedback Resistor

Focus:

For a power supply to be stable, certain gain and phase margins are needed. The conventional approach of making Bode plot measurements is preferred for its simplicity, assuming the user has access to the top feedback resistor. But how can the loop response be measured when the top feedback resistor is inaccessible within a molded module? Or when a device does not require a top feedback resistor and instead uses an output voltage sense pin? This article presents two variations on the conventional loop response measurement method, which use the same instrument but with some changes in the connection to the regulator and (in one case) some simple additional circuitry. Example results for one of the novel methods are checked against those obtained with the conventional method.

What you’ll learn:

• How to make Bode measurements of voltage regulator stability when the top feedback resistor is inaccessible
• How to make Bode measurements of voltage regulator stability when the regulator uses an output voltage sense pin

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Author & Publication:

Adam Huff, Xu Zhang and George (Zhijun) Qian, Analog Devices, San Jose, Calif., How2Power Today, Jun 13 2025

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