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A Bode Plot Without Access to the Control Loop

Updated: Mar 3, 2022

November 23, 2021

This extreme measurement seems more impossible than difficult. The traditional stability assessment method for voltage regulators is the Bode plot. Access to the control loop is required and a small signal is injected into the control loop, usually through a resistor, placed at the top of the voltage sense divider, for this purpose as shown in Figure 1.

Figure 1 Signal injection locations for the Bode plot measurement of a voltage regulator. The example shown here is used to indicate that most modern voltage regulators include multiple control loops, and in the best case, only one is accessible.

Without access to this voltage divider, there isn’t a place to inject a signal into the control loop, and therefore the Bode plot is seemingly impossible.

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