PICOTEST Application Note | Non-Invasive Phase Margin Assessment

Application Note | Non-Invasive Phase Margin Assessment
Application Note Picotest ::  Non-Invasive Assessment of Voltage Regulator Phase Margin...

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Application Note Picotest :: Non-Invasive Assessment of Voltage Regulator Phase Margin without Access to the Control Loop.

Application Note Picotest :: Non-Invasive Assessment of Voltage Regulator Phase Margin without Access to the Control Loop.

Many voltage regulators are of the fixed output variety and include the voltage divider internal to the regulator. There are definite advantages to this approach, which allows active voltage trimming and also minimizes the physical space required. A disadvantage to this approach is that the voltage divider is not available for Gain-Phase or "Bode" measurement. This leads many to believe that it is not possible, or even necessary to evaluate the stability of such a regulator. Neither of these beliefs is true.

Digital systems often contain high-powered fast switching loads. Interconnects and load step repetition rates and times impact the power system's stability. The loading can be difficult, if not impossible to replicate during the hardware development process and in simulation. Additionally, the stability varies throughout the system with varying impedances and filtering elements contributing to complex impedances presented to the power supply's outputs. While load step testing can be used to roughly indicate stability it does not provide an accurate phase margin number, and is subject to equipment issues and interactions with the circuit's impedance, depending on where the load step is injected.

It would be useful to be able to measure the stability of switching and linear regulators in a system, with the actual loading applied, though without the need to physically cut into traces to get to the control loops.

In this article we demonstrate how to determine the phase margin of a voltage regulator's control loop, without breaking any connections (non-invasively), and using only an Keysight E5601B Network Analyzer and the Picotest J2111A Current Injector. In order to validate the method, an LM317 regulator is used so that the Bode measurement can be obtained for comparison with the proposed, non-invasive method. This measurement technique is also performed on a fixed output voltage regulator. Several output capacitors are used in order to obtain a wide range of phase margin solutions..............



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