Loop Analysis Directly from Time-Domain Waveform with SEPIA

Published in Signal Integrity Journal, written by Masashi Nogawa.

Engineers can use a variety of methods to analyze the behavior of a voltage regulator module (VRM) using frequency and time-domain techniques, as we have seen starting from Part 6 in this series of articles.

Part 10 showed how the load transient response to a step change or impulse in the time domain can reveal the frequency-domain characteristics of the output impedance (Z_OUT) of a VRM. (See the full collection of articles here).

Another approach to using the load transient responses to step changes is to use the technique embodied by the Stability Evaluation for Power Integrity Analysis (SEPIA[i]) software, developed by Steve Sandler of Picotest. This analysis can be performed using simulations or bench-test waveform data recorded from physical devices.

The SEPIA method takes advantage of the assumption that any circuit that acts as a filter will demonstrate similar ringing profiles in response to a step load. If they share the same quality factor (Q) they will exhibit the same ringing profile as long as the underlying system demonstrates quadratic behavior. Take the plot in Figure 1 as an example, which was generated from a simulation described later in this article. With a Q of 5.52, it responds with 20 readily detectable ringing peaks. Variations in system setups will lead to different ringing frequency, amplitude and DC bias. But all of the systems with a Q of 5.52 will share the same numbers of peaks and with the same decay rate.

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