Assessing Closed Loop Stability in ADS with SEPIA

With contributing authors Heidi Barnes (Keysight) and Steve Sandler (Picotest)

Importing a SEPIA measurement into Keysight ADS gives you all of the information you need for your design to reach stability in less than a minute, even if it’s non-LTI. In this blog, we will discuss how to leverage these tools to get actionable insights on a design’s stability.

In a world where VRM control loops are no longer consistently linear, time‐invariant systems, the traditional approach of breaking the control loop to run a Bode plot no longer provides an accurate analysis. To truly understand the behavior of a non-LTI system, you need to simulate the entirety of power delivery network (PDN), including the PCB, the VRM, the decoupling network, and the ASIC. Keysight ADS is an ideal tool for this, and integrates seamlessly with the two methods of measuring a non-LTI system:  non-invasive stability measurement (NISM) and scope embedded power integrity analysis (SEPIA), both developed by Steve Standler of Picotest. 

Getting Started with SEPIA in ADS

SEPIA, which Sandler developed in 2013, is a measurement-based tool that extracts the quantitative stability of a VRM directly from the step load response. The step load is a gold standard of truth for power integrity engineers, and has been used to verify the results of Bode plots for decades. SEPIA converts step load responses into high-fidelity time-domain stability models that can be imported directly into Keysight ADS. ADS comes with a built-in function that allows you to create a SPICE model out of the measured standard time-domain transient waveform. It extracts the value of every individual component, output impedance, phase margin, Q, and both natural and forced resonance frequencies. Just by putting two markers on a waveform, you will get exactly what you need to know to fix your design’s stability. You can run SEPIA in ADS for free by pulling in and running the encrypted AL, which we have available for download here.

Case Study: Simulating LTM4637 Stability Time Domain using SEPIA

Figure 1 - LTM4637 Stability Time Domain Simulation Example using SEPIA in ADS

Figure 2 - LTM4637 Stability Simulation Results

Let’s look at an example of a simulation using a Sandler State-Space Average VRM Model (SSAM) of the LTM4637 in Keysight ADS (Figure 1). The test circuit (shown in Figures X) converts a 12V input to a 1V output, with R3 setting the output voltage and SRC2 providing control-loop compensation. To simulate a realistic environment, we’ve included SRLC1, a lumped-capacitor model representing the PDN, and R1 and R2 to account for some PCB copper resistance. If we apply a transient step response, we will see in Figure 2 that SEPIA calculated a Q = 4.051, highlighted by the green arrow, and a PM = 14.072 degrees, highlighted by the purple arrow.

SEPIA also extracts our SPICE model from the transient data. We can use the extracted parameters, Capacitance (207.192 µF), ESR (0.245 mΩ), Inductance (5.325 nH), and Series Resistance (1.006 mΩ), to accurately reconstruct the physical PDN.  We can now use these values to generate a passive SPICE model and re-run the time-domain simulation to validate it. Figure 3 shows the simulation setup for the extracted SPICE model.

Figure 3 - Simulation Setup for Extracted SPICE model for LTM4637 PDN

In Figure 4, our original step response recorded with SEPIA is shown in red, with the response from the extracted SPICE model shown in blue. As we can see, the State-Space Average VRM model is nearly identical to our extracted SEPIA model. 

Figure 4 - Simulation Result for LTM4637 State-Space VRM Model vs. Extracted SPICE model for LTM4637 PDN using SEPIA

The Future of Stability Measurement

At Signal Edge Solutions, we are already innovating ways to leverage SEPIA to make stability measurements even more efficient. We have created a test automation software integrating SEPIA that drives test equipment to measure the stability of a VRM or power supply across a range of frequencies, extracts a SPICE model, displays the impedance, Q, and phase margin, and provides a recommended capacitor fix to reach stability all in less than a minute. StepLoad Pro with SEPIA is compatible with equipment from all major suppliers. To learn more about stability measurements and what you can accomplish with SEPIA and test automation, subscribe to our blog and view our further reading suggestions below:

• Stop Breaking the Loop: Modern Closed-Loop Stability in ADS

• The Ultimate Guide to the R&S MXO5 for Power Integrity

• SEPIA ADS Workspace Download