Table of Contents
SEPIA: Stability Evaluation for Power Integrity Analysis
The SEPIA is a control loop stability analysis tool developed by Mr. Steve Sandler at Picotest.com.
The SEPIA extracts frequency-domain parameters of our target circuit/loop, directly from time-domain, load transient responses.
The SEPIA method is based on a simple fact that every single circuit sharing the same qualify factor Q exhibits the mathematically "similar" ringing to decay shape by assuming it is a quadratic system.
Copyright © 2014 Picotest
The "SEPIA" compiled binary, the EXE (Windows Executable) file, available from this repository is a copyright intellectual property of Picotest.com. All rights reserved.
Please contact info@picotest.com for more information.
- Download two (2) ".exe" files, in this repositry, into your working folder/directory.
- The SEPIA StandAlone program is NOT GUI software.
Prepare Windows command console first, for example cmd.exe, Powershell.exe, git-bash.exe,... - Run the UI (User Interface) program SEPIA_UI.exe with proper parameters.
See Usage / Example below for the detail.
Note
I - the author of this README file - is using git-bash.exe. Please pay attention how to use double quotation charactors on your console program.
Calling the SEPIA_UI.exe with no paramter is a handy way copy & paste this long parameter list.
prompt>> SEPIA_UI.exe
#######################################################################
## ERROR: The first parameter should be either one of "od" or "ud". ##
#######################################################################
Usage #1:: You have an over-dump response:
>> SEPIA_UI.exe od --d0="0 0" --d1="33ns 20.366mV" --d2="0.375us 26.316mV" --d3="2.015us 4.2168mV" --Istep="500uA" --Sim
or
Usage #2:: You have an under-dump response:
>> SEPIA_UI.exe ud --d0="0 0" --p1="1 3.077us -38.81mV" --p2="1.5 10.43us 29.3mV" --Istep="-2A" --Sim
We use the example response curve below.
From the curve, we pick two (2) of ringing peaks by specifying the peak numbers.
The main / first peak is #1 (number one). We call it overshoot / undershoot.
The peak following the #1 at the opposite direction is #1.5 (one and half).
The next peak following #1 and #1.5, in the same direction of #1 is #2 (number two).
...
And we have this important information: load step current amplitude
Then, we also need a data point just before the response starts as our reference point, call it #0.
Optionally, we can feed a converging data point call it #9 where "9" is just repsenting a big number.
When your VRM under test has a certain load regulation differnece before and after the response, this #9 helps.
When you have enough peaks, it is recommended to use #1 and #2.
prompt>> SEPIA_UI.exe ud --d0="99.9us 0V" --p1="1 103.1us -38.8mV" --p2="2 118.2us -22.2mV" --d9="350us 1.87uV" --Istep="-2" --Sim
-------------------------
Copyright \xa9 2014 Picotest
The "SEPIA" compiled binary, the EXE (Windows Executable) file, available from this repository is a copyright intellectual property of Picotest.com. All rights reserved.
Please contact info@picotest.com for more information.
-------------------------
##############################################
SEPIA for the "under-dumped" system below
Voltage waveform starting from 0 V at 99.9 us,
1.0-th Peak of -38.8 mV at 103.1 us,
2.0-th Peak of -22.2 mV at 118.2 us,
caused by a load-step -2 A
##############################################
SEPIA Results
Q = 5.6276
Zo = 22.306 mOhms
L = 53.186 nH
C = 106.9 uF
Rseries = 3.9636 mOhms
Rparallel = 125.53 mOhms
PhaseMargin = 10.154 deg
Loop BW = 66.225 kHz
##############################################
Actually, you can use other pair of peaks. Below example uses #1 and #1.5.
prompt>> >> SEPIA_UI.exe ud --d0="99.9us 0V" --p1="1 103.1us -38.8mV" --p2="1.5 110.4us 29.3mV" --d9="350us 1.87uV" --Istep="-2"
-------------------------
Copyright \xa9 2014 Picotest
The "SEPIA" compiled binary, the EXE (Windows Executable) file, available from this repository is a copyright intellectual property of Picotest.com. All rights reserved.
Please contact info@picotest.com for more information.
-------------------------
##############################################
SEPIA for the "under-dumped" system below
Voltage waveform starting from 0 V at 99.9 us,
1.0-th Peak of -38.8 mV at 103.1 us,
1.5-th Peak of 29.3 mV at 110.4 us,
caused by a load-step -2 A
##############################################
SEPIA Results
Q = 5.5921
Zo = 22.325 mOhms
L = 51.465 nH
C = 103.26 uF
Rseries = 3.9923 mOhms
Rparallel = 124.84 mOhms
PhaseMargin = 10.218 deg
Loop BW = 68.493 kHz
##############################################
The SEPIA is an analog analysis, we observe tiny amount of differneces depending on how you probe the datapoints.
In the same way, we can use #1 and #3.
Prompt>> SEPIA_UI.exe ud --d0="99.9us 0V" --p1="1 103.1us -38.8mV" --p2="3 132.7us -12.6mV" --d9="350us 1.87uV" --Istep="-2"
-------------------------
Copyright \xa9 2014 Picotest
The "SEPIA" compiled binary, the EXE (Windows Executable) file, available from this repository is a copyright intellectual property of Picotest.com. All rights reserved.
Please contact info@picotest.com for more information.
-------------------------
##############################################
SEPIA for the "under-dumped" system below
Voltage waveform starting from 0 V at 99.9 us,
1.0-th Peak of -38.8 mV at 103.1 us,
3.0-th Peak of -12.6 mV at 132.7 us,
caused by a load-step -2 A
##############################################
SEPIA Results
Q = 5.5872
Zo = 22.328 mOhms
L = 52.176 nH
C = 104.66 uF
Rseries = 3.9963 mOhms
Rparallel = 124.75 mOhms
PhaseMargin = 10.227 deg
Loop BW = 67.568 kHz
##############################################
We use the example response curve below.
From the curve, we pick three (3) datapoints one before and two after the overshoot/undershoot peak.
Regarding the 2 after peak probing, it's very preferrable to probe these 2 separated away.
Then, we also need a data point just before the response starts as our reference point, call it #0.
Note
For the over-dumped response, Avoid probing near the peak, as shown in the waveform.
prompt>> SEPIA_UI.exe od --d0="9.9us 0V" --d1="10.03us 20.4mV" --d2="10.38us 26.3mV" --d3="12.02us 4.22mV" --Istep="2"
-------------------------
Copyright © 2014 Picotest
The "SEPIA" compiled binary, the EXE (Windows Executable) file, available from this repository is a copyright intellectual property of Picotest.com. All rights reserved.
Please contact info@picotest.com for more information.
-------------------------
##############################################
SEPIA for the "over-dumped" system below
Voltage waveform starting from 0 V at 9.9 us,
Pre-Peak probing of 20.4 mV at 10.03 us,
1st Post-Peak probing of 26.3 mV at 10.38 us,
2nd Post-Peak probing of 4.22 mV at 12.02 us,
caused by a load-step 2 A
##############################################
SEPIA Results
Q = 426.87m
R = 22.465 mOhms
C = 9.5616 uF
L = 26.482 nH
PhaseMargin = 79.832 deg
Loop BW = 133.34 kHz
##############################################
*** Post Simulation
By adding "--Sim" parameter to the SEPIA_UI.exe program, it runs post simulation based on the SEPIA extracted result when you have the QSPICE installed.
Note
Due to the copyright, the source of SEPIA.exe is protected.
In this repositry, you can find the source Python code of the UI SEPIA_UI.exe program.
Inside the SEPIA_UI.exe, we call the core program SEPIA.exe by translating parameters from the command line parameters into a CSV format.
The SEPIA.exe takes the CSV format data, from its STDIN.
Note
You can use the SEPIA.exe program by feeding a CSV file
prompt>> SEPIA.exe < od.csv
or
prompt>> type ud.csv | SEPIA.exe
By implementing other interface library into the SEPIA_UI.exe, this SEPIA StandAlone program can do various automated / semi-automated loop analysis.
Examples:
- VISA libraries talking to measurement equipment (oscilloscopes, analyzers)
- Data plotting libraries to load datafile from bench equipment / simulators
The 
The PyQSPICE is a Python wrapper-module for the QSPICE, available from this Qorvo repository on GitHub. By using the QSPICE as a strong back-end simulator, it supports complex data processing and data plotting.
The SEPIA@QSPICE is a in-simulation probe implementation of SEPIA.
The QSPICE supports user-defined C++ and/or Verilog circuit blocks. The SEPIA processing routines are implemented as one of such user-defined circuit blocks, that is the DLL binary available.
Important
This SEPIA@QSPICE module is a SUB-SET of the SEPIA, where NOT ALL the SEPIA PROCESUREs implemented.