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question about the fra feature in LTspice

Category: Software
Product Number: LTC3311S
Software Version: 17.1.4

 I’m using the LTspice 17.1.4 and facing some problem in using the new fra feature.

  1. The setting of .fra gain vs. frequency function for SMPS:

(Results of an example LTC3311S .fra and LTpowerCAD)


After several times simulation, I summarized how to do the setting of .fra gain function. And the result is roughly same as the LTpowerCAD result. Is there any further improvement that I can do to increase the accuracy of the result?


2. Setting and location of .fra output impedance function for SMPS:

Is the setting method of gain plot still applicable for the impedance vs. frequency for SMPS?

If I want the output impedance of SMPS or the input impedance of SMPS where should I put this fra feature?

3. .fra compare with 1A AC current source method in SMPS:

I’m trying to compare the normal method output impedance result with .fra feature to check the result. I tried the 1A AC current source method (put an idea 1A AC current source at output, because of R=V/I, then Vout is the output impedance) in SMPS. But the result is not reasonable. How to test the output impedance without the .fra?


But when I try to use this to simulate the output impedance of op-amp. The result is looks like roughly same. But this time, the .fra feature seems not as good as 1A AC current source method (the magnitude in high frequency is larger than datasheet). Why is this happened?


(Output impedance of LT6016 from data sheet)


here are the schematics that I used for the simulation.

fra output impedance.ascoutput impedance of op-amp.ascSMPS output impedance with 1A.ascSMPS output impedance.asc


Thank you very much for your help!!!!!!