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Different results for FRA using LTpowerCAD and LTspice, Specifically LT8643S and LT8390

Category: Software


I have a different results for LT8643S based step-down converter.

LTpowerCAD report closed loop bandwith 100kHz, LTspice 130kHz.
Which of them can be trusted?

See attachment, please.

Anyway... LTspice FRA settings is very poorly documented, I can not find any application notes or other sourses.

Only single basic and very short information available at

Added LT8390 to keywords. Expanded subject.
[edited by: MStokowski at 8:39 PM (GMT -4) on 12 Mar 2024]

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  • Hi  ,

    Thank you for providing these files, we will investigate the cause. We already have a few hints what might be missing. But to give you a conclusion, it is likely that the LTspice model is missing something and must be fixed so what I said earlier about who to trust still holds true which is LTpowerCAD.

    A fix on the LTspice model of LT8643S is coming out soon. We will let you know once it becomes available.

    We appreciate your help in ensuring the highest quality of our LTspice models.



  • Hi ,

    We are on some finishing work to update the LT8643S model.
    We are now able to match the LTpowerCAD gain/phase plot.

    We will also be updating the models for the LT8608S and LT8390.

    For your question:LTpowerCAD vs. LTspice. Which of them can be trusted?

    LTpowerCAD simulation results have been demonstrated to be matching actual hardware test results of various ADI parts.
    The new LTspice FRA tool is also a good tool, well-designed to make measurements following the industry-accepted control loop theory.
    It just so happened that the model you encountered, the LT8643S has a missing internal parameter that affected its results in the frequency domain analysis. Nonetheless, the LT8643S model is still representing the actual device in time-domain analysis.

  • Hi ,

    Please be advised that the updated model for LT8643S is now available in the library. Please sync your library by going to Tool>Update components

    This result matches the result from LTpowerCAD.

  • How about LT8390? As mentioned, it has the same issue. Any fix?

    Also, LTpowerCADII exhibits very strange behavior when switching between the "Power Stage Design" tab and the "Loop Comp. & Load Transient" tab. It appears to alter the results of the load transient plot. Has anyone else experienced this issue, or is it just me?

  • Hi  ,

    Apologies for the delayed response. We will look into the LT8390 discrepancy.

    About LTpowerCADII, I fear your question regarding LTpowerCADII may languish here at the end of this LTspice thread.

    To remedy, can you open a new Question…

    Category: "Software"

    Product: "LTpowerCADII"

    Subject: LTpowerCADII exhibits very strange behavior when switching between the "Power Stage Design" tab and the "Loop Comp. & Load Transient" tab

    Keywords: LTpowerCAD

    The LTpowerCAD team can address your question directly.