Different results for FRA using LTpowerCAD and LTspice

Question

1805.FRA.zip 
 
 Hello 
 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 
 ez.analog.com/.../generate-a-bode-plot-for-an-smps-in-ltspice-17-1

John Kevin · Accepted Answer

Hi Levandovsky , 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.

PaulDaria · Answer

Hi Levandovsky , 
 
 You can find more information about the FRA feature using the help file. Just type in FRA and all related topic will show up. 
 FRA is a new feature of LTspice and thus app notes about this feature are still limited. 
 
 with regards to the difference between LTpowerCAD and LTspice result. It depends, LTpowerCAD is limited to CCM mode of operation. also, it can be likely that there is a bug on the LTspice model. 
 What I can do on the other hand is to consult the part owner and let them answer the question for you. 
 
 Thank you. 
 
 Regards, 
 PaulDaria

PaulDaria · Answer

Hi Levandovsky , 
 
 Someone is already working with LT8643S Bode plot mismatch issue of LTspice and LTpowerCAD. With regard to which to trust better, I would suggest LTpowerCAD,. Simply because it's came in first and have already higher reliability over the LTspice FRA tool. We are still figuring out what might have caused the plot mismatch. 
 
 Thanks. 
 
 Regards, 
 PaulDaria

PaulDaria · Answer

Hi Levandovsky , 
 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. 
 
 Regards, 
 PaulDaria

John Kevin · Answer

Hi Levandovsky , 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.

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Different results for FRA using LTpowerCAD and LTspice

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