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WEBENCH® Tools/TPS54340: Autocompensate does not seem to be working

Pradeep Mohan
Intellectual 725 points
Part Number: TPS54340
Other Parts Discussed in Thread: TPS54361, TPS54360

Tool/software: WEBENCH® Design Tools

When running the Bode Plot Simulations using the default compensation components recommended by the WEBENCH design, the Low Frequency Gain is highlighted in Red and recommends using the AUto compensate. When using auto compensate, the software always returns the value of 1pF for the parallel compensation capacitor. By the manual calculations this value does not seem to be right. 

How do I proceed further?

Is there any other information I can provide that would help identify the issue?

Are the any other tools offered by TI to calculate values for the compensation circuit?

  • 0
    TI__Mastermind 25590 points

    Pradeep,

    Can you share your WEBENCH file so I can take a look? (On WEBENCH page, top right, click "Share Design - Share With Public - Get New Shareable Link")

    We have an excel calculator available on the product page. See the, "Tools and Software" tab, scroll down, and click "TPS54360 and TPS54361 Family Design Excel Tool"

    -Sam

  • 0 in reply to Samuel Jaffe
    Intellectual 725 points

    Hello Samuel,

    thank you for your response.

    https://webench.ti.com/appinfo/webench/scripts/SDP.cgi?ID=3A9D68FDEF5BE2F2

    I did find the Excel link already on the product page and was able to calculate the compensation component values. I used Manual Compensation using these values. The cross over frequency is not exactly the same as the targeted cross over frequency from the excel sheet, but now simulating the Bode plot shows the parameters are all within the accepted range. 

  • 0 in reply to Pradeep Mohan
    TI__Mastermind 25590 points
    Pradeep,

    I'm trying to recreate what you're seeing. So far:

    Your link has Ccomp2=15pF.

    I used your specs (VIN=9-42V, VOUT=3.8V, IOUT=3A) and made my own WEBENCH sim and I got the same thing you saw with the low-frequency gain (requesting re-comp) but my Ccomp2 was 6.8pF, not 1pF. After re-comp (clicking Auto-Comp, then Apply changes to Design) Ccomp2=1.24pF (and there was a change to other comp values as well). Is it possible that you did not click the "Auto Compensate" and "Apply changes to design" (I missed these buttons at first, so it's not uncommon). This would explain why the values did not change.

    As for the accuracy of the values, no calculator will ever be perfect. Some take more into consideration than others so it's not alarming to see small discrepancies between these tools. They should all give you similar and dependable results for your design.

    Let me know if you have further questions,
    -Sam
  • 0 in reply to Samuel Jaffe
    Intellectual 725 points
    Hello Samuel,

    thanks a ton for the prompt response. The 1pF value is exactly my problem. The manual calculations (using Datasheet) and excel sheet calculations result in at least a value of Ccomp2=14pF or around that range.

    Would you recommend that I go ahead with the values obtained from the spreadsheet tool, input the values in the WeBENCH designer and if the Bode plot does not throw errors then proceed with it?

    Thanks again!
  • 0 in reply to Pradeep Mohan
    TI__Mastermind 25590 points
    Pradeep,

    Yes, I would suggest exactly what you said.

    I'll dig into this some more and get back to you with what I find, but for now, at least, you can keep working.

    -Sam
  • 0
    TI__Expert 6860 points
    Pradeep ,

    Webench is trying to optimize crossover frequency for the design . In your design, , Ccomp2 can be eliminated completely which can give best crossover Freq + better phase margin combo .
    The High Frequency noise would in itself attenuated due to inductor pole .

    ---Ambreesh