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WEBENCH® Tools/TPS50601-SP: Webench Re-Comp question

Mark Guastaferro
Genius 11645 points

Part Number: TPS50601-SP

Tool/software: WEBENCH® Design Tools

Using Webench for a TPS50601 design.  My Webench generated design has one 47 uF output cap, ESR = 35 mOhm. 

I wanted to add more output caps in parallel, so I used the "create Custom Part" to force the quantity to change from 1 to 3. The schematic properly shows the output cap as 47 uF, qty = 3. But when I go to the Re-Comp feature, it shows Cout in the Power Stage Components as 47 uF with 11.67 mOhm ESR.  Thus the tool appears to be dividing the ESR by 3 as I expected,  but the total capacitance stayed the same. I expected is to be 141 uF.  

Is this a bug in the Re-Comp tool? If it is, is this also impacting the phase/gain margin calculations shown in the Ops Val results?  The margin results did change after adding the output caps, but I don't know if this is just because of the ESR change or if the correct total capacitance was included. If I manually enter 141 uF for the Cout value in the Re-comp tool, I get a different phase/gain margin result. 

  • 0
    TI__Mastermind 28870 points

    Mark,

    This mostly might have to do with the recent capacitor derating updates and we might need to rethink on how to incorporate the derated cap value for custom caps. When you create a custom cap, since we do not know what the derated cap value is, we assume the "Total Derated Cap" value in the custom cap form as the correct value. If it is a regular cap from a manufacturer, we evaluate the derated value and use that for the calculations. So you will also have to ensure you are updating the "Total Derated Cap" value as re-comp and opvals use this value for the loop calculations. Can you please try updating that value in the form and confirm you get expected results (opvals loop results and recomp loop results should match. Also you should see the "Total Derated Cap" value match in the Cout value under "Power Stage Components" tab in Re-comp)

     


    Regards,
    Amod

  • 0 in reply to Amod Vaze
    TI__Genius 11645 points
    Hi Amod,
    Yes this was the problem. I had to manually change the Total Derated Value.
    I assumed the custom component tool would take the derated value for one cap and multiply it by the quantity to get to the Total Derated Value. This is what it does for the ESR.
  • 0 in reply to Mark Guastaferro
    TI__Mastermind 28870 points

    Hi Mark,

    Sorry about the confusion. Thanks for confirming that your design changes are working and please continue to update the Total Derated Cap for custom caps for now.

    I can probably try to do the same (like ESR) but then the Total Derated Cap is also an entry field so will need to change this form a little bit and accept the "Derated Cap" value only and then update the total value automatically.


    Regards,
    Amod