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Original question:

LM5158: Webench Vs Quickstart Calculator

LM5158: Discrepency bewteen Webbench and calculator

Clément Letonnelier
Expert 2796 points
Part Number: LM5158
Other Parts Discussed in Thread: LM5157

Tool/software:

Hello

I have the same issue, the compensation values that are output by Webbench and the calculated suggestion in the sheet differs.

What am I missing ?

webench.ti.com/.../SDP.cgi

LM5157-58_Excel_Quickstart_Calculator_for_Boost_Converter_Design_V1_0_0.xlsx

Best regards,

Clément

  • 0
    Expert 2796 points

    I believe it's due to the output capacitor, I had put the big capacitor value we have further on the output of the component in the excel file.

    But I believe I should not take into account for the close loop compensation computations.

  • 0 in reply to Clément Letonnelier
    TI__Expert 4090 points

    Hi Clement,

    Thanks for using the E2E forum.

    Kindly, use the same power stage component values in both tools.

    The output cap and its ESR have a prominent effect on the close loop compensation calculations.

    Best Regards,

    Hassan 

  • 0 in reply to Hassan Jamal
    Expert 2796 points

    Hello,

    Indeed, that's what I believed.

    But you agree that you are talking about the capacitors which are directly on the buck output ?

    I am asking the question as we intend we have a big capacitor of 680uF further on the line to buffer power input loss.

    Best regards,

    Clément

  • 0 in reply to Clément Letonnelier
    TI__Expert 4090 points

    Hi Clement,

    I am talking about the output cap of the boost converter. The output cap is the part of control loop and changing its value would impact the close loop stability.

    If you have a big cap at the output of boost in excel calculator, you need to use same value in web bench because same formulation is used in both tools. 

    Best Regards,

    Hassan 

  • 0 in reply to Hassan Jamal
    Expert 2796 points

    Hassan,

    Yes of course I need to put the same values.

    But then my question is, is the stability endangered by the effecive capacitive load on the supply or just the output capacitors close to the part ?

    Best regards,

    Clément

  • +1 in reply to Clément Letonnelier
    TI__Expert 4090 points

    Hi Clement,

    Adding excessive output capacitance can shift the dominant pole and reduce phase margin, potentially making the system unstable if the compensation is not properly adjusted.

    Moreover, If the output capacitor is too large, the system’s ability to respond to fast load transients might be reduced. The controller may not react quickly enough, causing momentary voltage deviations.

    kindly, design your compensation network while considering the output cap. 

    Best Regards,

    Hassan