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LM5122: How to actually calculate a correct compensation RC value with the quick calc?

Weiwei Su
Prodigy 230 points
Part Number: LM5122

Tool/software:

Hi Team,

I'm using the provided quick calculator to calculate the appropriate comp RC value, something I don't really understand is, if I set the output capacitance and the selected crossover frequency first, the calculated RC value does not reflect the correct values of the crossover frequency displayed in the Bode Plot. It will always lower than the selected crossover frequency. If I try to adjust the output capacitance to match the selected crossover frequency, the calculated RC values will be changed again, and If applied, the calculated crossover frequency will then again way different than the selected.

So how exactly should I figure out the correct RC value? Should I set the output cap value first then adjust my way to match the frequency?

Also, while input voltage could also affects crossover frequency, and my application here would have 2 typical input supply voltage (12-24V), and of course I could not change RC in the fly, so does it affects much on the converter performance?

  • +1
    TI__Mastermind 29520 points

    Hello Weiwei,

    Thanks for using the e2e forum and pointing this out.
    The same question was raised just a few days ago by another customer.

    It seems the compensation calculation has a mismatch within its calculation and therefore does not return the correct bode plot graph.
    We are currently working on a new calculator version with a fixed compensation.
    I expect the new version to be online within the next two weeks.

    In the meantime, I would kindly ask you to use our power stage designer tool for compensation calculation:
    https://www.ti.com/tool/de-de/POWERSTAGE-DESIGNER

    The tool comes with a compensation calculation as well and all missing input values can be found within the electrical characteristic table of the LM5122 datasheet.
    Please let me know if you need additional help using this alternative tool.

    I am very sorry for the inconvenience.

    Best regards,
    Niklas