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LM5155: RCOMP and CCOMP in SNVA866 application note

Oba
Genius 17555 points
Part Number: LM5155
Other Parts Discussed in Thread: TL431

Tool/software:

Hello,

My customer has a question for RCOMP and COMP in SNVA866 application note.

The below is from the application note. In this figure, there is a compensation network and CCOMP and RCOMP are put on both COMP pin side and TL431 side.
They are wondering if "CCOMPs and RCOMP" on the both side  should be the same values. Should they be the same value?
If so, they would like to understand why the same values for both side is better.

Regards,
Oba

  • +1
    TI__Mastermind 31880 points

    Hi Oba,

    Thanks for using the e2e forum.
    The two compensations fulfill different function. The RC and the output side is the compensation for the regulation loop. This RC should always be place and has a strong impact on the stability behavior and bode plots. The compensation values calculated in the application note and quickstart calculator are for these components.

    The other RC directly at the COMP pin of the device is an additional compensation specifically for the optocoupler. This RC is optional, but can be placed if stability improvements are necessary.

    It is possible to place the same values for both RCs, as the app notes suggests, but the two RCs are not dependent on each other, so different values can be used depending on application requirements.

    Please let me know if further explanations are requested.

    Best regards,
    Niklas

  • 0 in reply to Niklas Schwarz
    TI__Genius 17555 points

    Hello Niklas,

    The customer has an additional question.
    Do you have any ducument like an application note which desribes how to select RC values on COMP pin side?

    Regards,
    Oba

  • 0 in reply to Oba
    TI__Mastermind 31880 points

    Hi Oba,

    There is actually an app note in the working right now on this topic.
    I will ask for an update on the current status of the app note and will get back to you beginning of next week.

    Best regards,
    Niklas

  • 0 in reply to Niklas Schwarz
    TI__Mastermind 19205 points

    Hi Oba,

    Until now there is no document or calculator on how to select these values.

    However, we are currently working on an appnote for this topic.

    RCOMP and CCOMP (at the COMP pin on the primary side) are used in order to reduce the centerband gain of the inner loop, so that the total phaseshift of the inner loop stays roughly constant.

    As Niklas said, these parts are optional and are not always necesseary. I recommend to have placeholders for these parts in the design.

    Best regards

    Moritz