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LM5155: Questions for LM5155

Part Number: LM5155
Other Parts Discussed in Thread: TL431

Hi Experts,

In LM5155 application note SNVA866A, what's the purpose of Rcomp and Ccomp which are connected Vcomp?

If it's for compensation purpose, how it contribute to Zero and Pole?


  • Hello,

    Thank you for using the E2E forum.

    The main compensation of an isolated flyback is done on the secondary side at the TL431 voltage reference. However, in some cases, RCOMP and CCOMP on the primary side are also needed. RLED is responsible for the mid band gain (AVM). If the needed value for RLED is higher than the maximum possible RLED (depending on the optocoupler, Vout and the voltage reference), RCOMP (on primary side) has to be used. CCOMP on the primary side influences the high frequency pole, as it is in parallel to Copto.

    Please note, that the values for Rcomp and Ccomp on the primary side dont have to be the same like Rcomp and Ccomp on the secondary side. They are independed from each other.

    For detailed explanation and formulas i recommend the following video:

    Also the following document might be useful for understanding the flyback powerstage and the isolated feedback via optocoupler: 

    If you have further questions, please feel free to ask.

    Best regards,


  • Thanks Moritz. Is there a excel or mathcad based tool available for the equations shown in the video?

    If yes, can you please share?


  • Hi Neo,

    Unfortunately i dont have a tool like this.

    Best regards,