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UCC28740: VS, CS pin apply question

Jefferson Chen
Prodigy 10 points
Part Number: UCC28740

Please tell me how to fine tune the R4 that series connection to CS pin.

And I add C20 in parallel to VS pin for eliminate the noise from aux winding. Can you suggest the limit value for C20. 

  • 0
    TI__Guru 73795 points

    R4 is between CS pin and MOSFET source pin, right? If so, that is RLC and equation 3 in the ds shows how to select its value initially. Fine tuning is to change the value to make high line constant current close to low line. So need to try increase its value first and test until ok. When increase RLC, Rcs may need to adjust so a correct Iocc can still be achieved.

    VS pin is recommended no external capacitors. But if really needed, up to 10 pF may be ok. We suggest to optimize the layout and minimize the VS pin external capacitance.