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UCC28056: question about paralleled components in reference design

Thomas Stalcup
Prodigy 20 points
Part Number: UCC28056


I'm working on implementing a PFC pre-regulator based on the example circuit in the UCC28056 datasheet and the UCC28056BEVM-296 evaluation module.  I don't understand some of the design choices and I'm looking for some help.

In the UCC28056 datasheet, figure 36 shows an example schematic.  There are three resistors in parallel for current sensing - RCSa, RCSb, RCSc.  Their values are 0.13, 0.13, and 453k.  I don't understand why the 453k RCSc resistor is included.  It seems that in parallel with two 0.13 ohm resistors, it has negligible impact on the result.  

Thanks!

Thomas

  • +1
    TI__Genius 11350 points

    Hi Thomas,

    This is a good question. I believe it's in there as a placeholder in case you find you need to adjust RCS once the board is working. Unfortunately, this was not mentioned in the EVM User Guide or the datasheet. I believe the person who wrote the datasheet just copied it over (surprising since it's an oddly specific value).

    Anyway, I would at least put a DNP for RCSc (R11 in the EVM) in your design in case you find it necessary to fine-tune RCS. Or you can calculate your best estimate, similar to what was shown in the datasheet in eq (22) & (23):

    Thank you,

    Ray

  • 0 in reply to Ray DiCecco
    Prodigy 20 points

    Thanks, I assumed as much but just wanted to make sure I wasn't missing something.