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UCC28780: MathCad compensation calculations no help

Gerrit Barrere
Expert 1245 points
Part Number: UCC28780

I'm using the MathCad worksheets in sluc644 to do a power supply design.  Toward the end of the "Neutron..." worksheet there are a series of calculations for the compensation network components.  The design approach in the worksheet is almost completely different than in the data sheet and there are no comments or explanations in the worksheet.

The worksheet also has a fixed value of 68 kOhms baked into two of its formulas, which affect the downstream results.  Without knowing what that value represents, and with no explanatory text for the rest of it, it is difficult to trust the worksheet results.

If the worksheet either used the data sheet equations or explained its approach (and its fixed resistance value) it might be usable.  Unfortunately, it looks like I will need to study section 8.2.2.7 in the data sheet instead, and enter the equations myself.  

  • 0
    TI__Genius 9710 points

    Dear Gerrit,

    So far as I know the Rbias1 68k ohm is not a fixed value.

    As you mentioned, the equation on the datasheet and Mathcad is hard to deal with it.

    Unfortunately, the UCC28780's definer is out of office. I will double check with him and get back to you soon.

    Best Regards

    Kevin

  • 0 in reply to Kuan Wei Chen (Kevin Chen)
    Expert 1245 points

    The 68kOhm fixed value appears in the formulas for R.bias1_min and f.cr(V.in,CTR).

  • 0 in reply to Gerrit Barrere
    TI__Genius 9710 points
    Dear Gerrit,

    I double check with the UCC28780's definer, the 68k Ohm is the IC's inside parameter, so the worksheet use 68k Ohm as fix value.
    For the datasheet's equation is base on ABM mode, when work sheet's version is base on SBP mode. You can check both of the equation to see which result get smaller Rbias1. Use smaller Rbias1 to make sure have enough bias current for normal regulation.

    Best Regards
    Kevin
  • 0 in reply to Kuan Wei Chen (Kevin Chen)
    Expert 1245 points

    Thank you for the reply, Kevin.  I don't see 68k anywhere in the data sheet, or in any diagrams which have anything to do with the compensation network.

    The point is that the worksheet is not reliable, and I am trying to point out some of its problems.  It has no explanatory text and uses the mysterious 68k value with no explanation.  I still have no idea what the 68k represents.

    "Checking both equations for smaller Rbias1" is a really cheesy undocumented hack.  How is the user supposed know that they should do this?  It also means they should enter the data sheet equations in addition to the worksheet.  Why doesn't the worksheet give you the actual Rbias1 which you need?

  • 0 in reply to Gerrit Barrere
    TI__Genius 9710 points
    Dear Gerrit,

    Sorry for the inconvenient.
    The 68k Ohm is the FB pins pull up resistor.
    This resistor is IC design's circuit, so you aren't able to find it in the datasheet.

    I think is because the time to market, so some documents are not perfect for end users.
    I will feed back this kind of issues to team member, to mitigate this kind of issue in the future.

    Best Regards
    Kevin