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TPSM8287A12: output cap selection 22uF 0603 VS 0805

David Reed1
Intellectual 365 points
Part Number: TPSM8287A12

Tool/software:

Hi when simulating the TPSM8287A12BASRDVR in webench it always selects 0805 22uF 2X for output caps, when I try to change it to the same as the input 22uF 0603 GRM188R60J226MEA0D it is not available in the database. Is there a specific reason this component is not an option for this design? the ESR is higher for the 0805 parts VS the 0603 parts.

webench.ti.com/.../SDP.cgi

Thanks,

Dave

  • 0
    TI__Guru**** 150690 points

    Hi Dave,

    Can you send a screenshot of your design?  The link didn't come through your post.

    When I create the default TPSM8287A12BAS design, for example, I get this.  The input caps are GRM21BR61A226ME44L.

    Webench uses an algorithm, based on its database of components, to pick each component based on cost, availability, size, and power loss.  You are still able to pick a different passive component.

    One guess at why that 0603 doesn't show up for a possible Cin (it's not clear if you're talking about Cin or Cout) is that it is only rated to 6.3Vin.  The default Webench design requires the Cin to have 10% margin to the IC's 6V rated Vin: 6.6V.  You can change these design criteria by clicking 'edit' in the BOM tab.

    Murata also has the GRM188C81A226ME01D, which is a 10V cap, but it doesn't look like that one is in the Webench database.

    Chris

  • 0 in reply to Chris Glaser
    Intellectual 365 points

    Hi Chris,

    Apologies on the link, trying again with a public link

    Sorry if not clear in my note, yes, the output caps, and the tool selects the 22uF 0603 for other designs so I believe they are already in the database. The only reasons I could think of were higher cost, but it is negligible, or potentially for loop stability the higher ESR is desired.

     

  • 0 in reply to David Reed1
    TI__Guru**** 150690 points

    Thanks for clarifying, Dave.  I still didn't see a link come through.  What Vin, Vout, and Iout is your design for?

    Did it give the option for 3 of those output caps?  Webench calculates the effective capacitance, based on the voltage applied (the output voltage), so 2 may not be enough.

    There is no requirement for a minimum ESR or something.

    Chris

  • 0 in reply to Chris Glaser
    Intellectual 365 points

    Chris,

    not sure why my links are deleted / truncated

    Customize TPSM8287A12BASRDVR ‑ 3.2V-3.4V to 1.05V @ 8Aedit
    Input: 
    DC 3.2 V - 3.4 V
    Output: 
    1.05 V at 8 A
    Temp: 
    30 °C
    There are two of the 22uF and three of the 47uF on the output.  The tool selects for the input  the 0603 22uF X5R 6.3V but picks the larger 0805 6.3V X5R for the output. That is what I am trying to understand/rationalize. We prefer min board area and that made no sense to me especially given that it has already selected the smaller same voltage same x5R for the input.
    Dave
  • 0 in reply to David Reed1
    TI__Guru**** 150690 points

    Hi Dave,

    I think I understand now.  The default design at those conditions gives:

    Cin: 2x 10uF, 0603

    Cout: 2x 22uF, 0805

    Coutx: 3x 47uF, 0805

    Each of these component selections requires different amounts of effective capacitance.  Cin requires 5uF per cap, while Cout requires 20uF per cap.  Coutx requires 3 times as much capacitance as is used for Cout.  So, that is where the values come from.

    Webench assumes that the load (and its Coutx) is placed 'not near' Cout.  Therefore, it requires (with some margin) meeting this requirement that the Coutx be twice as large as Cout.

    If you design your PCB, such that the load's input caps are right next to the output caps of the power module, then you don't need to meet the 2x requirement. 

    To get smaller Couts or Coutxs to show up, you need to edit the required minimum Cout in the BOM and then pick new caps:

    Let me know if I need to explain more.

    Thanks,

    Chris