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TPSM84424: Layout advice and use of AGND vs PGND

Irene Hu
Prodigy 30 points
Part Number: TPSM84424

Hi!  I am trying to design a layout for the TPSM84424.  I used the EVM to evaluate it and thought the performance was good for my application.

I noticed that some EVMs for other parts have gerbers posted online. I would like to use the TPSM84424-EVM design as a starting point, but some of the traces shown in the User Guide are a bit blurry. Is it possible to get gerbers for this module (or Eagle files if by chance that's what you used)? Alternatively, there is a sample layout given in the spec sheet for the TPSM84424; gerbers or CAD files for that would also be super helpful as an alternative, since the parts I am confused about are the same on both of these layouts.

I was also wondering if you have any advice for designing a layout for a 2-layer board.  The recommended layout seems to be a 4-layer board where one layer has an AGND plane and 3 layers have PGND. WIth a 2 layer board, I only have one layer that can have an uninterrupted plane.  I could try to split the bottom layer into two localized planes, or I could try to do one of the planes on top, working around the part and all the top traces, and it would just have to cut the other plane with vias and bottom traces when I need to route it.  I don't know how feasible either of these solutions are; it looks very difficult.  What would you recommend, or do you think this is a part that requires 4 layers?

Finally, I noticed that it is recommended to use AGND with analog circuitry since PGND is noisy. However, the EVM and spec sheet diagrams seem to imply that PGND should be the return path for VOUT. My circuit is basically analog (with a few ADCs thrown in) so should I use AGND or PGND with the rest of my circuit?

Thanks!

  • 0
    TI__Genius 17905 points

    Hi Irene,

    See attached gerbers of the EVM layout./cfs-file/__key/communityserver-discussions-components-files/196/TPSM84424EVMGerber.zip

    I would recommend using 4 layers like the EVM and datasheet examples. As you have found, it will be challenging to route everything on just top and bottom layers without cutting up VIN, VOUT, PGND, and AGND pours that ideally should be as low impedance as possible.

    Ultimately your load circuits should tie to PGND, since PGND is the return current path for the power stage.

  • 0 in reply to Kris F
    Prodigy 30 points

    Hi Kris,

    Thanks so much for your quick response!  It is very helpful.

    Just to confirm, then, is the AGND is basically only relevant to this part and there to make the part work properly?  So do I really only need an AGND pour locally around this part, and I can use the rest of that layer for something else in the rest of my board?  Or can you describe in what ways I might want to connect AGND to other parts of my circuit?  My end design will have several power modules (a few TPSM84424 and then some other DC/DC converters for other voltages) and they are driving different loads; their loads may be physically mixed with each other, except I'll keep the analog loads separate so they don't have digital return currents in their ground plane.  In a design like this, would it make sense to just have local AGND pours for each module in e.g. layer 3, and then PGND / global GND planes in the rest of layer 3 that's not occupied by AGND pours?  (In addition to one or two more full ground planes in other layers for PGND / global GND).

    Thanks!

  • 0 in reply to Irene Hu
    TI__Genius 17905 points

    Hi Irene, yes, you can keep a local AGND pour for each module then fill the rest of that layer with PGND.