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BQ25120A: Connection required for PGND

Paul Mellor
Prodigy 160 points
Part Number: BQ25120A

In the datasheet pin functions section (page 4) it tells us to connect PGND to the ground plane.

Yet in the layout example (page 63) it shows PGND connected only to SYS via the bypass capacitor, with no ground plane connection.

Is it necessary to connect PGND to the ground plane, or can it be connected simply as shown in the layout example?

  • 0
    TI__Genius 12485 points
    Paul,

    Depending on your system design, I would recommend the DC/DCs connected to the same GND plane and leave your analog plane isolated.

    However, you will have to connect them together later.

    regards,
    gautham
  • 0 in reply to Gautham
    Prodigy 160 points

    Gautham,

    Many thanks for replying. I'm truly sorry, but I don't follow what you mean... for example: "However, you will have to connect them together later. " 

    We have used the reference design largely as-is. We do not have any load on PMID. We have our MCU on SYS, and a couple of components (Bluetooth module, flash storage) running from LS/LDO. IPRETERM, ISET, and ILIM are all connected to ground as we use the I2C to programme these. Vin in from USB. There's a single cell LIPO battery on Vbat.

    So in development we connect A1 (GND), D5 (GND), and A5 (PGND) to the same ground plane. However your reference layout - which we would like to follow - shows A5 (PGND) isolated from any ground plane and connected via SYSCap to SYS (and VINLS).

    So we need to know: do we follow the reference design as-is and leave PGND isolated OR do we connect it to the ground plane. As you can imagine, it's very important we get this right as it is for our production boards.

    Many thanks,

    Paul.

  • 0 in reply to Paul Mellor
    TI__Genius 12485 points

    Paul,

    The two GNDs will still need to be connected atleast at one point to keep them at a same potential. 

    So if you are using just one GND through out your entire design, you can connect both to the same net. if you are having sensitive blocks in your design you are connecting to AGND and separating the planes, then you can look into splitting the GNDs. 

    You cannot fully isolate them. Hope this clarifies. 

    regards,

    Guahtma

  • 0 in reply to Gautham
    Prodigy 160 points

    Gautham,

    Yes I think that solves the problem. So I think you are saying YES, I need to connect PGND and GND to the same ground plane.

    Perhaps the reference layout diagram could be updated, as it shows PGND left fully isolated, and connected ONLY to SYS (via SYSCap), which I think you are saying is wrong.

    Many thanks,

    Paul.