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BQ24123: Blind vias ok under center GND pad?

Part Number: BQ24123

Hello,

   I'm working on a design that has a BQ24123 on it, and on the opposite side is a large SMT connector that prevents me from putting nice GND vias under the center pad. So right now there are no vias under the pad, they're all about 100mil away from the package.  Board is 10 layer, with GND on layer 2 (beneath top) and also on layer 7 & 9. So I was thinking that one way to fix this would be to use blind vias from top layer down to the GND plane right below. Would this be helpful? In the existing design (with gnd vias 100mil away) the charging current has to be reduced in order to pass emissions. Would using blind vias beneath the center pad reduce emissions?

   Also, if we do use these blind vias to GND beneath the pad, should they be tented?

  • Hi Derek,

    I think blind vias are fine. The point of the vias underneath the IC pad is to use the ground planes to dissipate heat that is generated by the power losses in the converter. If the GND on layer 2, 7 and 9 create a shorter ground path for the high frequency switching signals, then it will improve EMI as well. Best way for layout is to make sure that the input caps and output caps are as close as possible to the IC pins and GND (look at our layout example on figure 25 page 30 of the datasheet). If you want to pass EMI with higher current, I suggest leaving space for an RC snubber on SW node. You can read more on that here: e2e.ti.com/.../calculate-an-r-c-snubber-in-seven-steps

    For the blind vias underneath the power GND pad, they should be filled but not tented. Tented is only needed when you don't have a pad right on top of the via.

    Regards,
    Steven
  • Hello, where do I connect a snubber on the BQ24123, since it has internal FETs? Also, do you have values that would be good to start with? I saw the seven step document above but wanted to see if you had recommended values.
  • Hi Derek,

    Connect the RC snubber on the OUT pin. I would start with about 300pF and 5 ohm, but you should follow the steps on the link I sent, it will give you a better starting point, because the RC snubber will depend on the PCB parasitics unique to your design.

    Regards,
    Steven