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PMP21611: How to connect chassis ground

Nikos Agianniotis
Prodigy 40 points
Part Number: PMP21611

Tool/software:

Hi,

I wanted to get some advice regarding connecting the chassis ground of a metal enclosure that contains the PCB of PMP21611. Normally, taking into account ESD and EMI considerations, I would make a low inductance connection of the chassis ground to the signal ground. However, in the case of the PMP21611, there is the common-mode choke that (at least for me) makes things a bit confusing, because of the VBATT- at the input and the GND at the output of it and because chassis ground is essentially the same as VBATT- (at least in our case).

The extra complication that I have in my case is that I'm using this circuit to supply a wireless transceiver with an antenna that is connected to the board via a UFL to SMA cable attached on the enclosure. Therefore, the ground on the SMA connector (which is the signal ground) is connected via the cable to the enclosure (chassis ground). But the enclosure is at the same potential as the "-" of the power in. Essentially, what I'm doing is that I connect VBAT- with GND. I suppose there no meaning in doing that, since then the choke's purpose is diminished or nullified. I've actually also observed failures (perhaps due to noise) in some cases when the connection I described above is there.

I guess what I'm trying to understand is this. If I want to keep the choke (which I think it's a good idea to do), is there a way to still keep the metal enclosure or should I better switch to one with non-conductive material (or maybe isolate the SMA connector from the enclosure)?

  • 0
    TI__Mastermind 40455 points

    Hi Nikos,

    Usually the common mode choke is the last line of defense for EMI. So looking at the schematic of PMP21611, we would place connections to the chassis (through a cap) on the right side of the common mode choke (the IC side). 

    Note: in one of our more recent designs we're working on, we put options for both cap chassis connections on the left (unpopulated for now) and right side (populated) of the common mode choke and so far the EMI looks okay. 

    Now ESD mitigation (vs EMI mitigation) is the similar concept (creating a low impedance path to steer energy to prevent damage to ICs). So if common mode ESD is sent on the vbat side, then a cap to chassis on the left side of the choke would help steer it away from the IC/load. This is where you'll need to find the right balance through testing.