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BQ78350: Struggling with common grounds

Part Number: BQ78350
Other Parts Discussed in Thread: BQ76920, , BQ76200

Hi TI Experts,

I am posting this because I need help on a system, I am designing a power unit using Li-Ion batteries, this will be a supply for other products,


I saw on the BQ78350/BQ76920 datasheets that these components are used in kind of stand-alone battery packs, like RC's cars battery packs, what I want to say is you have the same terminals to charge or discharge the batteries, am I right ?

But, in my case I have the CC-CV charger and two buck-boosts included on the same PCB. As you may know the BQ76920 commands N-channels FET to charge or discharge the batteries, so I use 3 differents grounds :
- one for the input buck-boost and the CC-CV charger. (GND0)
- one for the BMS part. (GN1)
- one for the output buck-boost. (GND2)

But it creates some issues, the first one is that I use a "boot alternative" in order to wake up the BQ76920/BQ78350 without push-button, so I have to set the same reference through GND0 and GND1, I freshly noticed that because I was using a laboratory supply and my computer connected on the SMB bus and there are both connected to the grid and without that the BQ76920 don't wake up when I connect an Input from a floating voltage. The second one is when I want to connect the BMS part and the second buck-boost we can hear a little spark noise and it creates a OCD error for the BQ78350 but if I connect my oscilloscope to the output of the converter it works well.

So I have to solve these issues.

Two options for me :
- First, I stay in this multi ground configuration but I find a way to have the same 0V reference without current flux, and I don't know how to do that.
- Second, I redesign my card in order to have the same GND everywhere and I control the charge and the discharge with the Positive terminal of the battery. I saw we can add a BQ76200 for that, can you explain to me why do I need it in this config ? Please.

So what's for you the best choice ? I think it would be better if all circuits had the same GND.

Also I tried to split charge and discharge circuit in order to be able to do both at the same time, but I'm not sure if I did it well and if I shouldn't just let the two FETs one behind the other and connect In and Out at the same place.

Thanks for your help

Lucas

  • Hello Lucas,

    Pack+/Pack- pair is used as the only connection to charge and discharge batteries. Just follow the EVM schematic and then you can then use the Pack- connection as ground for the charger.

    Hopefully that simplifies things for your design.