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BQ76942: I2C Grounding

Oscar Walsh
Prodigy 150 points
Part Number: BQ76942

Hi, I'm working on a design incorporating the BQ76942 and have a question about how things are grounded. 

Using the BQ76942EVM schematic as a reference, it seems the BQ76942 should be grounded to BAT- (BQ76942's VSS connects to BAT-), while the MCU that it communicates with is grounded to PGND (MCU's QGND connects to PGND), i.e. the 'real' ground that the outside world sees.

This means the grounds on each side of the I2C bus are separated by the current sense resistor (R24 on the EVM schematic). As per the datasheet, this resistor is to be sized such that the voltage across it doesn't exceed +/- 200mV (in my case, this means a 25mOhm resistor). This means that, worst case, the BQ76942 ground and the MCU ground could be ~200mV apart, right? 

1. Is this ground separation OK for I2C comms, even with a higher separating resistance? In my use case, the sense resistor should be 25mOhm (as opposed to 1mOhm on the EVM) for best measurement resolution.

2. If so, why? If not, how should things be grounded to avoid the problem?

I don't have much experience with designs with multiple grounds, and just want to make sure the I2C communication is as robust as possible. 

Thank you!

  • 0
    TI__Mastermind 27321 points

    Hello Oscar,

    Please let me explain how we recommend to ground the BMS.

    1- The negative terminal of the battery pack (BAT-) should connect to the SRP side of the sense resistor (R24). 

    2- The IC ground (VSS) gets fed by BAT- through a Net-Tie.

    3- The negative terminal of the charger/load side of the BMS (PGND) should be connected to the SRN side of the sense resistor (R24).

    Notice how the sense resistor ground path is isolated from the ground of the IC (VSS). This is to avoid noise and get the best accuracy as the BMS highly relies on the sense resistor for current measurements and protections.

    As for the I2C comms concerns, communication on the I2C lines should not be an issue since the BQ76942 uses a high side driver for the FETs and there's no much noise in the ground of the IC (VSS). Just make sure the SDA/SCL lines are connected to a recommended 10k pull-up resistor value.

    Regards,
    Jose Couso