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BQ76905: BQ76905 GND connection

Panagiotis Verras
Prodigy 115 points
Part Number: BQ76905

Tool/software:

Hello,

I'm in the design process of a self contained system that is using 2S cells and integrating BQ76905 for protection and management.

I'm having a little trouble with where to GND the application part of the circuit. 

I've noticed that the BQ76905 GND is placed behind the current measurement shunt. Hence any current flowing through the direct connection will not be accounted for in the current measurement.

Connecting my MCU and application circuit to the PACK- will solve the issue but the I2C signals will be referenced now through the shunt and that might create noise problems. 

How should this be handled? Can the BQ76905 be grounded after the current measuring shunt? Is the grounding of the I2C signals referenced in such way acceptable?

Thank you very much!

Panagiotis

  • 0
    TI__Mastermind 38350 points

    Hello Panagiotis,

    Panagiotis Verras said:
    I've noticed that the BQ76905 GND is placed behind the current measurement shunt. Hence any current flowing through the direct connection will not be accounted for in the current measurement.

    That would be correct.

    Panagiotis Verras said:
    Connecting my MCU and application circuit to the PACK- will solve the issue but the I2C signals will be referenced now through the shunt and that might create noise problems. 

    There will not be any noise issues, but there can be some modulation of the I2C signals. Typically this would not have much of an effect unless there is a very large voltage drop across the shunt resistor. But even with a 1-mOhm sense resistor, 100-A would correspond to just a signal shift of 100-mV, I believe most I2C line should be able to handle such case.

    Panagiotis Verras said:
    Can the BQ76905 be grounded after the current measuring shunt? Is the grounding of the I2C signals referenced in such way acceptable?

    The device was designed to have its Vss to the BAT- side of things, so I would not advice to shift that ground. I think it'd be okay to connect the MCU ground on other side of the shunt resistor.

    Best Regards,

    Luis Hernandez Salomon