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BQ34Z100 AGND & GND

Stuart Graham
Prodigy 165 points

Hello,

Can somebody advise if its essential for the AGND and GND to come from BAT-?   Can it come from PACK-?

This is because my microcontroller will be powered from BAT+/PACK+ and PACK- and if the I2C data ground comes from GND where as my microcontroller 0V comes from PACK-, there will be potential to upset the reading of SRP/SRN.

Thanks,

Stuart

  • 0
    TI__Genius 13700 points

    Hello Stuart,

    I will give you question a whirl but for future support of this device I would recommend posting in the Gauge Forum here:

    http://e2e.ti.com/support/power_management/battery_management/f/180.aspx

    I would strongly recommend that you follow the datasheet recommendations for the grounding connections of the gauge. The reasoning behind this is that the analog sensing that occurs via the SRN & SRP pins relative to the ground potential of the device is far more sensitive than the I2C communication. A properly sized sense resistor will result in an offset of ground potential between the gauge and an external microcontroller of 125mV or less and that much of an offset would only occur at a max charge or discharge rate with a sense resistor sized to be as large as possible.

    This much of a voltage offset should not cause any issues with your I2C communication.

  • 0 in reply to Ryan May
    Prodigy 165 points

    Hello,

    Apologies - i had clicked onto the wrong forum and not noticed but thanks for having a tackle of the query anyway.  I will repost in the other forum and link to your response for reference there.

    Im not worried that I2C comms will be upset by the ground offset caused by the sense resistor, i want the current consumption of the microprocessor to be accounted for by the fuel gauge which would mean having its 0V coming from PACK-.   

    This would mean the I2C ground for the CPU is PACK- but the I2C ground for the fuel gauge is AGND/GND.   My main concern was these grounds being potentially different by -0.125v to +0.125v would via the fuel guage IC internal circuit, result in the current sense readings being ofset by a leakage path.   

    I am assuming the I2C input impedance would be high enough no to cause an issue but i dont know.

    Will see what the correct forum thinks.

    Thanks,

    Stuart