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BQ76930: BAT - or PACK - or other ways to connect GND

Eohan George22
Intellectual 620 points
Part Number: BQ76930
Other Parts Discussed in Thread: BQ20Z90,

This page has some reference on connecting GND to BMS: 

In this, page 6 shows that the BMS ground could be connected to PACK - ( or after the shunt resistor)

However, this forum(https://e2e.ti.com/support/power-management/f/196/t/585790) says its should be BAT - 

How do we think about this ? What is the right way to ground BQ chips in the BMS?

  • 0
    TI__Guru 54715 points

    Hi Eohan,

    The right way to connect GND depends on the part and the system. 

    With the BQ20z90 designs common at the time the application report was written, the system currents were moderate and cells optimized for energy storage had limited short circuit current. Connecting the electronics reference at the PACK- side was common and has a couple advantages:

    • Communications levels referenced to PACK- are not modulated by the system current, other than the long impedance shown in the figure.
    • BMU (in the figure above) current from the cells can be measured by the gauge.

    With small currents, Cell0 and the sense inputs don't go far below VSS or GND of the BMU.  Sense inputs can typically measure some below VSS, any loss of voltage on the bottom cell due to VC0 dropping is accepted.

    As power cells dropped the internal cell resistance and with higher load on batteries, system currents and short circuits increased the voltage excursion of these pins, high short circuit currents could drive sense inputs into the abs max limit.

    Many devices intended for high current system use such as BQ76930 have VSS or GND referenced at the battery side of the sense resistor.  The sense inputs are then positive with respect to VSS with load currents which are typically much larger than charge currents.  Drawbacks are now the benefits above:

    • Communications reference to PACK- are now modulated by the load current.
    • BMU current is not measured by the gauge device.

    Each particular BMU device has its unique characteristics, some may have limited range of some pins which may make GND placement more critical.  For the BQ76930 see figures 22 to 24 of the application note http://www.ti.com/lit/pdf/slua749 which can be found in the product folder http://www.ti.com/product/BQ76930/technicaldocuments .