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bq77908A drawing current on VC8

Marek Roszko
Intellectual 380 points
Other Parts Discussed in Thread: BQ77908A

I'm having an issue where the bq77908A is drawing current on the VC8 pin. It is in a 8 cell configuration. We connect the circuit by connecting VSS and BAT/PACK+ first and then the cells from bottom to top.

Once attached,  we see the VC8 pin drawing current and it usually comes out on VC9 (measured on the sense resistor), other times it is a significant current to see on the VC8 series resistor but nothing appears on VC9 and all other signals are fine.

Is there anything that may cause the bq77908A to behave this way? I have disabled the flag for cell balancing on this still occurs. This also occurs both on simulation resistors(where the current draw causes under/over voltage faults) and live cells.

  • 0
    TI__Guru 54715 points

    It sounds like you somehow apply an unexpected high voltage to VC8 during connection resulting in a shorted balance transistor to VC9 or a short to VSS.  You may have to probe and look at the voltage during connection to determine the cause.  Of course the probe can alter the voltage on the pins.  If it does not show up at connection, you may need to look at the end of board test to see if somethig is being applied after the board has passed. The abs max differential voltage to VC9 is 9V while the single ended voltage to VSS is 5V, so it would seem like a substantial voltage..

    Normally when the cells are connected BAT-/VSS and BAT+/PACK+ the input capacitors will divide the voltage evenly across the inputs, then they will drift due to leakage and bias currents to a point the pin bias current begins to change. The voltage will remain stable unless some external signal acts on this.  A residual voltage on VC8 when the top cell is connected will still receive a step when a higher voltage cell is connected..  When cells are connected bottom up, there will be some pulse on the lower cells as an upper cell connects, but it will drop off quickly with the lower cell input connected. If the capacitor from VC8 to VSS is smaller than the other inputs, the voltage can be larger and could be a problem.  If the cell inputs make then break bottom up in quick successon, the voltage on the bottom cap can be pumped up by the sequence.

    If you connect cells while in a fixture, you may be able to limit voltage swings by limiting zeners in the fixture.  If you choose to add circuit protection for the input to the battery board, since there is no margin between the VC8 abs max and the induced voltage swing during balancing, you will need to compromise on a zener value or place the zener or TVS device at or toward the cell side of the input resistance