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BQ76920: What should I do with an unused cell pin?

Paul K52
Intellectual 640 points
Part Number: BQ76920
Other Parts Discussed in Thread: BQ4050

Hello,

recently there was a post about the minimum amount of cells that should be connected to this chip. I read that it is recommended, to use more than 2 cells. The BQ76920 is designed to use 5 cells.

In my case Im having a battery, which has 4 cells in series, so I think this should be also okay, I guess? 

My question is, what should I do with the input pin which is not connected with a cell? Should I connect it to ground, leave it unconnected or should I do something else with this pin?

Thanks for helping.

  • 0
    TI__Guru 54715 points

    Hi Paul,

    The bq76920 will work with 4 cells.  See the data sheet table 8-1.  The input for the unused cell is shorted to the next pin.  Application note bq769x0 BMS Configurations for Cordless Appliances has figures which show reduced cell count for other family parts although not for 4 cells, see figure 3 for example.

    If you will be using only 4 cells rather than a range of applications with a common architecture, consider the bq4050, it may provide a more compact 4 cell solution.

  • 0 in reply to WM5295
    Intellectual 640 points
    Hello,
    refering to the data sheet table 8-1, this means: If I want to use 4 cells, VC3-VC4 is interal shorted together. How does the chip know, that there are only 4 cells?

    So my balancing cable, which provides 5 wires (+,Cell2,Cell3,Cell4,Ground), should be connected to the VC5,VC4,VC2,VC1,VC0 of the chip.

    Because VC4 and VC3 are shorted together, I should measure the voltage between VC4 and VC2 to get a cell voltage at this pins.

    Is this correct?
  • 0 in reply to Paul K52
    TI__Guru 54715 points
    Hi Paul,
    Please see the data sheet for the bq76920. Note the parameter UVMINQUAL, and any description. Below UVMINQUAL (as when shorted) the input will not be used for UV. Your host which implements balancing control will need to know that the shorted cell is not supposed to be there.
    Yes, your connections at the chip seem correct.