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BQ76925 absolute maximum ratings - exceeded by design?

Timothy Starr
Expert 2445 points

Hello TI Gas Gauge experts-

We are using the BQ26925 is a design and would like to take advantage of the open cell detect feature. On page 17 of the data sheet it says "In a normal system, closing a cell balancing FET will cause 2 cell voltages to appear across one cell input." This is up to 8.4V, well in excess of the absolute maximum rating of 6xn at VC1 (6V) given on sheet 4 for cell input range.

What's up with that?  It seems like we are exceeding the rating of the device, just like the data sheet tells us to!

Thanks in advance for clearing this up, and Best Regards,

-Tim Starr on behalf of BA@IW

  • 0
    TI__Guru 54715 points

    Sometimes it is hard to tell what an author intended.  Open cell detection seems to be more of a method than a feature. The statement that "In a normal system, closing a cell balancing FET will cause 2 cell voltages to appear across one cell input." seems to apply to a normal open cell system since the table 3 below the description shows "normal" as 1.5 X the cell voltage.

    Your point is good.  An open cell test is not something you would want to run at maximum voltage on all boards before attaching cells or in the product at at high voltage if the cell connections are repairable as you would overstress parts.  The author may have expected this was understood.  

    If the pack has maximum recommended 4.4V cell voltages and the test starts at the top of table 3:

    For VC0 open, while observing cell 2 voltage, turn on BAL_1.  VC0 would pull to 4.4V exceeding its 3V abs max.

    For VC1 open, while observing cell 3 voltage turn on BAL_2.  VC1 would pull to 8.8V as you indicate exceeding its 6V single ended abs max.  The VC0 to VC1 9V differential limit is not exceeded.

    For VC2 open, while observing cell 4 voltage turn on BAL_3.  VC2 would pull to 13.2V exceeding its 12V abs max limit.

    For VC3 open, while observing cell 5 voltage turn on BAL_4.  VC3 would  pull to 17.6V, less than the 18V abs max.

    So the limiting factor would seem to be 3V max per cell to be able to fail the test and not exceed abs max on the IC.