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BQ76940EVM: External FETs balancing (operation when shortening a cell)

Nikola Felkaroski
Prodigy 40 points
Part Number: BQ76940EVM
Other Parts Discussed in Thread: BQ76940

Hello,

I am using the BQ76940 as a voltage/temp monitor for a 9-12 cell configuration with a different host MCU that will also drive external FETs for balancing. I understand that we need to bypass the existing balancing FETs on the EVM. My questions are the following:

1) What happens if the external balancing FET shorts a cell from the series (while keeping block voltages above Vshut), and in the worst case scenario, what would happen if one block falls below Vshut during balancing, but say i use R74 instead of R75 so that REGSRC gets its voltage from VCx10 instead of VCx5 (since the cells are just 2.3V)?

Thank you in advance.

  • 0
    TI__Guru 54715 points

    Hi Nikola,

    If you are connected to cells balancing a cell will not short the cell input since the cell will have low impedance and provide significant current.  With a resistor divider balancing will short or at least alter the cell voltage.  

    1. If you pull a group into VSHUT it will likely reset that group.  If you go in slowly where there is still some supply voltage it will reset.  If this happens on the bottom group you will need to re-boot the part.  If it happens on an upper group the part will give an XREADY error which will need cleared when voltage is restored. 

    Powering REGSRC referenced from VC10X with a low voltage battery may be a good choice to maintain good REGSRC voltage for FET drive outputs.  It won't help with VSHUT since that is on the cell group voltages.