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BQ77908 pull up for Cell-to-cell differential pin

cjayet
Prodigy 40 points
Other Parts Discussed in Thread: BQ77908A, BQ34Z100, BQ77PL900

Dear TI support,

I will do a multi cell balancing configuration (4s or 6S).
My charger will detect which battry is connected and set the configuration with MOSFETs that are controlled by a µcontroller.
But I have a problem because in the datasheet, it say that Cell-to-cell differential unused (here VC4 to VC1 for 4S or VC2 to VC1 for 6s) should be tied together and pull up to the most positive cell
(http://www.ti.com/lit/ds/symlink/bq77908a.pdf  p36-37) but like that, if something goes wrong (wrong detection of the type of battery), some cell can be in short circuit (due to my MOSFETs controlled by the µcontroller).

My question is: Can I just pull up every  unused cell to the most positive used cell without tied them together?

Regards.

Cédric

  • 0
    TI__Genius 13700 points

    Hello Cedric,

    First, this device is intended to be incorporated on the PACK side not the charger side of the system. This is important for the sense resistor and low side charge/discharge FETs and how their placement will influence their operation.

    The primary reason for why it discusses shorting them together is that it is expecting that this part will be permanently integrated into the pack it is protecting and shorting the unused pins to a single pull-up will reduce the part count and simplify the design

    On to your question.

    Looking at the EVM diagrams and the discussed pull-up sensitivity (100-1000 ohms is acceptable) for the unused pins, individual pull-ups should work fine. http://www.ti.com/lit/ug/sluu854/sluu854.pdf

    Also as long as the BQ77908A is configured in its registers for the proper number of cells the unused pins will not even be read and the pull-ups are simply present to keep the pins from floating around.

    Let me know if you have any other questions.

  • 0 in reply to Ryan May
    Prodigy 40 points

    Thank you Ryan for your answer.

    So, Is it possible if I can get the temperature and the information of the current from the BQ34Z100 (which is incorporated on the pack) and send them with a DAC+ µcontroller (at the godd range) to the BQ77908A?
    Does the BQ77908A really need to have the number of cells to balance on the EEPROM ( With 3 times writting, it would be hard to do a universal charger).

    If I can't, do you have a product which can do balancing (BQ77PL900 perhaps?) with a changing configuration (4S and 6S) without to write in a EEPROM, because I can't incorporate it on the pack.

    Regards.

    Cédric