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BQ77915: 12S pack setup using four chips okay?

John M
Expert 2135 points
Part Number: BQ77915

Hello, our client just asked if we could add support for packs down to 12S using the four-chip bq7791501 design we were working on for their standard li-ion 18S-20S packs.

Technically this should be possible, with three cells per chip, but I was wondering if there were any practical concerns that aren’t covered in the datasheet or app notes? Thank you!

Regards, John M.

  • +1
    TI__Expert 8526 points

    Hi John,

    Since the BQ77915 was designed for 3-5S cells, 3 cells per device should be okay.  The main aspect I would be concerned about is VDD voltage.  Please ensure that the range of voltage the customer is expecting the cell to operate in will not result in too low of a VDD voltage.  

    I recommend looking through the electrical characteristics of the device to ensure the customer's cells will not cause VDD to fall below threshold voltages, such as Vpor or Vshut, which can lead to SHUTDOWN or prevent the device from powering up.  Please see sections 9.4.1.1, 9.4.1.5, and 9.3.14 for more information on SHUTDOWN, power on reset, and charging inhibition.

    Finally, double check the signal from the CTRC and CTRD pins to ensure they are strong enough to be recognized by the lower devices.  This is critical for FET switching.  Also, please note the connection of the CCFG pin.  This pin must have the appropriate connection to match the cell number:

    Best,

    Andria

  • 0 in reply to Andria McIntyre
    Expert 2135 points

    Hello Andria,

    Our initial review of all the thresholds looked okay but we will re-check all to be sure and the CCFG lines will be set. I am a little concerned with your mention of CTRC and CTRD though. If we never approach the VDD values that can otherwise cause problems (as you mentioned above) is there something that could affect CTRC/CTRD levels badly enough to interfere with good FET switching?

    We’re worried about telling the client that 12S is possible only to find out in the prototypes that there are problems.

    Cell voltages should never drop below 2.5V, resulting in a 7.5V minimum VDD, since we have an audible alert circuit as part of the BMS to let the user know that the pack is dropping too far down in voltage (when in extended storage, for example). This should prevent cell self-discharge from ever draining the pack too far (below POR, SHUT, and charge inhibition).

    The bq7791501’s should shut off the discharge FETs if a cell hits 2.8V though, stopping use of the pack when all the VDD’s are even higher than 7.5V.

  • +1 in reply to John M
    TI__Expert 8526 points

    Hi John,

    I think your current design should not cause an issue.  Checking the CTRD/C signal strength is just good practice.  We normally recommend placing devices that will have higher cell counts higher in the stack (ex: for 13S and 3 devices, bottom device: 4S, middle device: 4S, top device: 5S) to ensure the CTRD/C voltage is as strong as possible.  

    Because of this recommendation, I thought it wise to advise you to check on the CTRD/C pins.  That being said, if your VDD levels are adequate, then you should not see issues.  This recommendation is more for optimizing--rather than meeting--signal strength requirements.

    Best,

    Andria

  • 0 in reply to Andria McIntyre
    Expert 2135 points

    Hello Andria,

    Sounds good. Thank you for your patience in dealing with all my questions! Please consider this issue resolved.

  • 0 in reply to John M
    TI__Expert 8526 points

    Hi John,

    It is no problem at all!

    Best,

    Andria