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BQ77915: pack+ and pack- inquiry

Renan Santos Adriano
Mastermind 22360 points
Part Number: BQ77915

Hello team,

My customer is working with the device BQ77915 for 7s battery. They wanted to confirm how much power supply is sufficient (pack+ and pack-) should they use? They need 4.2V for each cell.

Regards,

Renan

  • 0
    TI__Guru 54715 points

    Hi Renan,

    For a 7S pack using BQ77915 2 devices would be needed.  The recommendation is to have fewer cells on the bottom part to maintain good drive levels for CTRC and CTRD to the bottom device.  This does mean only 3 cells are available to power the FET gates.

    Using the cells over a 4.2V to 3V range the top BQ77915 would have a supply range of 16.8 to 12V, while the bottom BQ77915 would have a supply range of 12.6 to 9V. Both are within the recommended VBAT Supply voltage range.  Charging the 7 cells to 4.2V would be 29.4V.

  • 0 in reply to WM5295
    TI__Mastermind 22360 points

    Hello Willy,

    Thank you for this response.

    Should they adjust the power circuit according to the max voltage? Shall they consider giving the same voltage for the 2 ic? Or should feed it with 2 different voltage sources?

    Thanks in advance.

    Renan

  • 0 in reply to Renan Santos Adriano
    TI__Guru 54715 points

    Hi Renan,

    Cells have a low impedance and will provide the power in an assembled pack.  With the full pack a voltage/current is supplied at the PACK terminals for charging, it would be one voltage.

    In development it is common to simulate the cell voltage for board test using resistors dividing the supply voltage.  It can be best to provide one supply per device to provide the best defined voltage for each device.  If using divider resistors of a few hundred ohms one supply may be sufficient, be sure to check for voltage shifts due to load changes.  When testing cell balancing the cell balance current will overwhelm a resistor divider and change the various input voltages.  It may be best to switch a low impedance source across the various resistors as each input is tested.