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combining fuel gauge and protection

Boothby
Prodigy 130 points
Other Parts Discussed in Thread: BQ34Z100, BQ77908A, BQ77PL900, BQ76925, BQ76PL536

trying to protect a 5-cell Lipo and incorporate a fuel gauge chip.  I have chosen the b134z100 as the fuel gauge and the bq 77910a as the protection.  Is this a good combination?  the protection is only for over voltage, undervoltage, over current, - not for cell balancing. just seems a little over complicated for the functions i need but all i could find on the TI website. both would be incorporated into the battery pack. wondering what the interconnection between these two circuits would be given they both have current sense resistors on the low side.

  • 0
    TI__Expert 6470 points

    Boothby,

    Why not look at the bq77908? Should be a little more economical. Unless you know your cells will stay balanced, you will need the cell balancing function in order to get excellent gauging accuracy with the bq34z100. Yes, the only interconnection is that both ICs tap off the same low side sense resistor. There is no communication between the two independent ICs.

    Regards,

    Doug Williams

  • 0
    Prodigy 130 points

    Thank you Doug for the quick response.  I don't see much difference between the bq77908 and the bq77910 except for the number of cells.  balancing will be done by a separate charging system (commerical).  The only areas to cover are the overcurrent, short circuit current, undervoltage.

  • 0 in reply to Doug Williams4811
    Prodigy 130 points

    Doug,

    Does TI offer anthing besides the bq77908a to protect 5 lipo cells from 1) undervoltage, 2) overcurrent?  I am not using the cell balancing feature of the bq77908a or anything related to charging.  Can I leave pins 14, 19-21 unconnected?  What is the reason for EEPROM and not flash memory as with the bq34z100?

    thanks -Boothby

  • 0 in reply to Boothby
    TI__Guru 54715 points

    If you browse the product selector, there are relatively few selections for 5 cells. If you want to customize the behavior of your protection you could use an AFE such as the bq76925 or AFE function of the bq77PL900, but you will need a micro-controller since these will either not.provide automatic voltage protections or at the desired voltages  The bq76PL536 could also be used with a micro-controller, external current monitor and comparator.

    Pin 14, CHGST is required to wake the part from shutdown, for programming and certain fault recovery as indicated in the datasheet.  If you do not need charger signaling or shutdown, consider connecting CHGST to a pull-up voltage derived from BAT or PACK+. 

    Pin 19-21 are related to the CHG driver.  If this is not used: CPCKN (19) should be connected to VSS. CCAP (21) should have a 0.1 to 1 uF cap connected to CPCKN = VSS as noted in the datasheet for stability. CHG (20) can be left unconnected.

    A gauge such as the bq34z100 needs to update information during operation and over the life of the pack, so it needs a memory such as flash to be repeatedly re-written.  A protector such as the bq77908A would typically be configured once at production and operate with those values over the life of the pack, so the write-limited EEPROM is suitable for this application.