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BQ27500 / BQ2057 design guidelines?

Tom Smith
Expert 2930 points
Other Parts Discussed in Thread: BQ2057, BQ27500-V130, BQ27500

BQ27500DRZT-v130

- I need to understand how the BQ27500-v130 and BQ2057 play together in our design.  Currently, we are using a dumb battery with an external NTC to control charging over temperature.  The fuel gauge wants a thermistor input as well but I don’t think we want to parallel the charger and fuel gauge on one thermistor.  We can’t rely on the processor to disable the charger at high/low temp because the device needs to charge when the processor is off.  Can the temp input on the fuel gauge be unused?

-Is an eval available, I see the BQ27500EVM, is there an EVM for the BQ27500-V130?

-Tom-

  • 0
    TI__Intellectual 2100 points

    -You can use the gauge to control the charger autonomously: This is documented in the following application notes:

    Using the bq24075 Charger and bq27500/1 Fuel Gauge  (PDF  173 KB)

    and

    Key Design Considerations for the bq27500 and bq27501  (PDF  527 KB)

     

    -The bq27500EVM is an EVM for the bq27500-V130.   Do note that the difference between different versions of bq27500 are firmware only, and therefore, you can upgrade/downgrade between bq27500's.  See the following application note:

    Updating Firmware with the bq2750x and EVM (Rev. A)  (PDF  955 KB)

     

     

  • 0 in reply to Charles Herder
    Expert 2930 points

    THANKS Charles!

  • 0 in reply to Tom Smith
    Expert 2930 points

    Charles,

    I’ve been reading through the data sheet a bit as well.  My concern with using the 27500 to control the charger is that it seems you also have to use the BAT_GD signal to control the system power during initial power up for calibration so how do you shut down the charger for, say over temp, without shutting down the rest of the system?  Right now, I am leaning toward running the charger autonomously and letting the 27500 gate system power and monitor low batt conditions.

    Comments....

    -Tom-

  • 0 in reply to Tom Smith
    TI__Intellectual 2100 points

    Tom,

    Use of the BAT_GD pin to control system startup is optional.

    The delay is introduced so that the gauge can get an accurate OCV measurement before everything turns on. 

    If you leave the gauge powered and running while your system is OFF, then you are OK, because this measurement is taken before the system boots.  Also, if you can guarantee that your system will not pull more than ~C/5-C/10 or so for the first few seconds of operation, then you also will not run into a significant problem, because the IR drop across the cell internal impedance will not be significant.

    If neither of these conditions is true, having the charger run autonomously is OK as well.

    Thanks,
    Charles

  • 0 in reply to Charles Herder
    Expert 2930 points

    Charles,

    Here is what I am thinking: Since the system will be disabled by default at the time the battery is first installed, we may be able to avoid most of my concerns.  How long does the OCV process take?  Upon initial battery insertion, the gauge will be powered but the system will not.  The system will not be powered until an external signal is applied at some later (perhaps significantly so) time.  Do you see any issues with that arrangement?
    Relative to the charger, is only rated for operation from -20 to +70 while we may need to operate down to -25.  However, if charging is only enabled between 0 and 45, do we care?

    -Tom-

  • 0 in reply to Tom Smith
    TI__Intellectual 2100 points

    Tom,

    I do not see any issues.  The gauge takes a maximum 2 to 3 seconds to take the first OCV sample. 

    You can program the gauge with the range of temperatures for which charging is enabled.  Do you really need to charge your battery at -25C?  Otherwise, I don't see any issue with selecting the 0-45 degree temperature range.

    Thanks,
    Charles