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BQ24072 - Over-discharge protection?

Scott A Miller
Prodigy 70 points
Other Parts Discussed in Thread: BQ24072, BQ24075

I've used the BQ24072 charger in a couple of designs, but now I need to use it with unprotected Li-ion cylindrical cells that must be protected from over-discharge.  Can someone suggest a good circuit for this that will work with the BQ24072?  Or perhaps recommend another battery management IC that might have this function built in?

Thanks!

Scott

  • 0
    TI__Genius 13700 points

    Hello Scott,

    I would recommend posting a more detailed question about what sort of pack you want to design in the gauges forum where the right people will have a higher likelihood of seeing the post.

    http://e2e.ti.com/support/power_management/battery_management/f/180.aspx

    Ryan May

  • 0 in reply to Ryan May
    Prodigy 70 points

    Is that really a gauge question?  I don't have any particular need to gauge the state of charge, I just need it to shut off completely (but still allow the charger to function) when the voltage drops below a preset.

    It's a single, removable 14500 cylindrical cell.

  • 0 in reply to Scott A Miller
    TI__Guru**** 167196 points

    The IC was designed expecting the battery to have a protector.  Therefore, the device checks for a short-circuit on the BAT pin by sourcing IBAT(SC) (7.5mA typical) to the battery and monitoring the voltage. When the BAT voltage exceeds VBAT(SC) (1.8V typical), presumably because the battery protector has closed, the battery charging continues to precharge and finally fast charge.

    When power is applied, if you change to bq24075/9 with SYSOFF, you could use an external supervisor to monitor the battery voltage and take SYSOFF high to disconnect the battery from the OUT pin, thereby disabling charge.

    When power is not applied, the battery FET is powered from the battery itself and will eventually turn off around 2V, which is the internal FET's Vgs voltage plus a diode drop. 

  • 0 in reply to Jeff F
    Prodigy 70 points

    I looked at the SYSOFF function, and I was under the impression that it would prevent the battery from charging as well.  The MCU can monitor the battery voltage, but I don't see an obvious way to accomplish the disconnect behavior when there's no other source of power to the MCU.

    I'm surprised TI doesn't have some simple IC to handle the low-voltage disconnect.  Or is there one that I just haven't found yet?  It seems like this should be a very common requirement, but I'm not finding simple solutions.

  • 0 in reply to Scott A Miller
    TI__Guru**** 167196 points

    SYSOFF disconnects the battery from the OUT and so there will be no charging or discharging. 

    These ICs were designed with the expectation of the battery having is own, internal protection FETs that will open and disconnect that battery from the IC to prevent over-discharge.

    When no power is connected, the IC uses the battery voltage to turn on/off that FET.   Below 2.5V, the battery FET will be begin to turn off because there is not enough gate drive to keep it on. 

    For a more precise turnoff, you can add a external PMOS between the battery IC BAT pin, with gate driven by a voltage supervisor like TPS3839K33.