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BQ24195: Recovering from deeply depleted battery

Eddie Koh
Genius 10635 points
Part Number: BQ24195

What is the expected behavior of BQ24195 when:

1. Battery deeply depleted till 1.0V

2. User plugs in USB, supplying 5V to Vbus of BQ24195

Currently, based on Section 8.3.2.1 of datasheet, Vsys should be regulated above "minimum system voltage (default 3.5V)". However, customers is seeing it totally dead at 0V. What might cause this?

  • 0
    TI__Genius 10385 points

    Eddie,

    Is there a SYS load applied when VBUS is plugged in? Do you still observe SYS collapse when the system load is removed and the adapter is plugged in?

    Best Regards,

    Ricardo

  • 0 in reply to Ricardo
    Prodigy 10 points

      Hi Ricardo, my name is Hugh and Eddie has suggested we comment directly to you using this forum.
    To answer your questions above, yes there is a sysload when VBUS is plugged in. We completed a test with SYS load removed and found that SYS did not collapse when VBUS is applied and that the battery would then be charged successfully. We tried this with batteries with at voltages around 0.9volts and again at 0.7volts.

    Based on email comment "if a large SYS load (>200 mA) is applied at VBUS plug in then SVS is not able to regulate to VSYSMIN and the SYS rail collapses.". Our steady state current consumption during startup is around 80ma, so we are not over 200mA. What is the profile of the 200mA? We guess that it could be reached momentarily due to initial inrush current?

     Based on the email comment “It has been observed that with no battery or significantly discharged battery, if a large SYS load (>200 mA) is applied at VBUS plug in then SVS is not able to regulate to VSYSMIN and the SYS rail collapses. Allowing SYS to get settled and then applying a large SYS load is fine.”  it seems this is a known behaviour.  Could you explain the behaviour further , as we would need to understand in depth to determine and evaluate any available solutions?  Could you outline any suggested solutions that TI may have developed with other customers? 

  • 0 in reply to Hugh McDonald
    TI__Genius 10385 points

    Hugh,

     Our steady state current consumption during startup is around 80ma, so we are not over 200mA. What is the profile of the 200mA? We guess that it could be reached momentarily due to initial inrush current?

    It is likely that the transient load is collapsing the SYS rail.

    Could you explain the behaviour further , as we would need to understand in depth to determine and evaluate any available solutions?  Could you outline any suggested solutions that TI may have developed with other customers? 

    In the absence of a battery or in a deeply discharged battery a large enough transient on SYS can collapse the SYS voltage regulation. This is not an issue when VBAT is greater than VSHORT. When in the VSHORT condition the BATFET acts as a constant current sink pulling from SYS, but if SYS has not had a chance to regulate or has collapsed, then this constant current sink will not initiate. 

    I would suggest adding either an integrated or discrete load switch as described here:

    Or reducing the SYS load upon startup in your software implementation.

    Best Regards,

    Ricardo