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BQ24780S Inserting Battery Current Spike

Akash Patel
Expert 6245 points
Other Parts Discussed in Thread: BQ24780S

Hi team,

We are currently using the BQ24780S in our design and we have a 2S Li-Ion configuration. The battery is only charged when using a 9V adapter.

When using a 5V adapter or a USB port on a laptop, we are able to run our system without the battery. However, once a battery is inserted, as this battery voltage (6.0-8.4V) is higher than the adapter or USB voltage (5V), we see a large current spike in our system (see graph below) which we have seen to cause laptops to shut down and issues with power adapters.

Are there any register or hardware changes that might help with this spike?

(The graph below shows 3A transient (using a 3A supply) when a battery is inserted)

Regards,

Akash Patel

  • 0
    TI__Guru 56650 points

    BATFET has a body diode. So, the system can still get power during input source removal due to this BATFET's body diode.

    Without adding another FET to block this body diode current path, the battery can power the system from this body diode. Any register change cannot remove the power FET's body diode. So, please think about adding another FET and its control logic to block the battery when 5V source is available.

    Also, do you allow the system temperary loss power between 5V source is removal and the extra FET turn-on? 

  • 0 in reply to Wang5577
    TI__Expert 6245 points
    Hi Wang,

    We are not removing the input source during this. This if the user *accidentally* plugs in the battery when we have the device plugged into a 5V source which is not able to charge the higher potential battery.

    Regards,
    Akash Patel
  • 0 in reply to Akash Patel
    TI__Guru 56650 points
    That is good. We don't need to worry the system temperary loss power at 5V source removal.
    The system has to add a switching to block the battery if 5V source is available.
  • 0
    Prodigy 50 points
    The idea of using a series element like a FET in the battery path or a diode in the charger path may add noise to the system voltage and affect charging.

    Could we instead use an opamp to detect when the current through the input current sense resistor starts to go negative and use it to drive an n-ch FET, which would pull down the gate drive to the input ACFET? That would immediately force the input FET off before the current spike gets anywhere near its maximum.

    Any problem with driving the ACDRV to ground through a 4k resistor?
  • 0 in reply to Glenn Garbeil12
    TI__Guru 56650 points

    When 5V source is available, the extra BATFET's series FET blocks the battery discharge current.

    When charger can be eanbled with other high voltage source, the extra BATFET's series FET should be fully on. We tried the extra BATFET's series FET for other project. So far, we don't see any application issue on that extra BATFET's series FET because is always fully on.

    ACDRV can be pulled down to disconnect input source the system. But, the charger could be latched off. We have to recycle the input power (VCC<UVLO or ACDET<0.6V) to reset the latch-off status. This ACDRV turn-off method doesn't reduce the battery discharge current for that 5V source application.