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BQ24155: Charger input VBUS gets lifted to +6V when removed from power supply

user4592198
Prodigy 40 points
Part Number: BQ24155

Hi,

We have noticed that sometimes when the charger is finished with charging and we remove the device from the power supply. The VBUS input voltage (instead of dropping to 0V as normally happens) rises to +6V

It only seems to happen when the battery is fully charged.

See below a screenshot of a measurement:

Ch1 (yellow): VBUS ; Ch2 (blue): PMID ; Ch3 (purple):VREF ; Ch4 (green) mcu signal related to battery measurement.

In this case I plugged in the power supply -> Channels 1,2 and 3 rise to 5.15V.

4 seconds later the CH1, 2 and 3 start to oscillate

2 seconds later I unplug the power supply and Ch 1, 2 and 3 oscillate at a higher frequency while there is no power source except the battery on the output of the charger.

below zoomed in on the last section:

Can anyone explain how this can happen and what we can do to stop this.

In this situation our device keeps thinking it is in the charger and doesn't  respond to other user inputs, and this can last for up to 1 minute.

Thanks for any help,

Bart

  • 0
    TI__Guru**** 151946 points
    Bart,

    How much capacitance is still connected to VBUS when power is removed, i.e. are you removing the wall adapter from the wall so that the adapter's internal capacitance is still attached to the charger through the power cord? Is the battery close to full charge? If so to both, then the charger is likely boosting back. What is the size of the CBOOT capacitor from BOOT to SW? Try increasing to 33nF. This should stop the boost back.
  • 0 in reply to Jeff F
    Prodigy 40 points

    Hi Jeff,

    Thanks for you response.

    When the charger is removed it is disconnected from the circuit so only 2 x 1uF is on VBUS.(see below circuit) This situation only occurs when the battery is fully charged (and not always). I tested with a increased bootstrap capacitance (added 22nF on top of the 10nF) but I can still reproduce the problem. 

    Do you have any other suggestions?

  • 0 in reply to user4592198
    Prodigy 40 points

    Hi Jeff,

    I think we found the problem, in our system the software uses the gauge to decide if the battery is fully charged(in this case 90% charge = full) when this happens the software changes the Battery regulation voltage from 4.10V to 4.05V. when we don't change the Battery regulation voltage and leave it at 4.10V the boost back symptoms are no longer seen.

    Could you confirm that the change in Battery regulation voltage can cause these boost back symptoms?

  • 0 in reply to user4592198
    TI__Guru**** 151946 points
    Bart,

    Yes! If VBATREG is lowered to a voltage below the current battery voltage, the charger will try to lower the battery voltage to that new regulation voltage.

    Also, please change the boot capacitor from 10nF to 33nF to prevent the other cause of boost back.
  • 0 in reply to Jeff F
    Prodigy 40 points
    Hi Jeff,

    Thanks for your support!