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BQ25303J: Issue with reverse current and boost to the input

Jörg Bornwasser
Prodigy 30 points
Part Number: BQ25303J


Tool/software:

Dear all,

i have an issue with BQ25303J. I'm using it for charging a single cell LiIon battery.

The issue is that when the cell voltage is below ~4 V and I start charging, than stop charging by removing power from the input, BQ25303J is boosting the cell voltage back to the input, delivering a stable voltage of 4,05 V. This leads to BQ25303J not turning off and powering itself. Even a pulldown resistor on the input side doesn't help until decreasing it's value to ~47R (than it turns off). But even when turned of, there is a voltage present at the input side, which is around ~0,5 V below the cell voltage. In this case it can be pulled down by a resistor and some mA Current flow.

When fully charging the LiIon Battery (CutOff voltage is designed to be 4,1 V (which works), BQ25303J turns off normally.

This leads to the fact that i have to fully charge the battery. If i stop during charging, BQ25303J will not turn off, still switching, thus boosting the cell voltage to the input side, draining the battery.

What is wrong here? In the following i put a screenshot of the designed circuit.

On the Input side, which is IN+ and BAT- i connect a 12 V DC Power supply for charging. On the Output side, a single LiIon cell is connected all the time.

I would be very thankfull about support in this case.

Thanks in advance,

Jörg

  • 0
    TI__Guru 59471 points

    Hi Jörg,

    We will check and get back to you.

    Thanks,

    Ning

  • 0 in reply to Ning T
    TI__Guru 59471 points

    Hi Jörg,

    We are unable to duplicate your observation on the EVM. You may try to disable the device via /EN pin when the boosting happens.

    Thanks,

    Ning

  • 0 in reply to Ning T
    Prodigy 30 points

    Hi Ning,

    thanks for trying.

    There is perhapps one more information valueable for you, i left it out because i already checked if it would have an impact but it didn't:

    I have three of this circuits, shown above, in parallel. They are in parallel on the input and output side.

    But because i already presumed, that this could be the reason for the issue i did try the following:

    I seperated on of the three channels via a diode on the input side so that the circuit can only draw current but not source.

    Still this channel sourced it own input to 4,05 V.

    In hindsight, perhapps this test was not enough, the other channels could still deliver (4,05-0,5V)=3,55 V, perhapps avoiding it's shutdown.

    So, of course I'm not fully aware of how the internal circuit of BQ25303J is working. Could it be, that, due to little difference in turn off speed of the three channels that they keep each other alive? But therefore i added a resistor to the input for fast discharge and voltage drop on the input side, still it didn't work. I fully understand that with this DC/DC topology, boosting the output back to the input is possible, but if Q1 would turn off, there should be no backflow possible. Thus, it seems not to turn off.

    Regarding your suggestion to turn off via EN-Pin: Pulling up is somewhat hard when not sourcing the input. Of course, because of the residual 4,05 V it could be possible for some time, but then still this is a standalone device without  µC. I do not have the information when to turn off the device (and when to turn on).

    Regarding the datasheet, it seems like the devices are boosting up to "VVBUS_OP ".

    Do you think, paralleling could be the reason for this behavior even it still appears with the diode i tested?

    Thanks in advance

    Jörg

  • 0 in reply to Jörg Bornwasser
    TI__Guru 59471 points

    Hi Jörg,

    It is likely related to three of this circuits in parallel. It's better to verify with operating one circuitry alone by isolating it from other two circuits (i.e. the other two circuits are disconnected completely).

    The current design has /EN low enabling the charger. It should be changed to /EN high enabling the charger so that /EN goes low (i.e. disable charger) when VBUS goes to low.

    Thanks,

    Ning.