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BQ25155: Using this battery management without battery - /MR keeps resetting the power rails

Lukas Brchl
Prodigy 90 points
Part Number: BQ25155


Hi,

as discussed in https://e2e.ti.com/support/power-management/f/196/t/900346, we would like to know, if it is possible to use BQ25155 without battery. When we do that, then /MR keeps getting charged and discharged (oscillating). And when we set a flag for charging disabled, the /MR goes high after the start and immediately to low (which then causes hardware reset). Is there some configuration that could be applied so the BQ25155 can run without a battery?

Best regards,

Lukas

  • 0
    TI__Mastermind 21590 points

    Hello,

    /MR is internally pulled to VBAT so when battery is not present, you'll see oscillation on /MR which comes from the capacitor on BAT getting charged and discharged.

    However, this should not cause a HW reset because /MR is not low long enough to trigger the HW Reset timer

  • 0 in reply to Raheem
    Prodigy 90 points

    > /MR is internally pulled to VBAT so when battery is not present, you'll see oscillation on /MR which comes from the capacitor on BAT getting charged and discharged.

    This is true only if the charging is enabled. If it is disabled, then the /MR gets charged once and then goes to low forever.

    So the question: Is it possible to operate BQ25155 normally without a battery?

  • 0 in reply to Lukas Brchl
    TI__Mastermind 21590 points

    Yes. You can operate the BQ25155 without a battery. All power rails will be powered as expected. 

  • 0 in reply to Raheem
    Prodigy 90 points

    Hi,

    I am not complaining about the power rails, they are working as expected. The problem is \MR. I will repeat the behavior once again, considering there is no battery connected.

    • When using default settings the \MR keeps oscillating because the capacitor on BAT getting charged and discharged. This is not appropriate as it makes electrical noise and triggers many faults interrupts.
    • When we keep the default settings and we disable the charging functionality, the \MR goes once up and then goes lows forever. This means that it will keep restarting the power rails forever because there is an HW reset set on button long press. And of course, we want to keep the functionality of the HW reset.

    So my question is, how to set up properly the BQ25155 without a battery that it won't oscillate on \MR and won't restart because the \MR is low?

  • 0 in reply to Lukas Brchl
    TI__Mastermind 21590 points

    Hello,

    VBAT will have to be present for the /MR functions to work to properly since /MR is internally pulled up to VBAT. The device will not be able to properly register a high to low transition on /MR with VBAT toggling. However, this should not cause a HW Reset on the device

  • 0 in reply to Raheem
    Prodigy 90 points

    Well, that's what I am trying to explain. When VBAT is not present and charging is disabled in settings, the \MR pin goes low which keeps HW resetting forever. So is this expected behavior or not? Seems that it would be best if you could try it yourself.

  • 0 in reply to Lukas Brchl
    TI__Mastermind 21590 points

    Hello,

    This is expected behavior, The capacitor on VBAT will not be charged when charging is disabled.

  • 0 in reply to Raheem
    Prodigy 90 points

    Ok, thanks! I think we are heading to the finish.

    Still considering the situation that no battery connected

    1. So if disabling charging is not an option, is there a way to keep the capacitor at \MR charged via some settings or HW modification? 
    2. If not, is it a problem that \MR keeps oscillating? Couldn't it cause some HW issues or should we accept that as a fact? 
    3. And also, when it keeps oscillating it generates a lot of faults/interrupts. Should we just mask them via registers or is there a better way how to deal with this issue?
  • 0 in reply to Lukas Brchl
    TI__Mastermind 21590 points

    Hello,

    One way around this is to disable termination. This will keep the capacitor on VBAT at the VBATREG voltage and will allow the use of /MR pins.

    However after a reset, the register will default to having termination enabled. 

    Yes you can mask the fault when the oscillation occurs.

  • 0 in reply to Raheem
    Prodigy 90 points

    Hi Sharafadeen,

    thanks for the reply. Disabling the Termination solves the oscillating issue.