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BQ24161B: Unexpected behavior with a fully charged battery

Robert Wiegand
Prodigy 60 points
Part Number: BQ24161B

I am seeing some unexpected with a few parts when I connect a charging source and the battery is already fully charged.

The battery charger will report that it's charging but there is no current going into the battery. Both the Status/Control register and the STAT output indicate charging.

This condition can persist up to several hours before the part recovers and reports not charging.

Not all parts do this, but we have a few that act like this, so it's not just one defective part.

  • 0
    TI__Guru*** 146715 points

    Hi Robert,

    I am assuming there is no load on SYS?  If not, then is the converter switching (i.e. pulses on SW pin)?  Is the SYS output showing a squarish waveform as shown in datasheet figure 16?  If so then the converter is in reverse boost prevention mode as explained in datasheet section 9.3.16.5 Reverse Boost (Boost Back) Prevention Circuit.

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Prodigy 60 points

    There is a load on SYS but it's pretty small.

    The SYS line doesn't look quite like the datasheet diagram, but it is pulsing.

    Enabling EN_NOBATOP as suggested in the datasheet does seem to make the problem go away.

    What's the risk to disabling the reverse boost prevention circuit?

  • 0 in reply to Robert Wiegand
    TI__Guru*** 146715 points

    Robert,

    With EN_NOBATOP disabled, the reverse boost prevention circuit and the redundant battery OVP circuit (which stops converter switching) are disabled.  The primary battery regulation loop prevents battery overvoltage so the redundant OVP circuit is just that, redundant. However, because of the forced CCM to improve transient response, there is a risk that the buck converter works in reverse as a boost converter and pushes current back to the input.  The charger has input OVP protection so it will not be harmed by the resulting voltage spikes on top of the source's output voltage. Your input power source may not be as forgiving.

    Regards,

    Jeff 

  • 0 in reply to Jeff F
    Prodigy 60 points

    Increasing the termination current fixed the problem.

    The datasheet says:

    "With termination enabled and ITERM < 150mA or with a low impedance battery, the likelihood of activating the reverse boost prevention circuit by a load transient or even the inductor ripple current is higher."

    Is the termination current value of 150mA independent of battery size? 150mA might be large for some batteries.

  • 0 in reply to Robert Wiegand
    TI__Guru*** 146715 points

    Robert,

    Actually the issue is a function battery impedance.  Higher impedance batteries will not see this issue.  By increasing ITERM, we are simulating a higher Z battery.

    Regards,

    Jeff