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BQ24070: DPPM preventing 'charge done' state on STAT pins

Jon Stevens1
Prodigy 30 points
Part Number: BQ24070
Other Parts Discussed in Thread: TPS63031

I have inherited this design. Schematic for PMIC section is included here. Bare boards are fabricated, so I must use this copper. Everything works, except that with R12 at 37k4, and with a GeB 404176 3.7V LiPo battery (1600mAh) with protection circuit included, BQ24070 will not indicate "charge done" on STAT1/STAT2. Changing R12 to 36k0 worked on a test unit. STAT1 and STAT2 indicated charge done as per the table in the datasheet. But across 50 units with R12 at 36k0, I had a few that would still never get to charge done and would continue to indicate "fast charge in progress" on the STAT lines. On each of those units, I changed R12 to 35k7 and each one was then able to reach charge done on STAT lines. LiPo battery seems to get to right around 4.185V max, whether PMIC indicates charge done or not. With these constraints, how can I set this design so that it will always reach the charge done state on the STAT pins?

Thank you.

  • 0
    TI__Mastermind 23220 points

    Hi Jon,

    Are you able to measure the current going into the battery when you see that behavior? Are you also able to measure the voltage at the IN, OUT, and BAT pins when you see this? 

    Is there a pullup on these pins?

    Best Regards,

    Anthony Pham

  • 0 in reply to Anthony P Pham
    Prodigy 30 points

    Hi Anthony,

    Have not tried to measure current going into the battery yet, but I can try to set that up soon and let you know what I see. 

    V_IN = 5.3V

    V_OUT = 4.39V

    V_BAT = 4.183V

    We are running a Fanstel module with a Nordic nRF52833. STAT pins go straight to GPIOs on the '833, and I've confirmed that internal pull-ups are enabled for those GPIOs. Nordic product spec shows the internal pull-ups as 13k0 typical. Output of the PMIC runs to a TPS63031, and the Fanstel module is running at 3.3V. I usually put pull-ups right on the board in these cases, just to be sure, but this should work in theory since the STAT pins are open drain. I also usually pull up to the output of the PMIC and then translate if need be to avoid any back-powering, depending on the design. But I don't sense that is the problem in this case - since I can see a 'charge done' pattern on STAT pins with R12 at 35k7. 

    Thank you,

    Jon

  • 0 in reply to Jon Stevens1
    TI__Mastermind 23220 points

    Hi Jon,

    Jon Stevens1 said:
    Have not tried to measure current going into the battery yet, but I can try to set that up soon and let you know what I see. 

    Great, looking forward to your update if possible. It'd also be nice to see the current at the IN and OUT pins as well. If adjusting the VDPPM threshold is helping the device perform better, the DPPM behavior will most likely be seen (battery current reduced to maintain the OUT voltage).

    Jon Stevens1 said:
    I can see a 'charge done' pattern on STAT pins with R12 at 35k7

    Based on your resistor values it looks like:

    RDPPM [kohm] VDPPM [V]
    35.7 4.105
    36 4.140
    37.4 4.301

    With 37.4kOhm, this threshold is most likely causing the device to enter DPPM which will disable termination causing the behavior your seeing since CHARGE DONE state occurs with termination 

    Best Regards,

    Anthony Pham

  • 0 in reply to Anthony P Pham
    Prodigy 30 points

    Anthony,

    Best I can tell using Joulescope, I see right around 2, 2.5mA going into the battery. Are you theorizing that perhaps DPPM is always reducing battery charge current to maintain PMIC OUT voltage, for some reason, and therefore it's simply taking much longer for the battery to fully charge, resulting in no 'charge done' indication on STAT pins? In the past, I have left this particular setup in place, charging, with R12 = 37k4, for roughly 24 hrs or more to see if it would reach charge done, and it did not. But depending on where the battery really is in its charge cycle, that may simply not be enough time to top it off. At 2mA for 24 hrs, that's only 48mAh. Battery capacity is 1600mAh.

    Interested to read what you think about this, and if you think populating R12 with 35k7 resistor is a suitable solution.

    Thank you,

    Jon

  • 0 in reply to Jon Stevens1
    TI__Mastermind 23220 points

    Hi Jon,

    DPPM would be the reason why you aren't seeing the device terminate the charge current and changing the STAT pin to show a charge complete. 

    Jon Stevens1 said:
    Are you theorizing that perhaps DPPM is always reducing battery charge current to maintain PMIC OUT voltage

    Not necessarily always but as needed to maintain the PMIC OUT voltage to the threshold you set based on the R12 resistor value. The higher your threshold, the more current is needed to maintain the voltage at OUT. It's similar to a voltage supply with a current limit; As the load approaches the current limit threshold, the voltage will drop. To maintain the voltage, the current output needs to be reduced or the threshold needs to be increased.

    It's the same for your scenario. Because you're lowering the voltage threshold for DPPM, the device does not need to divert as much current from BAT to OUT to maintain the voltage. 

    Jon Stevens1 said:
    you think populating R12 with 35k7 resistor is a suitable solution

    Ultimately, it is your decision but my input is that it really depends on your priority on the output or the battery. 35k7 will work but you'd most likely need to reduce the charge current to keep out of DPPM. This will help maintain your OUT voltage.

    If you want to prioritize your battery, then you'll want to adjust the R12 resistor until you see CHARGE DONE at the cost of reduced OUT voltage. With R12 at 35.7kOhm, you're allowing VOUT to drop to 4.105 V before reducing charge current. If you're fine with that, then yes this is a suitable solution.

    Best Regards,

    Anthony Pham

  • 0 in reply to Anthony P Pham
    Prodigy 30 points

    Anthony,

    Ok, explanation is understood. I think that we can tolerate a VDPPM threshold lower than 4.105V in favor of timely battery charge completion.

    If the threshold value is causing the device to enter DPPM and that is disabling termination, then I am confident I can simply lower the value of R12 to ensure charge done in every case. I just wanted to be sure there wasn't anything else going wrong. I would like to keep the charging current maximum at 500mA, but I may even be able to justify dropping that value by about 20 - 40% to help with this.

    Thank you for your help,

    Jon