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Why does my bq24314c fault pin trip prematurely?

Ankur Aggarwal
Prodigy 60 points
Other Parts Discussed in Thread: BQ24060

Hi TI,

In my current design (schematic proprietary unless shared under NDA), I'm using the bq24314cdsgt as a front-end protection chip for my LiPo charger, which is also one of yours (bq24060) .

Anyway, despite my rechargeable LiPo battery voltage being around 3.8V, and the input power to the bq244314 coming from a USB connection (so input overvoltage shouldn't be a problem), the fault pin goes low (connected to LED-resistor combo) when I plug in my PCBA, and, from what I can see from ammeters I've used in series with my battery, as well as from the status pins of the bq24060, charging cuts off as well. But surely my battery should be able to charge up beyond 3.8V with a lithium polymer chemistry, no?

The battery is not directly connected to VBat. Instead, VBat is connected to the output of an op-amp through a 100kOhm resistor. This op-amp is configured in non-inverting feedback such that when the battery voltage (Vin- for the op-amp) actually reaches 4.2V, Vbat will exceed reach around the 4.44V voltage limit. So 3.8V should definitely not be a problem, correct?

ILim and CE are pulled down to GND through resistors, 44.2kOhm (for a 600mA current limit) and 10kOhm respectively.

Now, since the op-amp's power rails are connected to USB Power and GND, I can imagine there being some weird transient voltages that might affect VBat when I first plug in USB. But surely these should settle out, with Fault no longer being pulled down, right?

I've tried various schemes where I program my micro to try to pull up CE, wait a bit, and pull it down if I detect that the board is plugged into USB power, but none of these seems to work sustainably.

Interestingly, my team also noticed that when you plug in the USB cable very slowly into the brick, it is less likely to trip the Fault pin, at which point the battery begins charging successfully. Is this a clue as to what's going on?

Please let me know if I can clarify anything else. I'll try to disclose what I can to share our findings and help solve this problem.

Best Regards,

Ankur Aggarwal

  • 0
    TI__Expert 4640 points
    Hi Ankur,

    First question, why are you using an op amp to feed a false OV to the VBAT pin? Have you simply tried removing the op amp and see if the battery charger up?

    2nd - Have you tried leaving CE pin floating or directly grounded (i.e. no resistor)?

    3rd - What is the charging voltage of your battery? (should be in the datasheet of the battery)

    My general guess is that the op amp is causing your problems with stopping charge.

    Miguel
  • 0 in reply to Miguel Angel Rios
    Prodigy 60 points
    Thanks for the followup, Miguel.

    Our batteries are somewhat custom and don't really have datasheets. In either case, their safe voltage range is between 2.7V and 4.2V, generally speaking.

    Regarding the VBAT pin, I tried removing the grounding resistor in the feedback path of the op-amp, effectively changing the gain to 1 and reproducing the battery voltage at VBat. No luck.

    I also tried cutting out the grounding resistor of CE (the datasheet did mention internal grounding, after all). Still no luck.

    Any additional thoughts?
  • 0 in reply to Ankur Aggarwal
    TI__Expert 4640 points
    Hi Ankur,

    Have you tried to remove the subsystem with the op amp altogether and use a resistor with a power supply to mimic the battery to see if the behavior continues?

    Miguel