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Original question:

BQ25792: VSYS cylcing between 12.5V, 9.5V, and 9.1V when no host or battery present.

BQ25792: VSYS unstable when battery is removed and adapter remains powering the device

Austin Christensen
Prodigy 10 points
Part Number: BQ25792
Other Parts Discussed in Thread: TPS25750, BQ25798

I am running into issues with the BQ25792 charge controller when the battery is removed but adapter still connected.  I am using a 2s battery and shooting for 8.4V setting on charger.  With the battery connected everything looks great.  VSYS is stable and no issues.  As others have found; if the battery is removed and the adapter is still powering the device, the BQ appears to power cycle.

I understand that this is a known issue and can be addressed by either disabling charge (difficult for my application) or adding additional capacitance to the BAT line on the order of ~200uF to soak up the voltage spike that occurs when the charger tries to charge the non-existent battery.

I have added between 160uF and 1000uF in ~80uF steps and have continued to see the VBAT peak to the 8.8V range before dropping.  Below are the traces with 468uF attached.

  

I have monitored the fault registers on the BQ and have picked up value 0b00100000 on REG20 indicating a VBAT fault so I believe I am indeed fighting the right issue.  No other faults were seen in on the device.

My questions are: 

Should the apparent "reset" and VBAT voltage drop behavior cease when sufficient capacitance is available? 

If the "reset" behavior remains present, will the device get damaged over time?  

What is the best way to determine the capacitance to apply?

Follow-on question: Is the BQ25798 compatible with the TPS25750?  I assume not or it would be an option in the GUI?

  • +1
    TI__Guru**** 152856 points

    Hi Austin,

    The BQ25792 needs charge to be disabled when the battery is removed, either using the /CE bit or pin or from TS pin fault due to missing thermistor.  Otherwise, the BAT pin capacitance overcharges, causing a BATOVP then SYS OVP event.  These events cause the converter to shutdown briefly. Lower charge current <1A and adding > 220uF on BAT pin prevents this with most configurations.  The easiest solution is to replace with BQ25798 which doesn't have this issue and which is pin 2 pin and register to register compatible with BQ25792.

    Regards,

    Jeff