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Original question:

BQ25120: BQ25120 i2c can interface can disable in the middle of i2c transaction?

BQ25120: after VBatt < BUVLO event, device enters ship mode, but exists sporadically if battery voltage recovers to > BUVLO (3.23V)?

Philip Gruebele
Prodigy 120 points
Part Number: BQ25120

VIN=0, MR is floating in my test case, and BQ25120 appears to be entering ship mode once VBatt drops to below 3.2V (with 1uA current as expected in our design).  However, the battery voltage slowly recovers and once it hits 3.23V, I am seeing strange current transients.  It seems the BQ25120 is exiting ship mode when VBatt > 3.23V and then re-entering it immediately when VBatt once again goes below 3.2V.  Current changes constantly in the range of 1uA to 120uA. It looks like the BQ is powering up sys and unregulated briefly and waking our MCU, then entering ship mode again?

This is the behavior I would expect if MR is pulled low, but I am leaving this pin floating and it has internal pull-up.  The documentation is not clear on what state is entered when VBatt < BUVLO.  It only mentions "lockout" but it's unclear what state that is.

Questions:

1. What am I missing here?

2. What state is entered when VBatt < BUVLO, and under what conditions is that state existed?

Note that this is a BQ25120 not BQ25120A.

Cheers

Philip

  • 0
    TI__Mastermind 21590 points

    Hello Philip,

    What you're seeing is actually BUVLO and not shipmode. When VBAT is below the BUVLO threshold, the device does not startup when in BAT only mode. Can you please replace the BQ25120 with the BQ25120A. 

  • 0 in reply to Raheem
    Prodigy 120 points

    You are implying that the bq25120a fixes this behavior.  Can you be more specific regarding how this is changed?  If the bq25120a does not fix this then how should one go about avoiding this constant power toggling?

  • 0 in reply to Philip Gruebele
    TI__Mastermind 21590 points

    Hello,

    The post below shows the differences between the BQ25120A and the BQ25120.

    https://e2e.ti.com/support/power-management/f/196/t/632901?BQ25120A-Differences-between-the-BQ25120A-and-BQ25120

    Are you testing this with a real battery or a sourcemeter on the BAT pin.

  • 0 in reply to Raheem
    Prodigy 120 points

    OK, so based on the list of changes it appears BQ25120A will not change this issues.

    This happens both with a battery and with a bench power supply.  With the bench supply I can see that once VBat < 3.0V, SYS output shut off.  Then Once voltage goes back up to >3.2V, SYS turns on again.  With the battery what I see is that around 3.1V, larger current spikes cause voltage droop below 3.0V which shuts down SYS.  Then the battery voltage recovers slightly to 3.23V and SYS turns on again - only to turn off again shortly thereafter when voltage droops again.

    I thought about having the MCU check battery voltage when it powers on and if VBAT < 3.2 without VIN it could tell the BQ25120 to enter ship mode.  The problem is that SYS power-on itself causes large transients which can cause VBat to droop below 3.0V again, so that would not be very reliable.  I also thought about changing the 3.2V threshold that causes SYS to turn back on, but there do not appear to be any register settings to control this. Currently it seems the only way to fix this is to change BUVLO to a much lower voltage like 2.8V and have the MCU read the voltage from the BQ and enter ship mode if VBat < e.g. 3.0V.

    The BQ expected behavior would be that VBAT < 3.0 without valid VIN would cause the BQ to enter ship mode state instead of the "lockout" non-defined state, which could then be exited via standard methods of MR or valid VIN.

  • 0 in reply to Philip Gruebele
    TI__Mastermind 21590 points

    Hello Philip,

    As stated in the previous linked post i shared, there were improvements made to the /MR pin.

    I'll suggest changing to the BQ25120A. 

    If you can communicate with the device, you can reduce BUVLO on startup and then set the device to shipmode.