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BQ76952: Source-follower mode SFMODE_SLEEP active during NORMAL mode

Christopher Churavy
Intellectual 260 points
Part Number: BQ76952
Other Parts Discussed in Thread: BQSTUDIO

Using the BQ75692 EVK:

With SLEEP disabled via SLEEP_DISABLE() subcommand,

if I configure SFMODE_SLEEP = 1 in Settings:FET:Chg Pump Control and then issue an ALL_FETS_ON() subcommand, I observe a voltage drop across the CHG and DSG FETs with only a DMM connected on the output.

If I configure SFMODE_SLEEP = 1, there is no voltage drop.

My assumption is that the DSG FET is being driven via the weak source-follower configuration even though I have explicitly disabled SLEEP mode, contrary to the description in the TRM.

What am I missing?

  • 0
    TI__Expert 8526 points

    Hi Christopher,

    The problem you are describing sounds interesting.  I would like to ask a few more questions to clarify the issue.  You did not specify in your question how much voltage drop you are seeing.  Is it in units of mV, or something more significant like volts?  Additionally, how much current is flowing?

    It is possible that the device is still just in sleep mode and is set to have the CHG FET off in sleep.  This can cause the body diode to have a drop across it while it is still in sleep.  There also could be a protection occurring that turned off the CHG FET.

    Please double check to ensure that the device is really not in sleep mode and that no protection faults are present.  Then record the voltage and current levels.

    Best,

    Andria

  • 0 in reply to Andria McIntyre
    Intellectual 260 points

    Hi Andria,

    The voltage drop is 4 V on the EVK.  The only load is a multimeter to measure the voltage so nearly 0 current.

    The device is not in SLEEP.  I deliberately send the DISABLE_SLEEP() command and verify that SLEEP is disabled via BatteryStatus() prior to observing this behavior.  The FETs are also verified to be ON via FetStatus().  No faults are active.

    The voltage drop only happens after first enabling SLEEP, then waking from SLEEP via DISABLE_SLEEP(), when source-follower mode is enabled.  Also DEEP_SLEEP is not active.

    Here is a printout from BqStudio showing the voltage drop (Stack vs. Pack) and some strange bits in the Alarm Status which don't appear to correspond with the actual SafetyStatus register values.  I confirmed these bit values are correct by sending independent commands to the device as well.

  • 0 in reply to Christopher Churavy
    TI__Expert 8526 points

    Hi Christopher,

    We have attempted to simulate your problem on our EVM, and the problem is not appearing.  Could you please send me your gg.csv file so I can see all your register settings?

    Best,

    Andria

  • 0 in reply to Andria McIntyre
    Intellectual 260 points

    Hi Andria,

    I don't know that the gg.csv file is relevant in this case because the settings are reconfigured on startup.  The issue seems like a sequence of event problem..  I too was unable to recreate the issue using bqStudio alone (previous image was take after disconnecting from our host MCU and connecting to bqStudio).  The symptoms appear to line up with turning the DSG fet on during SLEEP.  However, in our setup, enabling the FETs after disabling SLEEP after a HWD alert seems to have the same effect.

    I have found a work around for the moment that is sufficient for my purposes.  In my application SLEEP is only enabled when we know the FETs will be off, so rather turning the DSG FET off satisfies our power requirements and there is no issue with a voltage drop after waking back up.

    If I find time to dig into this more, I will post a follow up for anyone experiencing a similar issue. 

  • 0 in reply to Christopher Churavy
    TI__Expert 8526 points

    Hi Christopher,

    I am happy to hear that you found a workaround for your system.  Please keep us updated on further developments in your system if/when they arise!

    Best,

    Andria