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Original question:

BQ78350-R1A: Charge FET toggling

BQ78350-R1A: Charge FET open in a narrow cell voltage range

Jeffrey Wright
Expert 2875 points
Part Number: BQ78350-R1A
Other Parts Discussed in Thread: BQ78350

Hello;

My client reported something curious in how the 78350 is controlling the CHG FET when the cell voltage is low. I'm probably missing something obvious, but could not find an answer. 

Specifically, when they discharged their battery to where the cell volts were above 2.5V, and they reduced the discharge current to below the discharge threshold (programed for 2A), the 78350 opens the CHG FET...but then closes it again after a second(?)

BUT, when they discharged their battery to where the cell volts were below 2.5V (but still above its CUV threshold of 2.3V), it continues to keep the CHG FET on until the discharge current  falls below the threshold, but does NOT close it again until Vcell recovers/rebounds to ~ 2.9V ???  The issue is that, if a discharge terminates with cell volts  CUV < Cell Volts < 2.5, the CHG FET would be open, and disallow a charger from recharging the battery until/unless the cells rebounded by themselves to ~2.9V.  Is this the expected behavior? I can find no documentation for this behavior.

There is no UV fault triggered. This is repeatable, and it appears that this is voltage based, in the range of 2.5 to 2.9.

As a work-around, I modified FET Options.CHGFET defaults to 1 = disabled at EOC.  But we do want the to open the CHG FET at EndOfCharge, so this may be undesired.

Thanks

  • 0
    TI__Guru* 81035 points

    Hi Jeffrey,

    Are they able to capture the status registers of the BQ78350 during this behavior. This will usually shed light on internal conditions that might trigger the decision to open the CHG FET. 

    If they are going into Sleep mode for example, the default setting for the device (see the SLEEPCHG bit description) is to turn off the CHG FET while the device is in Sleep mode. I think this bit is set to the default value in the last version of the data flash settings I have seen from them.

    Best regards,

    Matt

  • 0 in reply to Matt Sunna
    Expert 2875 points

    Hi Matt;

    Yes, I was able to duplicate the behavior on my simulator, and it was not entering sleep mode, nor was any AFE status flag thrown. That's why it was so odd. My band-aid solution keeps the fet on, but like I said, that's not the desired behavior.  I'll try reproducing it again and send some screen captures....

    Thanks

  • 0 in reply to Matt Sunna
    Expert 2875 points

    Hi Matt;

    I've uploaded a screen capture of OperationStatus(), and it shows it's not going into sleep mode. Curiously, I did find that this behavior is masked if I clear bit 0 in FET Options.PCHG_EN (???), despite the fact that we're not using PCHG

    What's odd is that the CHG FET is opening at the end of a discharge , but only if Vcell is below 2.9V..?

    There are no Alerts or Status flags thrown.

    And since SMBus is pulled high via 2 R's (u/C is the master), I read that sleep will not be entered: SMBus low for BusTimeout(1) 

    What could cause the 920 to open the CHG FET at the end of discharge, but only if Vcell is below ~2.9V???

    Signed, purer Wahnsinn!!

    5444.Doc1.docx

  • 0 in reply to Jeffrey Wright
    TI__Guru* 81035 points

    Hi Jeff,

    I think the PCHG_EN bit might be a big clue. This directs the device to go through 2 different charging phases that are shown in Section 5.2 of the TRM (pre-charging and then fast charging which is with the CHG FET on). When the device is in the pre-charge phase, it will use the PCHG pin to control a pre-charge FET (if there is no pre-charge FET in your battery design, it would do nothing). Once the voltage is above the Pre-charge Recovery voltage (default is 2.9V), then it will turn on the CHG FET for fast charging.

    Matt

  • 0 in reply to Matt Sunna
    Expert 2875 points

    Thanks Matt;

    Yes...sounds like that's it. I guess the 78350 must automatically transition to its charge algorithm at the end of a discharge.