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bq78350 - bq76930: Undocumented CHG pin periodical behavior

Don Selinsky
Prodigy 30 points
Part Number: BQ76930
Other Parts Discussed in Thread: BQ78350-R1, BQSTUDIO

Hi,

In a scenario where the bq76930 is configured with 6 cells, with all other settings being OK, data flash protection and PF settings all cleared to ease testing, we only had difficulty with CHG pin.

When 0x001F is written to 0x0 to set the pin high, the pin would go high for several hundred milliseconds, then low for over 10 seconds, then high, and continues this periodically.

To isolate the pin behavior, we removed Q5 in the PWR524A EVM which is connected to the CHG pin. We temporarily had some difficulty in resoldering it back, so what was described was only and exactly the behavior of CHG pin alone.

 

As Q5 is removed, the CHG is not applied to NFET Q4, so there is no actual charging current. We suspect the periodical behavior (the CHG is not always high) is due to some undocumented design: the DAC checks the current sensor, and drives CHG pin low and waits for several seconds for another test. We could not find any pertinent description in the documents.

 

This is urgent. Please kindly give us detail accounts of the CHG FET driver’s behavior.

 

 

Don

  • 0
    TI__Guru 54715 points
    Hi Don,
    The bqStudio talks to the bq78350-R1 which controls the FETs. The gauge does have a FET body diode protection feature which is not described in the TRM. The gauge experts will be better able to assist with the test mode and how it may interact with different parameter settings, I will attempt to move to their forum and rename so they recognize it..
  • 0 in reply to WM5295
    TI__Guru 73830 points
    The CHG_FET_TOGGLE option will just override the normal FET_EN control and it should turn the CHG FET on continuously. The pulsed operation that you describe looks like an over current condition where the gauge is disabling the FET and then retrying after the retry time. It is odd that the gauge is reporting a -1594mA discharge current and the DSG FET is disabled.
  • 0 in reply to WM5295
    Prodigy 30 points
    WM5295,

    Some update: although we have not been able to discharge battery pack low enough, we did added back Q5 and everything else (D2,D3,R7,R8,R9) with good connection. With these restored connections as in the EVM, the CHG pin still shows the periodically pulling low behavior (same as above).

    Is that "body protection feature" responsible for this? And could the design team share more detailed info (like a shcematic, via forum message)?

    We are happy waiting for this online.

    Don
  • 0 in reply to ThomasCosby
    Prodigy 30 points
    Thomas,

    1. "The gauge does have a FET body diode protection feature which is not described in the TRM". Could this be responsible for the periodical pulse at CHG? Could you describe more details of this (or via forum message)?
    2. "CHG_FET_TOGGLE": seems there is no this option in TRM. Is this just some acronym you used? I assume you are referring to 0x001F and 0x0020?
    3. We are checking the -1594mA problem now. Will update on this.

    Don
  • 0 in reply to Don Selinsky
    TI__Expert 6085 points

    Don,

    On the CHG FET, what is the resistance between the Source and Gate? You may require a fast shut down of the CHG FET. Try using a stronger pull-down with

    about 250K-400K.