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BQ76920: Unable to clear UV, SCD, and OCD bits in SYS_STAT (0x00)

Yean Khai Eng
Prodigy 40 points
Part Number: BQ76920

Hi,

I am working on a project and I am using the BQ76920 IC to monitor my Li-ion cells condition. The IC keeps reply the 0x0b when I am requesting data from 0x00, even no load is attached. I tried to write "1"s to corresponding bits but those bits never change to "0". May I know if there is any alternative solution that I can try to solve this problem? Thanks!

  • 0
    TI__Guru 54715 points

    Hi Yean,

    If you have a persistent fault the bits will clear and come back.  The delays for OCD and SCD are short and may be set again before you read, but UV delay time is long enough that the UV bit should be cleared long enough to see it clear before setting again.

    For UV check the inputs to be sure the voltage is in range, also check any shorted cells are below a voltage which would give a value below the UVMINQUAL.

    For current be sure there is no offset voltage on SRP/SRN. Also check that REGOUT is stable.

    If the bits really don't clear inspect the bus with a scope to be sure you are sending what you expect to the device.  If the transaction is correct and the bits don't clear perhaps the part has been damaged in some way.

        

  • 0 in reply to WM5295
    Prodigy 40 points

    Hi there,

    Thanks for the reply! 

    I have checked the UV value and it remains default value (UV_THRESH = 0x97, UV is around 2.5V). All cells voltages are around 3.8V (5 cells) and no shorted cell is found. 

    For current side, I connected SRP/SRN to ground. The REGOUT is at 3.3V. 

    However, the bits don't clear even I send corresponding bits to the IC. Will try to get a new BQ76920 IC and try it out again.

  • 0 in reply to Yean Khai Eng
    Prodigy 40 points

    Hi, 

    I replaced another BQ76920 IC and the SCD and OCD bits are cleared. However, the UV bit remains high even all cells voltages are above the undervoltage threshold. 

    May I know if there is any possible cause?

  • 0 in reply to Yean Khai Eng
    TI__Guru 54715 points

    Hi Yean,

    The UV fault takes time to set.  The default delay is 1s.  If the UV bit remains high after sending the clear either the clear command is not getting to the part due to some protocol issue or the part is damaged.  Be sure the controller is sending a "1" to clear.  You might send 0xFF.

    If the clear clears the UV bit but it comes back in 1s, either the IC pins do not see the same voltage as the cells, or an unused cell is above UVMINQUAL volts, or the top cell is not used (VC5 shorted to VC4.  The top cell must be used for proper operation) when reducing the cell count, or the part is damaged.

  • 0 in reply to WM5295
    TI__Guru 54715 points

    Hi Yean,

    Your other post brought up another possibility:  If you have a CRC version device it will not accept your status clear command bit without the proper CRC byte following.  You can read a single byte without the CRC.  Check the version of the part you are using.