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BQ76930: Cell Balancing and Discharge Driver not working

Lasse Froehner
Prodigy 40 points
Part Number: BQ76930

I am using a BQ7693006DBTR (no CRC) as an AFE for a 6s Li-Ion battery pack.
Although cell voltages read correctly, I am not able to activate any balancing circuit.

A few random remarks (not sure what is relevant):

  • the cells are connected as recommended in the datasheet (Table 8-2)
  • balancing should be achieved through an external N-channel balancing network like described in the Design Recommentations (Figure 6)
  • the balancing network works as expected when shorting the respective cell input pins (bypassing the internal balancing FET)
  • no fault is active in SYS_STAT
  • ADC is enabled (Bit 4 in SYS_CTRL1), CC is disabled (Bits 5-6 in SYS_CTRL2)
  • cell balancing bits (CBx in CELLBAL registers) read back as intended after writing
  • activating the CHG FETs work, activating the DSG FETs doesn't work (DSG_ON in SYS_CTRL2 reads back as '1', though)
  • load detection circuit works (LOAD_PRESENT in SYS_CTRL1)

Since both balancing and activating the discharge circuit don't seem to work, I suppose I've missed deactivating some kind of protection circuit, although I can't find any reference to anything similar anywhere.

What am I missing?

Kind Regards
Lasse

  • 0
    TI__Guru* 81034 points
    Hi Lasse,

    It's possible there is a fault that is opening the DSG FET. This could be an under-voltage, over-current, or short circuit fault (these faults disable the DSG FET but leave the CHG FET enabled). Can you read the SYS_STAT register to see if any faults are present?

    Thanks,
    Matt
  • 0 in reply to Matt Sunna
    Prodigy 40 points

    Hi Matt,

    Matt Sunna said:
    Can you read the SYS_STAT register to see if any faults are present?

    at no point any fault is active in SYS_STAT. After each command (enable balancing / FETs) I read back the SYS_STAT register contents, and during the whole time it stayed 0x00 (no faults active).

    KR Lasse

  • 0 in reply to Lasse Froehner
    TI__Guru* 81034 points
    Hi Lasse,

    I apologize, I missed your statement in the original post about SYS_STAT showing no faults. Can you send your schematic? From your description, the DSG FET should be on, so I am wondering if there is anything preventing the FET from turning on.

    There is another document that might be helpful (you may already know about this) that goes into more detail on the topics of FET Drive and Cell Balancing: www.ti.com/.../slua749a.pdf

    Best regards,
    Matt
  • 0 in reply to Matt Sunna
    Prodigy 40 points

    Hi Matt,

    Matt Sunna said:
    Can you send your schematic? From your description, the DSG FET should be on, so I am wondering if there is anything preventing the FET from turning on.

    sure, see below. It's basically the circuit from the Reference Design tiduar8b, tailored to our 6s pack.

    Matt Sunna said:
    There is another document that might be helpful (you may already know about this) that goes into more detail on the topics of FET Drive and Cell Balancing: www.ti.com/.../slua749a.pdf

    I am aware of that document, too, but have not been able to find an indication to why my FETs do not turn on.

    Best Regards
    Lasse

  • 0 in reply to Lasse Froehner
    TI__Guru* 81034 points
    Hi Lasse,

    The Precharge FET is grouped with the Discharge FETs. Is this intended? I don't think this is causing the issue you are seeing, but just wanted to bring it to your attention.

    From your schematic, I do not see anything that would prevent the DSG FET from turning on. I did notice that the signal names are different between the schematic pages: "CHG" and "DSG" become "CHARGE" and "DISCHARGE". Is there any circuit in between? Is it possible the DISCHARGE signal is shorted low? It might be good to measure the current into REGSRC to see if it changes when DSG is high vs. low.

    I also do not see any reason for balance not to work from the schematic. Writing 0x01 to the cell balance bits should cause the low cell to balance. A low impedance source is needed to see the current draw and for the LED to light.

    Matt