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BQ76930: Balancing external

Tomas Coggio
Prodigy 40 points
Part Number: BQ76930

HI,

IN SLUUB41.PDF

9.7 Can the bq769x0 support external balancing? Yes, external N-channel or P-channel FETs may be used. This allows maintaining large input filter resistors for better transient filtering. The system designer should be aware that some combinations of balancing may not work with external balancing.

For BQ76930 in a 7 Li Cell , balancing Reg 0x01 and 0x02  reset to 0x00. I'm using P-Channel FETs with 100 Ohm Resistors (see schematic) when I read Cell 0 (0x0C) I read Half the true Voltage. 

I'm unsure wether the port outputs a 0 or whether it disables internal drive to the port as I cannot find much info on this. In any case to turn on the Fets I need a 0 drive for P- Fets and a 1 if I had used N -Fets.

Because the note says not to balance adjacent Fets and if the drive is 0 at operation, is it possible the device has been damaged by these settings?

Regards,

Tom

  

BQCellBal2.pdf 

  • 0
    TI__Prodigy 590 points
    Hello Tom,

    Thank you for your questions. when the related bit has been set to 1, the balancing has been enable for that channel.

    I will invite our expert to help on your further questions.
  • 0
    TI__Guru* 81034 points
    Hi Tom,

    The circuit looks okay. As Jessica noted, when the bit has been set to 1 for a channel, balancing is enabled. This is illustrated in Figure 5 of this app note (www.ti.com/.../slua749a.pdf). It is possible to damage the device if all bits have been set to 1.

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

    Thanks Jessica & Matt for your response.

    The BQ's VC0 has a static internal short to VC1 this appears to be the problem as the return read voltage is half that of the battery as the current is divided by the 2 x1K resistors in the path via R46- VC1-short-VC0 - R58 to GND. The damage appears to be the result of the random connection to the device as the Bat GND is at the bottom end of the connector (not detailed in the schematic) it could be the last one to make if the connector is plugged in top - bottom.

    Thanks again.

    Tom