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BQ78350-R1: reduce the voltage ,SMBUS communication is disconnected

Laker Lian
Intellectual 290 points
Part Number: BQ78350-R1
Other Parts Discussed in Thread: BQ78350, BQ76930, , BQ76200, ISO1540

Hello:

I have a 6s  battery protection board is made of bq78350 and bq76930,when I reduce the voltage to 2.75 V/Cell, SMBUS communication is disconnected,but when I raise the voltage, the communication returns to normal.  when the communication is disconnected, the VCC voltage of Bq76930 is normal 2.5V for bq78350.

So I wonder what caused this?

,

  • 0
    TI__Guru 54715 points

    Hi Laker,

    Since you have 2.5V at the bq78350 the gauge is apparently still running.   One area to look may be the signal reference for your SMBus.  If you have used low side FETs and referenced the communication to PACK-, if there is a protection event the signal reference will be lost and the battery will not communicate.  CUV would be the apparent protection when reducing voltage, the default value in the bq78350-R1 is 2500 mV, you might check if your system has this set at 2750.  Another related area to look at is the REGSRC circuit and FET drive voltage.  If you have used the dropping transistor to provide REGSRC the voltage may be sufficient to provide the 2.5V REGOUT but too low to provide and adequate voltage to keep the FETs on.  If this is the issue, with 6 cells consider using the battery voltage for REGSRC, see section 9 in bq769x0 Family Top 10 Design Considerations apnote.

    With the low side switching and the communication interface there are often 3 areas of concern with protection: Possible damage to the circuits, leakage through the communication lines, and loss of communication.

    If communication is not needed during fault you may consider switching the comm lines such as figure 10 of Using the bq78350.  If communication is needed during fault you might consider isolation  or high side switching.  The ISO1540 isolator is an option for communication lines.  High side P-channel switches are possible or high side N-channel using a part such as bq76200 high side N-channel FET driver.

    Check the other signaling, if it is a different issue and you have questions let us know.