SLUA749A July   2015  – May 2016 BQ76920 , BQ76930 , BQ76940

 

  1.   bq769x0 Family Top 10 Design Considerations
    1.     Trademarks
    2. 1  Host Controller Choice
    3. 2  Cell Count
    4. 3  Device Architecture
    5. 4  Cell Balance
    6. 5  XREADY
    7. 6  FET Drive
    8. 7  Load Detect
    9. 8  Low-Side Switching Considerations
    10. 9  REGSRC Supply
    11. 10 Random Cell Connection – Within Limits
    12. 11 References
  2.   Revision History

1 Host Controller Choice

The bq769x0 is not a standalone protector and will require a host. The bq769x0 AFE will measure voltages and monitor voltage faults based on those voltages as well as monitor current faults. However, a host is required to set the registers for the protection thresholds and to turn on FET control outputs. If current measurement is desired, the host must read the values from the AFE. When faults occur, the host must clear the faults and recover when appropriate. While the AFE could be set and left to operate unsupervised, a fault would leave the battery disabled. The selection of the host will influence the appropriate AFE version.

The bq78350 is a gauge designed to control the bq769x0 AFE. It has pre-programmed behavior determined by the selection of available parameters. Protection limits can be set in the firmware. The gauging uses a Compensated End of Discharge Voltage (CEDV) algorithm which uses coefficients calculated from data collection runs with the pack design. Communication to the gauge is through SMBus. The gauge handles all communication to the AFE and does not share the I2C bus to the AFE or allow modification of the AFE registers. The bq78350 is a 2.5-V device and at its initial release supports the -00 and -01 2.5-V output AFEs, be sure to check the latest bq78350 information for options supported.

A customer provided MCU can be used with the AFE. This allows the MCU programming to provide special system behaviors for the battery. The MCU must set the protection registers, enable FETs, recover from faults, and provide a balancing algorithm, if desired. The MCU can implement a gauging algorithm and provide display or communication appropriate for the battery system. In addition to the cell voltage readings from the AFE the MCU may measure battery voltage for a real-time calibration as described in the bq769x0 datasheet and pack voltage, if desired. Any of the bq769x0 device options may be used as appropriate for the MCU.

Related to the MCU is the selection of a boot method for the AFE. The bq769x0 datasheet shows a simple concept of using a switch to boot the device when ready. If the MCU is powered from a separate regulator, it can put the AFE to sleep and boot the AFE as desired. With the bq78350, a circuit to provide a pulse is needed to boot the AFE and start the bq78350.