bq769x0 Family Top 10 Design Considerations

bq769x0 Family Top 10 Design Considerations

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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.

2 Cell Count

The bq769x0 family devices use a common register architecture to allow a host implementation to easily move across different cell count configurations. It would seem attractive to make a single board which could support any cell count supported by the AFE family. From the controller and firmware standpoint this should work well, however, there are hardware considerations which may make this unattractive. One is the inefficiency of having the board space for 15 cells when fewer cells are actually used. A second consideration is the package pitch difference between the bq76920 (0.65 mm) and the bq76930 and bq76940 (0.5 mm). A common footprint might be constructed with offset patterns similar to Figure 1, but this may have complications in component placement and routing. A third is the circuit structure differences resulting from the architecture of the family devices, details of this are described in following sections. A common board design between the bq76930 and bq76940 may be more practical; this was done on the bq76930 and bq76940 EVM boards. The cell count will generally indicate the device to use, see the datasheet for supported cell counts. In cases where there is an overlap in the cell count supported between devices such as 9 or 10 cells the selection might be based on the lower cost part or the desirability of an extra temperature sensor.

Figure 1. Common Footprint Concept