SLVU957A September 2019 – November 2021 BQ76922
The BQ76922 must have the top and bottom cells used for proper operation. Cell count can be reduced by shorting the unused cell inputs from the next-to-top down. Cell count can be reduced for basic evaluation by shorting unused cells at the input terminal block. Follow the recommendations in the datasheet for which cells to short. This works for both operation with the cell simulator and cells, but can have some side effects in transient tests because it parallels the shorted resistors to the cell IC where the capacitor provides a signal path to the used input. See Figure 5-4 for an example of simple reduced cell configuration for 3 cells. For the best evaluation with reduced cells in a transient environment, short the VCx pins at the capacitor and remove the unused input resistor. When using the cell simulator, shorting the unused cell at the terminal block is still required to eliminate the simulated cell voltage. Shorting the cell inputs at the terminal block screw terminals is suggested since it should be apparent if the board is re-used for a different cell count. Table 5-1 shows configuration recommendations for reduced cell count.
Unused Cell (Numbered from Bottom Cell 1) | Short Cell Input Terminals | Input Resistor To Remove | Replace Capacitor With 0 Ω | IC Inputs Shorted |
---|---|---|---|---|
Cell 4 | CELL4 to CELL3 | R4 | C5 | VC4 to VC3 |
Cell 3 | CELL3 to CELL2 | R5 | C7 | VC3 to VC2 |
When reducing the cell count with the top cell used, the resulting configuration of the BQ7718 secondary protector does not match its data sheet configuration. While the circuit typicaly function on the EVM, when implementing a design the configuration of the data sheet is recommended.
After changing the physical connection the Vcell Mode must be changed in Data Memory Settings for proper operation. The default setting of 0x0000 selects all cells and will be the same as 0x001F. For 3 cells as shown above set the Vcell Mode to 0x0013.