bq76920, bq76930, bq76940 AFE FAQ Question

Hi Matt,

If 2 adjacent cells are balanced, the VCx pin between the cells does not change significantly in voltage. The differential input voltage of the next cell effectively doubles.

==>What is the meaning?

When a cell is balanced, an internal FET puts a small resistance between the inputs. If all resistances match well and only 1 cell (cell(n+1)) is balanced VC(n+1) moves down by 1/2 the cell voltage and VCn moves up by 1/2 the cell voltage. If 2 adjacent cells (cell(n+1) and cell(n)are balanced, VC(n+1) moves down 1 cell voltage, VC(n) does not move, and VC(n-1) moves up 1 cell voltage. So VC(n+2) to VC(n+1) is 2x Vcell, VC(n+1) to VC(n) is 0, VC(n) to VC(n-1) is 0, and VC(n-1) to VC(n-2) is 2x Vcell.

As described at 9.5:

As an example, assume all cells are at 4 V. If cells 4 and 5 are balanced, the VC3 input pulls to VC4,

which is approximately the cell 4 voltage or 16 V.

VC0--(cell 1)--VC1--(cell 2)--VC2--(cell 3)--VC3--(cell 4)--VC4--(cell 5)--VC5

==>

VC3, VC4, VC5 at the same voltage?

Yes

what is the voltage of VC0, VC1, VC2, VC3, VC4, VC5 when cell4, cell5 are balanced?

VC5 = VC4 = VC3 = 16V

VC2 = 8V

VC1 = 4V

VC0 = 0V

How come VC3 input pulls to VC4? VC4 input pulls to VC3???

Because cell 4 connects between VC4 and VC3 and both cell 4 and cell 5 are being balanced.

VSS-VC0 above the absolute maximum is 3.6V? Does it mean that cell 1 cannot be balanced? because that VC0 pulls to the cell 1 value of 4 V?

Cell 1 can be balanced. If cell 1 is balanced and cell 2 is not, VC0 = VC1 = Vcell1/2. For 4V VC0 = VC1 = 2V and all is fine.

If you try to balance cell1 and the adjacent cell 2, VC0 = VC1 = VC2 = 4V and VC0 has exceeded abs max. Don't balance adjacent cells.