The BQ76922 device supports a random connection
sequence of cells to the device during pack manufacturing. For example, cell-4 in a
5-cell stack might be first connected at the input terminals leading to pins VC4B
and VC3A, then cell-1 may next be connected at the input terminals leading to pins
VC1 and VC0, and so on. It is not necessary to connect the negative terminal of
cell-1 first at VC0. As another example, consider a cell stack that is already
assembled and cells already interconnected to each other, then the stack is
connected to the PCB through a connector, which is plugged or soldered to the PCB.
In this case, the sequence order in which the connections are made to the PCB can be
random in time, they do not need to be controlled in a certain sequence.
There are, however, some restrictions to how the cells are connected during
manufacturing:
- To avoid misunderstanding,
note that the cells in a stack cannot be randomly connected to
any VC pin on the device, such as the lowest cell
(cell-1) connected to VC5, while the top cell (cell-5) is connected to
VC1, and so on. It is important that the cells in the stack be connected in
ascending pin order, with the lowest cell (cell-1) connected between VC1 and
VC0, the next higher voltage cell (cell-2) connected between VC2 and VC1,
and so on.
- The random cell connection
support is possible due to high voltage tolerance on pins VC1–VC5.
Note: VC0 has a lower voltage
tolerance. This is because VC0 should be connected through the
series-cell input resistor to the VSS pin on the PCB, before any cells
are attached to the PCB. Thus, the VC0 pin voltage is expected to remain
close to the VSS pin voltage during cell attach. If VC0 is not connected
through the series resistor to VSS on the PCB, then cells cannot be
connected in random
sequence.
- Each of the VC1–VC5 pins
includes a diode between the pin and the adjacent lower cell input pin (that
is, between VC5 and VC4A, between VC4B and VC3A, and so on), which is reverse
biased in normal operation. This means an upper cell input pin should not be
driven to a low voltage while a lower cell input pin is driven to a higher
voltage, since this would forward bias these diodes. During cell attach, the
cell input terminals should generally be floating before they are connected
to the appropriate cell. It is expected that transient current will flow
briefly when each cell is attached, but the cell voltages will quickly
stabilize to a state without DC current flowing through the diodes. However,
if a large capacitance is included between a cell input pin and another
terminal (such as VSS or another cell input pin), the transient current may
become excessive and lead to device heating. Therefore, it is recommended to
limit capacitances applied at each cell input pin to the values recommended
in the specifications.