Linear Technology DC2100A Demo Manual page 13

Quick start proceDure
A similar event occurs when Cell 6 is operating in the
Charge Mode and other cells controlled by the U1, the lower
LTC3300-1, and U2, the upper LTC3300-1 are operational,
an overvoltage can occur on Cell 6. The overvoltage on
Cell 6 will shut down the operation of Cell 1 – Cell 6 but
Cell 7– Cell 12 will continue to operate. The overvoltage
sensing circuit Q5, Q6, D12, D13 and R58 will turn off
the operations of Cell 7 – Cell 12 through the internal
overvoltage protection circuit within the LTC3300-1 of U2.
Cell Bypass Capacitors
The DC2100A contains bypass capacitors between the cell
connections and the stack connections. These capacitors
are intended to smooth the high current triangular wave
before they travel down the interconnection wires to the
cells within the stack and the secondary stack connections.
The RMS current rating of these capacitors is a critical
parameter as well as their capacitance with applied voltage
DEMO MANUAL DC2100A
and their physical size. The capacitance of MLCC capaci-
tors decreases with applied voltage and this must be taken
into account when selecting the capacitance value. The
value of the cell bypass capacitor is outline on page 31 of
the LTC3300-1 data sheet. Larger size, 1812 and above,
MLCC capacitors have a tendency to crack and short due
to thermal expansion and physical stress.
Each cell must have an equal capacitance between cells
to prevent an overvoltage condition across the cell when
randomly connecting cells to the LTC3300-1 battery bal-
ancer circuit. There are also capacitors across adjacent
cells that act a reservoir of charge for the cell's FET Gate
turn on circuit. These capacitors must also be of equal
value to maintain the balancing of voltage and a capacitor
of 1/2 the value needs to be connected between Cell 1 and
–
V of the lowest LTC3300-1 and from the top cell to the
cell below it to insure a voltage across all cells when the
battery stack is initially connected.
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