Advertisement
DEMO MANUAL DC2064A
Quick start proceDure
Cell Bypass Capacitors
The DC2064A contains bypass capacitors from the cell
connections and the stack connections. These capacitors
have a dual function of smoothing the high triangular
current wave before the current travels down the inter-
connecting wires to the cells and they also help balance
the voltage between cells when hot-plugging cells in a
random order. The RMS current rating of these capaci-
tors is a critical parameter for these bypass capacitors as
well as their physical size. These high triangular current
waveforms produce an RMS current that passes through
the capacitors which result in an internal heat rise. Larger
physical size MLCC capacitors have higher RMS current
rating due to their greater surface area to dissipate the
heat rise. The capacitance of MLCC capacitors decreases
with applied voltage and this must be taken into account
when selecting the capacitance value. If a connection is
lost during balancing, the differential voltage seen by the
LTC3300-1 power circuit on each side of the break may
increase or decrease in depending on whether the power
stage is charging or discharging and where the break
occurred. The worst-case scenario is when the balanc-
ers on each side of the break are active and balancing
in opposite directions. Here the differential voltage will
increase rapidly on one side and decrease rapidly on the
12
other. The LTC3300-1 contains an overvoltage protection
comparator which monitors the cell voltage and will shut
down all balancers before the differential voltage on any
cell input reaches the maximum absolute voltage rating.
Each cell node must have an equivalent capacitance across
it to prevent an overvoltage condition when randomly con-
necting cells to the LTC3300-1 battery balancer circuit. In
addition to the smoothing capacitors across each balancer
power circuit, there are capacitors across the Cx pins of the
LTC3300-1 to reduce high frequency noise on these pins
and capacitors across adjacent cells to act as a reservoir
of charge for the cell's MOSFET gate driver circuits. These
reservoir capacitors must also be of equal value to maintain
the balancing of voltage and a capacitor of 2x the value
of the reservoir capacitor must be connected between C1
–
and V
of the lowest LTC3300-1 and from the top cell to
the cell below it to insure an equal voltage across all cells
when the battery stack is initially connected. Figures 25
and 26 detail these capacitor connections and their values.
The reservoir capacitors must be large compared to the
capacitors across the Cx pins to force the MOSFET gate
charging current to flow through the reservoir capacitors.
An effective 10:1 ratio between these cell capacitors was
selected when considering that a capacitor across two
cells would result in a 5:1 ratio.
dc2064afa
Advertisement
