LT8584
11
8584fb
For more information www.linear.com/LT8584
Many systems use multiple battery cells connected in se-
ries to increase the available capacity and voltage. In such
systems,
the individual battery cells must be constantly
monitored to ensure that they operate within a controlled
range. Otherwise, the battery’s capacity and life span may
be compromised. Linear Technology offers the LTC680x
family series of multicell battery stack monitors (BSM) to
accomplish this task.
The LTC680x monitors each individual cell in the stack
and communicates this information through a proprietary
serial bus to a central processing unit. As a cell begins to
reach the upper charge limit, commands are issued to the
LTC680x to turn on that cell’s passive shunt, bypassing
the charging current to that cell and allowing the current to
continue to the rest of the cells. The passive shunt current
and/or power capability constrains the maximum charging
current for the battery stack. Using a passive shunt is also
inefficient, and the shunted current produces considerable
heat at higher charging currents.
The LT8584 solves the two limitations of passive shunting
balancers by actively shunting the charging current and
returning the energy back to the battery stack. Instead of
the energy being lost
as heat, it is reused to charge the
rest of the batteries in the stack. The architecture of the
LT8584 also solves the problem of reduced run time when
one or more of the cells in the stack reaches the lower
safety voltage threshold before the entire stack capacity
TiMing DiagraM
operaTion
is extracted. Only active balancing can redistribute the
charge from the stronger cells (cells with higher voltage)
to charge the weaker cells. This allows the weaker cells to
continue to supply the load, extracting greater than 96%
of entire stack capacity where passive balancing may only
extract 80%.
The LT8584 has an integrated 6A switch designed to operate
as a boundary mode flyback converter and provides 2.5A
average discharge current. The average discharge current
is also scalable by using multiple LT8584s to balance each
cell. Note that each battery in the stack requires an LT8584
active cell balancer.
The LT8584 flyback topology allows the charge to return
between any two points in the battery stack. Most applica
-
tions use
a module approach and return the charge to a
local
set of 12 series-connected cells monitored by a 12
channel BSM IC, where the output of the flyback converter
is designated as V
MODULE
. The entire battery stack is then
constructed using several 12-cell modules connected in
series. A second approach is to return the charge to the
entire battery stack, where the flyback output is desig
-
nated as V
STACK
. A final option is to return the charge to
an auxiliary power rail, designated as V
AUX
.
The LT8584 has two modes of operation—selectable by
the MODE pin—that can be integrated with the LTC680x
or other battery stack system. In simple mode, the LT8584
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