LTC3300-2_15 Datasheet, PDF (30/42 Pages) Linear Technology – Addressable High Efficiency Bidirectional Multicell Battery Balancer

LTC3300-2
APPLICATIONS INFORMATION
Sizing the Cell Bypass Caps for Broken Connection
Protection
If a single connection to the battery stack is lost while bal-
ancing, the differential cell voltages seen by the LTC3300-2
power circuit on each side of the break can increase or
decrease depending on whether charging or discharging
and where the actual break occurred. The worst-case
scenario is when the balancers on each side of the break
are both active and balancing in opposite directions. In
this scenario, the differential cell voltage will increase
rapidly on one side of the break and decrease rapidly
on the other. The cell overvoltage comparators working
in conjunction with appropriately-sized differential cell
bypass capacitors protect the LTC3300-2 and its asso-
ciated power components by shutting off all balancing
before any local differential cell voltage reaches its abso-
lute maximum rating. The comparator threshold (rising)
is 5V, and it takes 3µs to 6μs for the balancing to stop,
during which the bypass capacitor must prevent the dif-
ferential cell voltage from increasing past 6V. Therefore,
the minimum differential bypass capacitor value for full
broken connection protection is:
( ) _{CBYPASS(MIN) =}
I_CHARGE +I_DISCHARGE
6V – 5V
• 6µs
If I_CHARGE and I_DISCHARGE are set nominally equal, then
approximately 12µF of real capacitance per amp of balance
current is required.
•••
Protection from a broken connection to a cluster of sec-
ondary windings is provided local to each LTC3300-2 in
the stack by the secondary winding OVP function (via
WDT pin) described in the Operation section. However,
because of the interleaving of the transformer windings
up the stack, it is possible for a remote LTC3300-2 to still
act on the cell voltage seen locally by another LTC3300-2
at the point of the break which has shut itself off. For this
reason, each cluster of secondary windings must have
a dedicated connection to the stack separate from the
individual cell connection that it connects to.
Using the LTC3300-2 with Fewer Than 6 Cells
To balance a series stack of N cells, the required number
of LTC3300-2 ICs is N/6 rounded up to the nearest integer.
Since the LTC3300-2 address is 5 bits, the maximum N can
be is 192 cells. Additionally, each LTC3300-2 in the stack
must interface to a minimum of 3 cells (must include C4,
C5, and C6). Thus, any stack of between 3 and 192 cells
can be balanced using an appropriate stack of LTC3300-2
ICs. Unused cell inputs (C1, C1 + C2, or C1 + C2 + C3) in a
given LTC3300-2 sub-stack should be shorted to V^– (see
Figure 10). However, in all configurations, the write data
remains at 16 bits. The LTC3300-2 will not act on the cell
balancing bits for the unused cell(s) but these bits are still
included in the CRC calculation.
•••
•••
C6
C5
C4
LTC3300-2
C3
C2
V^{– C1}
+
CELL n + 4
+
CELL n + 3
+
CELL n + 2
+
CELL n + 1
+
CELL n
C6
C5
C4
LTC3300-2
C3
C2
+
CELL n + 3
+
CELL n + 2
+
CELL n + 1
+
CELL n
V^{– C1}
C6
C5
C4
LTC3300-2
C3
+
CELL n + 2
+
CELL n + 1
+
CELL n
C2
V^{– C1}
30
•
•
•
•
•
•
•
•
• ^{33002 F10}
(10a) Sub-Stack Using Only 5 Cells (10b) Sub-Stack Using Only 4 Cells (10c) Sub-Stack Using Only 3 Cells
Figure 10. Battery Stack Connections for 5, 4 or 3 Cells
For more information www.linear.com/LTC3300-2
33002f