LTC3300-1
32
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For more information www.linear.com/LTC3300-1
APPLICATIONS INFORMATION
Boosted Gate Drive Component Selection
(BOOST = V
REG
)
The external boost capacitor connected from BOOST
+
to
BOOST
–
supplies the gate drive voltage required for turning
on the external NMOS connected to G6P. This capacitor
is charged through the external Schottky diode from C6
to BOOST
+
when the NMOS is off (G6P = BOOST
–
= C5).
When the NMOS is to be turned on, the BOOST
–
driver
switches the lower plate of the capacitor from C5 to C6,
and the BOOST
+
voltage common modes up to one cell
voltage higher than C6. When the NMOS turns off again,
the BOOST
–
driver switches the lower plate of the capaci-
tor back to C5 so that the boost capacitor is refreshed.
A good rule of thumb is to make the value of the boost
capacitor 100
times that of the input capacitance of the
NMOS at G6P. For most applications, a 0.1µF/10V capacitor
will suffice.The reverse breakdown of the Schottky diode
must only be greater than 6V. To prevent an excessive and
potentially damaging surge current from flowing in the
boosted gate drive components during initial connection of
the battery voltages to the LTC3300-1, it is recommended
to place a 6.8Ω resistor in series with the Schottky diode
as shown in Figure 3. The surge current must be limited
to 1A to avoid potential damage.
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 LTC3300-1
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 LTC3300-1 and its associated
power components by shutting off all balancing before
any local differential cell voltage reaches its absolute
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 differential
cell voltage
from increasing past 6V. Therefore, the mini
-
mum differential bypass capacitor value for full broken
connection protection is:
C
BYPASS(MIN)
=
I
CHARGE
+I
DISCHARGE
•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 LTC3300-1 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 LTC3300-1 to still
act on the cell voltage seen locally by another LTC3300-1
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 LTC3300-1 with Fewer Than 6 Cells
To balance a series stack of N cells, the required number
of LTC3300-1 ICs is N/6 rounded up to the nearest integer.
Additionally, each LTC3300-1 in the stack must interface
to a minimum of 3 cells (must include C4, C5, and C6).
Thus, any stack of 3 or more cells can be balanced us
-
ing an appropriate stack of LTC3300-1 ICs. Unused cell
inputs (C1, C1 + C2, or C1 + C2 + C3) in a given LTC3300
-1 sub-stack should be shorted to V
–
(see Figure 11).
However, in all configurations, the write data remains at
16 bits. The LTC3300-1 will not act on the cell balancing
bits for the unused cell(s) but these bits are still included
in the CRC calculation.
Supplementary Voltage Regulator Drive (>40mA)
The 4.8V linear voltage regulator internal to the LTC3300-1
is capable of providing 40mA at the V
REG
pin. If additional
current capability is required, the V
REG
pin can be back-
driven by an external low cost 5V buck DC/DC regulator
powered from C6 as shown in Figure 12. The internal
regulator of the LTC3300-1 has very limited sink current
capability and will not fight the higher forced voltage.
