LTC3305_15 Datasheet, PDF (12/28 Pages) Linear Technology – Lead-Acid Battery Balancer

LTC3305
Operation
The LTC3305 balances batteries using an auxiliary cell or an
alternate storage cell as a charge reservoir. External NMOS
switches are controlled in a preprogrammed sequence
to connect each battery in the stack to an auxiliary cell.
Charge is transferred to or from the auxiliary cell when it
is connected to a battery.
The LTC3305 can operate in one of two modes pro-
grammable via the MODE pin.
Timer Mode (MODE = 0)
The balancing operation begins once the C_BOOST capacitor
is charged to at least 6.95V. The BAL pin is pulled low,
indicating that the part is enabled and balancing the bat-
tery stack.
The termination voltage, V_TERMINATE, is the difference in
voltage between the auxiliary cell and the battery connected
to the auxiliary cell for which a battery is considered to
be balanced. V_TERMINATE is programmed via the TERM1
and TERM2 pins to one of four preset voltages as shown
in Table 1.
Table 1. Termination Voltages
TERM1
0
TERM2
0
V_TERMINATE
±12.5mV
1
0
±25mV
0
1
±50mV
1
1
±100mV
The balancing operation begins with the negative terminal
of the auxiliary cell connected to the negative terminal of
BAT1, the lowest battery in the stack. Referring to Figure 1,
the bottom switches N1 and N9 that connect the negative
terminal of BAT1 to the auxiliary cell’s negative terminal are
first turned on. To turn on an external NMOS switch, the
9
NGATE1-9
NGATE6
NGATE5
N5
NGATE4
+
BAT4
LTC3305
N6
NGATE7
N4
NGATE3 ₊
BAT3
N3A
AUXP
PTC _N7
+
AUX N8
AUXN
N2
NGATE8
N9
N3B
NGATE2
NGATE1
+
BAT2
+
BAT1
N1
NGATE9
3305 F01
Figure 1. External Switch Arrangement
for a 4- Battery Balancing Application
current source at the NGATE pin connected to the gate of
the external NMOS is turned on and a gate source voltage
is developed across an external resistor.
After an internally set delay of 35ms, the voltages across
the auxiliary cell and BAT1 are compared by the termina-
tion sense comparator.
If the voltage difference between the auxiliary cell and BAT1
is less than the selected termination voltage, the battery
is deemed to be balanced with respect to the auxiliary cell
and the bottom switches are turned off by turning off the
corresponding NGATE pin currents. The next battery in
the stack is then connected to the auxiliary cell.
3305f
12
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