LTC3305_15 Datasheet, PDF (14/28 Pages) Linear Technology – Lead-Acid Battery Balancer

LTC3305
Operation
In timer mode, the balancing operation continues for a
maximum time, t_ON, that is programmed by a capacitor at
the CTON pin. After time t_ON the LTC3305 is put in a low
power off state for a fixed time, t_OFF, even though the bat-
teries may not be balanced. The t_OFF time is programmed
by a capacitor at the CTOFF pin. Once t_OFF times out, the
part is put in the ON state and normal operation resumes.
In the OFF state the BAL pin is in its high impedance state.
The BAL pin is also in the high impedance state if the part
is in shutdown.
Continuous Mode (MODE = 1)
In the continuous mode of operation the part functions in
much the same way as in timer mode with the following
differences.
1. There is no ON or OFF state. The CTON and CTOFF pins
must be tied to GND in continuous mode. The balanc-
ing operation continues even if the stack is in balance.
The balancing operation is terminated only if the part
is put in shutdown. The BAL pin is always pulled low
in continuous mode.
2. In timer mode, if the termination comparator senses
that a battery is balanced to the auxiliary cell with only
the bottom plates connected, the balancing operation
on that battery is terminated. This is not the case in
continuous mode. In continuous mode the top switches
are turned on and the balancing operation on a battery
is terminated only by a t_BAT time out. Since the auxiliary
cell remains connected to the battery until a t_BAT time
out occurs, its voltage can change before it connects
to the next battery in the stack. As a result, when the
stack is balanced and the DONE pin is pulled low, the
voltages across the individual batteries in the stack
may differ by more than the programmed V_TERMINATE.
In the worst case when the capacity of the auxiliary cell
is much smaller than the battery, the individual battery
voltages could differ by up to twice the programmed
V_TERMINATE when balanced.
Charge Pump Operation
The LTC3305 uses external NMOS devices as switches to
connect a battery to the auxiliary cell. The LTC3305 has a
charge pump that generates the higher voltage required
to turn on some of the external NMOS switches.
Two external capacitors C_FLY and C_BOOST, two diodes D1
and D2, and resistors R1 and R2 are required for charge
pump operation as shown in Figure 2. When the LTC3305
is enabled, the charge pump is turned on. C_FLY initially
charges with a current I_CHG through external diode D1,
resistor R1, and the internal NMOS switch N1 to GND. When
C_FLY is charged to 10.5V, an internal comparator switches
the internal NMOS switch off and turns on switch P1. C_FLY
connects to C_BOOST through diode D2, resistor R2 and the
internal PMOS switch P1. Charge is transferred from C_FLY
to C_BOOST with a current I_DISCHG. When C_FLY is discharged
to 9.5V, it is disconnected from C_BOOST, recharged back up
to 10.5V, and then reconnected to C_BOOST. In this fashion
the voltage across C_BOOST is built up.
I_CHG
D1
R1
I_DISCHG
C_FLY
CM
P1
N1
CP
BOOST
V4
D2
R2
CBOOST
LTC3305
3305 F02
Figure 2. Charge Pump Operation
3305f
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