LTC3305_15 Datasheet, PDF (13/28 Pages) Linear Technology – Lead-Acid Battery Balancer

LTC3305
Operation
If the voltage difference between the auxiliary cell and
BAT1 is greater than the selected termination voltage, the
top NMOS switches N2 and N7 that connect the positive
terminal of BAT1 to the auxiliary cell’s positive terminal
through the PTC thermistor are turned on. After a second
internally set delay of 35ms, the termination sense com-
parator starts monitoring the voltages across the auxiliary
cell and the battery. The battery stays connected to the
auxiliary cell until the voltage difference decreases to
V_TERMINATE or a t_BAT timeout occurs (t_BAT is the maximum
time that a battery remains connected to the auxiliary cell
and is programmed by a capacitor on the CTBAT pin). This
timer is reset each time the auxiliary cell is connected to
a different battery.
At this point, all switches are turned off and the next bat-
tery in the stack is connected to the auxiliary cell. After
the switches have been turned off, an internal 40ms delay
provides a break-before-make time after which the nega-
tive terminal of the next battery in the stack is connected
to the negative terminal of the auxiliary cell via its bottom
switches. Table 2 shows the top and bottom switches
used to connect each battery for different battery stack
configurations. When only the bottom switches are on,
there is a conduction path between the auxiliary cell and
the battery through the body diodes of the top switches.
Current will flow through this conduction path if the aux-
iliary cell and the battery are more than two diode drops
apart. The current is limited by the PTC resistor.
Table 2. Top and Bottom Switch Arrangement
EN1, EN2
BATTERY BEING
BALANCED
TOP
SWITCHES
BOTTOM
SWITCHES
1,1
(4 Bat App)
Battery 1
Battery 2
N2, N7
N3, N6
N1, N9
N2, N8
Battery 3
N4, N7
N3, N9
Battery 4
N5, N6
N4, N8
1,0
(3 Bat App)
Battery 1
Battery 2
N2, N7
N4, N6
N1, N8
N2, N9
Battery 3
N5, N7
N4, N8
0,1
(2 Bat App)
Battery 1
Battery 2
N7
N9
N5
N8
In this fashion each battery in the stack is connected to the
auxiliary cell and the batteries in the stack will be balanced.
An internal clamp circuit protects the LTC3305 when the
voltage difference between the battery being balanced
and the auxiliary cell is greater than 1V. With a 13.2V
difference, the clamp draws 475µA of current. In the worst
case scenario, a 16V difference may appear between the
auxiliary cell and a battery. The internal clamp draws 600µA
of current in this scenario.
In the state when both the top and bottom side switches
are turned on, the PTCFLT pin will be pulled low if the
voltage difference between the battery being balanced and
the auxiliary cell is greater than 1V. If during the balancing
operation the voltage difference becomes less than 1V, the
PTCFLT pin returns to its high impedance state.
The BATX and BATY pins indicate which battery in the
stack is currently connected to the auxiliary cell as shown
in Table 3. In shutdown, the BATX and BATY pins are in
a High Z state and the BAL pin is also in a High Z state.
Table 3. State of BATX and BATY Pins
STATE OF OPERATION
BATX
Battery 1 Connected
High Z
Battery 2 Connected
High Z
Battery 3 Connected
0
Battery 4 Connected
0
BATY
High Z
0
0
High Z
Once all batteries in the stack are balanced the DONE pin
is pulled low, the BAL pin is in its high impedance state
and the part is put in a low power OFF state.
A four-battery stack is deemed balanced when the ter-
mination sense comparator detects V_TERMINATE on five
consecutive cycles that connect each of the batteries to
the auxiliary cell using the bottom switches only.
For more information www.linear.com/LTC3305
3305f
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