LTC3305_15 Datasheet, PDF (18/28 Pages) Linear Technology – Lead-Acid Battery Balancer

LTC3305
Applications Information
Selecting the PTC Thermistor
A PTC thermistor is a type of resistor with a Positive Tem-
perature Coefficient that serves as a protection device by
limiting its current above a certain threshold. The PTC device
limits the peak current that transfers charge between the
auxiliary cell and the battery. When the voltage across the
PTC is small, the power dissipated in the PTC is small and
the PTC resistance remains constant. As the voltage across
the PTC increases, power dissipation in the PTC increases
which causes the PTC temperature to rise. When the tem-
perature reaches the Curie Temperature, further increases
in voltage will cause the PTC resistance to increase rapidly,
which limits the current through the device and thus limits
the power dissipation in the PTC. This behavior is shown
in the PTC Current Voltage Characteristics in Figure 5a. In
this fashion the PTC serves to protect the external NMOS
switches from operating outside of their SOA region.
As seen in the PTC Current Voltage Characteristics in Figure
5a, when the PTC has a small voltage or a high voltage
across it, the current flowing through it is small. For small
voltages, this is OK since the battery and the auxiliary cell
are close to balance. For high voltages, this slows down
balancing. To increase balance currents at high voltages
a power resistor can be placed in parallel with the PTC
device, as shown in Figure 5b. Additionally, multiple PTC
resistors may be connected in parallel to increase current
flow at all voltages.
PTC devices are manufactured in two styles: ceramic and
poly fuse. Only a ceramic style PTC device should be used
in this application. Poly fuse devices have a very limited
number of lifetime trip cycles and are not suitable in a
balancing application.
The PTC must be selected such that power dissipation
through the external NMOS switches never exceeds their
rated SOA power dissipation value. Refer to Table 4 for a
list of recommended PTC thermistors.
PTC Current Voltage Characteristics
CURIE POINT
VOLTAGE →
(a)
3305 F05a
Increasing Current at Large Voltage
PTC RESISTOR IN PARALLEL
WITH A RESISTOR
SINGLE PTC RESISTOR
RESISTOR ONLY
VOLTAGE →
(b)
3305 F05b
Figure 5. PTC Behavior
Table 4. Recommended Ceramic PTC Thermistors
PART NUMBER
RESISTANCE
MANUFACTURER VOLTAGE
(Ω)
PTGL7SARR47M1B51B0
Murata
16V
0.47
PTGLASARR27M1B51B0
Murata
16V
0.27
PTGLESARR15M1B51B0
Murata
16V
0.15
PTGL12AR1R2H2B51B0
Murata
30V
1.2
2381 663 51121
Vishay
30V
0.7
2381 663 51321
Vishay
30V
0.5
2381 664 52021
Vishay
30V
0.3
3305f
18
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