LTC4066/LTC4066-1
APPLICATIONS INFORMATION
NTC Thermistor
The battery temperature is measured by placing a negative
temperature coefficient (NTC) thermistor close to the bat-
tery pack. The NTC circuitry is shown in Figure 4. To use
this feature, connect the NTC thermistor (RNTC) between
the NTC pin and ground and a resistor (RNOM) from the
NTC pin to VNTC. RNOM should be a 1% resistor with a
value equal to the value of the chosen NTC thermistor at
25°C (this value is 10k for a Vishay NTHS0603N02N1002J
thermistor). The LTC4066/LTC4066-1 go into hold mode
when the resistance (RHOT) of the NTC thermistor drops
to 0.41 times the value of RNOM or approximately 4.1k,
which should be at 50°C. The hold mode freezes the timer
and stops the charge cycle until the thermistor indicates a
return to a valid temperature. As the temperature drops,
the resistance of the NTC thermistor rises. The LTC4066/
LTC4066-1 are designed to go into hold mode when the
value of the NTC thermistor increases to 2.82 times the
value of RNOM. This resistance is RCOLD. For a Vishay
NTHS0603N02N1002J thermistor, this value is 28.2k
which corresponds to approximately 0°C. The hot and cold
comparators each have approximately 3°C of hysteresis
to prevent oscillation about the trip point. Grounding the
NTC pin can disable the NTC function.
Thermistors
The LTC4066/LTC4066-1 NTC trip points were designed to
work with thermistors whose resistance-temperature char-
acteristics follow Vishay Dale’s “R-T Curve 2”. The Vishay
NTHS0603N02N1002J is an example of such a thermistor.
However, Vishay Dale has many thermistor products that
follow the “R-T Curve 2” characteristic in a variety of sizes.
Furthermore, any thermistor whose ratio of RCOLD to RHOT
is about 7.0 will also work (Vishay Dale R-T Curve 2 shows
a ratio of RCOLD to RHOT of 2.815/0.4086 = 6.89).
VNTC
15
RNOM
10k
NTC
14
RNTC
10k
0.74 • VNTC
–
+
–
0.29 • VNTC
+
+
0.1V
–
(4a)
LTC4066
TOO_COLD
TOO_HOT
NTC_ENABLE
VNTC
15
RNOM
121k
NTC
14
R1
13.3k
RNTC
100k
4066 F04a
Figure 4. NTC Circuits
0.74 • VNTC
–
+
–
0.29 • VNTC
+
+
0.1V
–
(4b)
LTC4066
TOO_COLD
TOO_HOT
NTC_ENABLE
4055 F03b
4066fc
20