Datasheet

LTC4055/LTC4055-1
17
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OPERATION
THERMISTORS
The LTC4055/LTC4055-1 NTC trip points were designed
to work with thermistors whose resistance-temperature
characteristics 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 R
COLD
to R
HOT
is about 7.0 will also work
(Vishay Dale R-T Curve 2 shows a ratio of R
COLD
to R
HOT
of 2.815/0.4086 = 6.89).
Power conscious designs may want to use thermistors
whose room temperature value is greater than 10k. Vishay
Dale has a number of values of thermistor from 10k to 100k
that follow the “R-T Curve 1.” Using these as indicated
in the NTC Thermistor section will give temperature trip
points of approximately 3°C and 47°C, a delta of 44°C. This
delta in temperature can be moved in either direction by
changing the value of R
NOM
with respect to R
NTC
. Increasing
R
NOM
will move both trip points to lower temperatures.
Likewise a decrease in R
NOM
with respect to R
NTC
will
move the trip points to higher temperatures. To calculate
R
NOM
for a shift to lower temperature for example, use
the following equation:
R
R
RatC
NOM
COLD
NTC
=°
2 815
25
.
•
where R
COLD
is the resistance ratio of R
NTC
at the desired
cold temperature trip point. If you want to shift the trip points
to higher temperatures use the following equation:
R
R
RatC
NOM
HOT
NTC
=°
0 4086
25
.
•
where R
HOT
is the resistance ratio of R
NTC
at the desired
hot temperature trip point.
Here is an example using a 100k R-T Curve 1 thermistor
from Vishay Dale. The difference between the trip points
is 44°C, from before, and we want the cold trip point to
be 0°C, which would put the hot trip point at 44°C. The
R
NOM
needed is calculated as follows:
R
R
RatC
kk
NOM
COLD
NTC
=°
==
2 815
25
3 266
2 815
100 116
.
•
.
.
•
–
+
–
+
R
NOM
100k
R
NTC
100k
NTC
V
NTC
15
0.1V
NTC_ENABLE
4055 F03a
LTC4055/LTC4055-1
NTC BLOCK
TOO_COLD
TOO_HOT
0.74 • V
NTC
0.29 • V
NTC
–
+
16
–
+
–
+
R
NOM
121k
R
NTC
100k
R1
13.3k
NTC
V
NTC
15
0.1V
NTC_ENABLE
4055 F03b
TOO_COLD
TOO_HOT
0.74 • V
NTC
0.29 • V
NTC
–
+
16
LTC4055/LTC4055-1
NTC BLOCK
(3a) (3b)
Figure 3. NTC Circuits