Datasheet

LTC4085-3/LTC4085-4
19
408534fb
(4a) (4b)
Figure 4. NTC Circuits
APPLICATIONS INFORMATION
Alternate NTC Thermistors
The LTC4085 NTC trip points were designed to work
with thermistors whose resistance-temperature charac-
teristics follow Vishay Dale’s “R-T Curve 2.” The Vishay
NTHS0603N02N1002J is an example of such a thermis-
tor. 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 6.0 will also work (Vishay Dale R-T Curve
2 shows a ratio of 2.816/0.4839 = 5.82).
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 42°C, a delta of 39°C.
This delta in temperature can be moved in either direc-
tion 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
NOM
=
R
COLD
2.816
•R
NTC
at 25°C
where R
COLD
is the resistance ratio of R
NTC
at the desired
cold temperature trip point. To shift the trip points to higher
temperatures use the following equation:
R
NOM
=
R
HOT
0.484
•R
NTC
at 25°C
where R
HOT
is the resistance ratio of R
NTC
at the desired
hot temperature trip point.
The following example uses a 100k R-T Curve 1 Thermistor
from Vishay Dale. The difference between the trip points
is 39°C, from before—and the desired cold trip point of
0°C, would put the hot trip point at about 39°C. The R
NOM
needed is calculated as follows:
R
NOM
=
R
COLD
2.816
•R
NTC
at 25°C=
3.266
2.816
• 100kΩ =116kΩ
–
+
–
+
R
NOM
10k
R
NTC
10k
NTC
V
NTC
9
0.1V
NTC_ENABLE
408534 F04a
LTC4085-3/
LTC4085-4
TOO_COLD
TOO_HOT
t7
NTC
t7
NTC
–
+
8
–
+
–
+
R
NOM
124k
R
NTC
100k
R1
24.3k
NTC
V
NTC
9
0.1V
NTC_ENABLE
408534 F04b
TOO_COLD
TOO_HOT
t7
NTC
t7
NTC
–
+
8
LTC4085-3/
LTC4085-4