Datasheet

LTC4089-3
20
40893f
For more information www.linear.com/4089-3
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 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 =116k
The nearest 1% value for R
NOM
is 115k. This is the value
used to bias the NTC thermistor to get cold and hot trip
points of approximately 0°C and 44°C, respectively. To
extend the delta between the cold and hot trip points,
a resistor (R1) can be added in series with R
NTC
(see
Figure 7). The values of the resistors are calculated as
follows:
R
NOM
=
R
COLD
R
HOT
2.816 0.484
R1=
0.484
2.816 0.484
R
COLD
R
HOT
[ ]
R
HOT
where R
NOM
is the value of the bias resistor, R
HOT
and
R
COLD
are the values of R
NTC
at the desired temperature
trip points. Continuing the forementioned example with
a desired hot trip point of 50°C:
R
NOM
=
R
COLD
R
HOT
2.816 0.484
=
100k (3.266 0.3602)
2.816 0.484
= 124.6k,124k nearest 1%
R1= 100k
0.484
2.816 0.484
3.266 0.3602
( )
0.3602
= 24.3k
The final solution is shown in Figure 7, where
R
NOM
= 124k, R1 = 24.3k and R
NTC
= 100k at 25°C
Figure 7. Modified NTC Circuit
APPLICATIONS INFORMATION
+
+
R
NOM
124k
R
NTC
100k
R1
24.3k
NTC
VNTC
15
0.1V
NTC_ENABLE
4089 F07
TOO_COLD
TOO_HOT
0.738 • VNTC
0.326 • VNTC
+
14
LTC4089-3