Datasheet

LTC4089/LTC4089-5
19
40895fc
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. 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.
The following example uses a 100K R-T Curve 1 Thermistor
from Vishay Dale. The difference between the trip points is
44°C, from before—and the desired cold trip point of 0°C,
would put the hot trip point at 44°C. The R
NOM
needed is
calculated as follows:
R
R
RatC
NOM
COLD
NTC
=°=
2 815
25
3 266
2 815
100
.
•
.
.
•kkkΩΩ=116
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
RR
R
NOM
COLD HOT
=
−
−
=
−
2 815 0 4086
1
0 4086
2 815 0
..
.
.
..
•
4086
⎡
⎣
⎢
⎤
⎦
⎥
−
[]
−RRR
COLD HOT 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
RR
k
NOM
COLD HOT
=
−
−
=
−
2 815 0 4086
100 3 266 0 3
..
•( . .
6602
2 815 0 4086
120 8 121 1
110
)
..
., %
−
=
=
k k nearest
R
00
0 4086
2 815 0 4086
3 266 0 3602
k•
.
..
•
..
−
⎛
⎝
⎜
⎞
⎠
⎟
−
()
−−
⎡
⎣
⎢
⎢
⎢
⎤
⎦
⎥
⎥
⎥
=
0 3602
13 3 13 3 1
.
., . %k k is nearest
The fi nal solution is shown in Figure 7, where
R
NOM
= 121k, R1 = 13.3k and R
NTC
= 100k at 25°C
Figure 7. Modifi ed NTC Circuit
APPLICATIONS INFORMATION
–
+
–
+
R
NOM
121k
R
NTC
100k
R1
13.3k
NTC
VNTC
15
0.1V
NTC_ENABLE
4089 F07
TOO_COLD
TOO_HOT
0.74 •
VNTC
0.29 • VNTC
–
+
14
LTC4089