Datasheet

LTC4081
19
4081f
APPLICATIO S I FOR ATIO
WUU
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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 different R-T curves, such as Vishay
Dale “R-T Curve 2”, is also possible. This curve, combined with
LTC4081 internal thresholds, gives temperature trip points of
approximately 0°C (falling) and 40°C (rising), a delta of 40°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 higher temperatures. To
calculate R
NOM
for a shift to lower temperature for example,
use the following equation:
R
R
RatC
NOM
COLD
NTC
=°
3 266
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 5325
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 2 thermistor
from Vishay Dale. The difference between the trip points is
40°C, from before, and we want the cold trip point to be 0°C,
which would put the hot trip point at 40°C. The R
NOM
needed
is calculated as follows:
R
R
RatC
NOM
COLD
NTC
=°
=
3 266
25
2 816
3
.
•
.
..
•.
266
10 8 62kk=
The nearest 1% value for R
NOM
is 8.66k. This is the value used
to bias the NTC thermistor to get cold and hot trip points of
approximately 0°C and 40°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 5). The values of the
resistors are calculated as follows:
R
RR
R
NOM
COLD HOT
=
−
−
=
3 266 0 5325
0 5325
3 266
1
..
.
.
−−
⎛
⎝
⎜
⎞
⎠
⎟
−
()
−
0 5325.
•R R R
COLD HOT HOT
where R
NOM
is the value of the bias resistor and R
HOT
and
R
COLD
are the values of R
NTC
at the desired temperature trip
points. Continuing the example from before with a desired
trip point of 50°C:
R
RR
k
NOM
COLD HOT
=
−
−
=
−
3 266 0 5325
10 2 816 0
..
•. .
44086
3 266 0 5325
88 887
()
−
=
..
.,.kkis %.
•
.
.
the nearest value
Rk
1
10
0 5325
326
1
=
66 0 5325
2 816 0 4086 0 4086
−
⎛
⎝
⎜
⎞
⎠
⎟
−
()
−
.
•. . .
,%.= 604 604 1Ω is the nearest value
4081 F05
R
NOM
8.87k
R
NTC
10k
V
CC
–
+
–
+
–
+
TOO COLD
TOO HOT
NTC_ENABLE
R1
604Ω
0.76 • V
CC
0.35 • V
CC
0.016 • V
CC
6
NTC
T
Figure 5. NTC Circuits