Datasheet
LTC4098/LTC4098-1
25
40981fc
–
+
–
+
R
NOM
100k
R
NTC
100k
NTC
0.1V
NTC_ENABLE
40981 F09a
LTC4098/LTC4098-1
NTC BLOCK
TOO_COLD
TOO_HOT
0.765 • NTCBIAS
0.349 • NTCBIAS
–
+
5
NTCBIAS
4
T
–
+
–
+
R
NOM
105k
R
NTC
100k
R1
12.7k
NTC
0.1V
NTC_ENABLE
40981 F09b
TOO_COLD
TOO_HOT
0.765 • NTCBIAS
0.349 • NTCBIAS
–
+
5
LTC4098/LTC4098-1
NTC BLOCK
T
NTCBIAS
4
(9a) (9b)
Figure 9. NTC Circuits
APPLICATIONS INFORMATION
By using a bias resistor, R
NOM
, different in value from
R25, the hot and cold trip points can be moved in either
direction. The temperature span will change somewhat due
to the nonlinear behavior of the thermistor. The following
equations can be used to easily calculate a new value for
the bias resistor:
R
NOM
=
r
HOT
0.536
•R25
R
NOM
=
r
COLD
3.25
•R25
where r
HOT
and r
COLD
are the resistance ratios at the
desired
hot and cold trip points. Note that these equations
are linked. Therefore, only one of the two trip points can
be chosen, the other is determined by the default ratios
designed in the IC. Consider an example where a 60°C
hot trip point is desired.
From the Vishay curve 1 R-T characteristics, r
HOT
is 0.2488
at 60°C. Using the previous equation, R
NOM
should be set
to 46.4k. With this value of R
NOM
, the cold trip point is
about 16°C. Notice that the span is now 44°C rather than
the previous 40°C. This is due to the decrease in tem-
perature gain of the thermistor as absolute temperature
increases.
The upper and lower temperature trip points can be inde-
pendently programmed by using an additional bias resistor
as shown in Figure 9b. The following formulas can be used
to compute the values of R
NOM
and R1:
R
NOM
=
r
COLD
–r
HOT
2.714
•R25
R1 = 0.536 • R
NOM
–r
HOT
•R25
For example, to set the trip points to 0°C and 45°C with
a Vishay Curve 1 thermistor choose:
R
NOM
=
3.266 – 0.4368
2.714
•100k =104.2k
the nearest 1% value is 105k:
R1 = 0.536 • 105k – 0.4368 • 100k = 12.6k
the nearest 1% value is 12.7k. The final circuit is shown
in Figure 9b and results in an upper trip point of 45°C and
a lower trip point of 0°C.