LTC3558
18
3558f
APPLICATIONS INFORMATION
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.
The upper and lower temperature trip points can be inde-
pendently programmed by using an additional bias resistor
as shown in Figure 4. The following formulas can be used
to compute the values of R
NOM
and R1:
R
r
r
R
R
R
r
NOM
COLD
HOT
NOM
HOT
=
=
–
.
•
.
•
–
•
2 714
25
1 0 536
R
R25
For example, to set the trip points to 0°C and 45°C with
a Vishay Curve 1 thermistor choose:
R
k
k
NOM
=
=
3 266 0 4368
2 714
100
104 2
.
– .
.
•
.
the nearest 1% value is 105k.
R1 = 0.536 • 105k – 0.4368 • 100k = 12.6k
the nearest 1% value is 12.7k. The fi nal solution is shown
in Figure 4 and results in an upper trip point of 45°C and
a lower trip point of 0°C.
3558 F03
R
NOM
100k
R
NTC
100k
–
+
–
+
–
+
TOO_COLD
TOO_HOT
NTC_ENABLE
0.765 • V
CC
(NTC RISING)
NTC BLOCK
0.349 • V
CC
(NTC FALLING)
17
NTC
20
V
CC
0.017 • V
CC
(NTC FALLING)
3558 F04
R
NOM
105k
R
NTC
100k
–
+
–
+
–
+
TOO_COLD
TOO_HOT
NTC_ENABLE
R1
12.7k
0.765 • V
CC
(NTC RISING)
0.349 • V
CC
(NTC FALLING)
17
NTC
0.017 • V
CC
(NTC FALLING)
20
V
CC
Figure 3. Typical NTC Thermistor Circuit
Figure 4. NTC Thermistor Circuit with Additional Bias Resistor
