Datasheet

LTC4090/LTC4090-5
23
4090fc
designed to work with thermistors whose resistance-tem-
perature characteristics follow Vishay Dale’s R-T curve 2.
The Vishay NTHS0603N02N1002J is an example of such
a thermistor. However, Vishay Dale has many thermistor
products that follow the R-T curve 2 characteristic in a
variety of sizes. Furthermore, any thermistor whose ratio
of r
COLD
to r
HOT
is about 7.0 will also work (Vishay Dale
R-T curve 2 shows a ratio of 2.815/0.409 = 6.89).
In the explanation below, the following notation is used.
R
25C
= Value of the Thermistor at 25°C
R
NTC|COLD
= Value of Thermistor at the Cold Trip Point
R
NTC|HOT
= Value of the Thermistor at the Hot Trip Point
r
COLD
= Ratio of R
NTC|COLD
to R
25C
r
HOT
= Ratio of R
NTC|HOT
to R
25C
R
NOM
= Primary Thermistor Bias Resistor (see Figure 8)
R1 = Optional Temperature Range Adjustment resistor
(see Figure 9)
The trip points for the LTC4090/LTC4090-5’s tempera-
ture qualifi cation are internally programmed at 0.29 •
VNTC for the hot threshold and 0.74 • VNTC for the cold
threshold.
Therefore, the hot trip point is set when:
R
NTC|HOT
R
NOM
+R
NTC|HOT
•VNTC= 0.29 • VNTC
and the cold trip point is set when:
R
NTC|COLD
R
NOM
+R
NTC|COLD
•VNTC= 0.74 • VNTC
Solving these equations for R
NTC|COLD
and R
NTC|HOT
results
in the following:
R
NTC|HOT
= 0.409 • R
NOM
and
R
NTC|COLD
= 2.815 • R
NOM
By setting R
NOM
equal to R
25C
, the above equations result
in r
HOT
= 0.409 and r
COLD
= 2.815. Referencing these ratios
to the Vishay Resistance-Temperature curve 2 chart gives
a hot trip point of about 50°C and a cold trip point of about
0°C. The difference between the hot and cold trip points
is approximately 50°C.
By using a bias resistor, R
NOM
, different in value from
R
25C
, 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
APPLICATIONS INFORMATION
Figure 8. Typical NTC Thermistor Circuit Figure 9. NTC Thermistor Circuit with Additional Bias Resistor
–
+
–
+
R
NOM
10k
R
NTC
10k
NTC
VNTC
6
0.1V
NTC_ENABLE
4090 F08
NTC BLOCK
TOO_COLD
TOO_HOT
0.738 •
VNTC
0.29 • VNTC
–
+
5
–
+
–
+
R
NOM
13.2k
R
NTC
10k
R1
1.97k
NTC
VNTC
6
0.1V
NTC_ENABLE
4090 F09
TOO_COLD
TOO_HOT
0.738 •
VNTC
0.29 • VNTC
–
+
5
NTC BLOCK