LTC4085 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC4085' PDF : 24 Pages View PDF
APPLICATIONS INFORMATION
VNTC
9
RNOM
10k
NTC
8
0.74 • VNTC
–
+
LTC4085
TOO_COLD
RNTC
10k
–
0.29 • VNTC
+
TOO_HOT
+
0.1V
–
NTC_ENABLE
VNTC
9
RNOM
121k
NTC
8
R1
13.3k
RNTC
100k
4085 F04a
(4a)
Figure 4. NTC Circuits
0.74 • VNTC
–
+
–
0.29 • VNTC
+
+
0.1V
–
(4b)
LTC4085
LTC4085
TOO_COLD
TOO_HOT
NTC_ENABLE
4085 F04b
Thermistors
The LTC4085 NTC trip points were designed to work
with thermistors whose resistance-temperature charac-
teristics follow Vishay Dale’s “R-T Curve 2”. The Vishay
NTHS0603N02N1002J is an example of such a thermis-
tor. 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 RCOLD
to RHOT is about 7.0 will also work (Vishay Dale R-T Curve
2 shows a ratio of RCOLD to RHOT of 2.815/0.4086 = 6.89).
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 2”. Using these as indi-
cated in the NTC Thermistor section will give temperature
trip points of approximately 3°C and 47°C, a delta of
44°C. This delta in temperature can be moved in either
direction by changing the value of RNOM with respect to
RNTC. Increasing RNOM will move both trip points to lower
temperatures. Likewise a decrease in RNOM with respect
to RNTC will move the trip points to higher temperatures.
To calculate RNOM for a shift to lower temperature for
example, use the following equation:
RNOM
= RCOLD
2.815
• RNTC
at
25°C
where RCOLD is the resistance ratio of RNTC at the desired
cold temperature trip point. If you want to shift the trip
points to higher temperatures use the following equation:
RNOM
=
RHOT
0.4086
• RNTC
at
25°C
where RHOT is the resistance ratio of RNTC at the desired
hot temperature trip point.
Here is an example using a 100k R-T Curve 1 thermistor
from Vishay Dale. The difference between the trip points
is 44°C, from before, and we want the cold trip point to
be 0°C, which would put the hot trip point at 44°C. The
RNOM needed is calculated as follows:
RNOM
=
RC O L D
2.815
• RNTC
at
25°C
= 3.266 • 100k = 116k
2.815
4085fd
19
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