Datasheet

LTC4160/LTC4160-1
24
41601fa
Therefore, the hot trip point is set when:
R
RR
NTCBIAS NTCBIAS
NT CHOT
NOM
NT CHOT
+
=•.•0 349
And the cold trip point is set when:
R
RR
NTCBIAS NTCBIAS
NT CCOLD
NOM
NT CCOLD
+
=•.•0 765
Solving these equations for R
NTC|COLD
and R
NTC|HOT
results
in the following:
R
NTC|HOT
= 0.536 • R
NOM
and
R
NTC|COLD
= 3.25 • R
NOM
By setting R
NOM
equal to R25, the above equations result
in r
HOT
= 0.536 and r
COLD
= 3.25. Referencing these ratios
to the Vishay Resistance-Temperature Curve 1 chart gives
a hot trip point of about 40°C and a cold trip point of about
0°C. The difference between the hot and cold trip points
is approximately 40°C.
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 non-linear behavior of the thermistor. The fol-
lowing equations can be used to calculate a new value for
the bias resistor:
R
r
R
R
r
R
NOM
HOT
NOM
COLD
=
=
0 536
25
325
25
.
•
.
•
where r
HOT
and r
COLD
are the resistance ratios at the de-
sired 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 above equation, R
NOM
should
be set to 46.4k. With this value of R
NOM
, r
COLD
is 1.436
and 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 “temperature 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,
R1, as shown in Figure 8. The following formulas can be
used to compute the values of R
NOM
and R1:
R
rr
R
RRr
NOM
COLD HOT
NOM HOT
=
=
–
.
•
.•
–•
2 714
25
10536 RR25
For example, to set the trip points to 0°C and 45°C with
a Vishay Curve 1 thermistor choose:
Rk
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 final solution is shown
in Figure 8 and results in an upper trip point of 45°C and
a lower trip point of 0°C.
APPLICATIONS INFORMATION
Figure 7. Standard NTC Configuration
–
+
–
+
R
NOM
100k
R
NTC
100k
NTC
NTCBIAS
0.1V
NTC_ENABLE
41601 F07
LTC4160/LTC4160-1
NTC BLOCK
TOO_COLD
TOO_HOT
0.765 • NTCBIAS
0.349 • NTCBIAS
–
+
3
4
T