Datasheet
LTC4099
29
4099fd
–
+
–
+
R
NOM
10k
R
NTC
10k
NTC
0.228 • NTCBIAS
OVERTEMP
4099 F07
LTC4099
NTC BLOCK
TOO_COLD
TOO_HOT
0.738 • NTCBIAS
0.326 • NTCBIAS
–
+
2
NTCBIAS
3
T
–
+
–
+
R
NOM
10.2k
R
NTC
10k
R1
845Ω
NTC
0.228 • NTCBIAS
OVERTEMP
4099 F08
TOO_COLD
TOO_HOT
0.738 • NTCBIAS
0.326 • NTCBIAS
–
+
2
LTC4099
NTC BLOCK
T
NTCBIAS
3
Figure 7. Standard NTC Configuration
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
=
a
HOT
0.4839
• R25
R
NOM
=
a
COLD
2.816
• R25
where a
HOT
and a
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 LTC4099. Consider an example where a
50°C hot trip point is desired.
From the Vishay curve 2 R-T characteristics,
a
HOT
is
0.4086 at 50°C. Using the prior equation, R
NOM
should
be set to 8.45k. With this value of R
NOM
, a
COLD
is 2.380
and the cold trip point is about 4°C. Notice that the span
is now 46°C rather than the previous 45°C. This is due to
the decrease in the temperature gain of the thermistor as
the absolute temperature increases.
The upper and lower temperature trip points can be in-
dependently programmed by using an additional bias
resistor as shown in Figure 8. The following formulas can
be used to compute the values of R
NOM
and R1:
R
NOM
=
a
COLD
– a
HOT
2.332
• R25
R1= 0.4839 • R
NOM
– a
HOT
• R25
For example, to set the trip points to 0°C and 50°C with
a Vishay curve 2 thermistor, choose:
R
NOM
=
2.816 – 0.4086
2.332
• 10k = 10.32k
the nearest 1% value is 10.2k:
R1
= 0.4839 • 10.2k – 0.4086 • 10k = 0.850k
The nearest 1% value is 845Ω. The final circuit is shown
in Figure 8, and results in an upper trip point of 50°C and
a lower trip point of 0°C.
Figure 8. Modified NTC Configuration
applicaTions inForMaTion