Datasheet

 2002-2013 Microchip Technology Inc. DS21737B-page 23
TC664/TC665
7.3 Temperature Sensor Design
As discussed in previous sections, the V
IN
analog input
has a range of 1.62 V to 2.6 V (typical), which repre-
sents a duty cycle range on the V
OUT
output of 30% to
100%, respectively. The V
IN
voltages can be thought of
as representing temperatures. The 1.62 V level is the
low temperature at which the system only requires 30%
fan speed for proper cooling. The 2.6 V level is the high
temperature, for which the system needs maximum
cooling capability, so the fan needs to be at 100%
speed.
One of the simplest ways of sensing temperature over
a given range is to use a thermistor. By using an NTC
thermistor as shown in Figure 7-3, a temperature vari-
ant voltage can be created.
FIGURE 7-3: Temperature Sensing
Circuit.
Figure 7-3 represents a temperature dependent volt-
age divider circuit. R
t
is a conventional NTC thermistor,
R
1
and R
2
are standard resistors. R
1
and R
t
form a par-
allel resistor combination that will be referred to as
R
TEMP
(R
TEMP
= R
1
* R
t
/ R
1
+ R
t
). As the temperature
increases, the value of R
t
decreases and the value of
R
TEMP
will decrease with it. Accordingly, the voltage at
V
IN
increases as temperature increases, giving the
desired relationship for the V
IN
input. The purpose of
R
1
is to help linearize the response of the sensing net-
work. Figure 7-4 shows an example of this.
There are many values that can be chosen for the NTC
thermistor. There are also thermistors that have a linear
resistance, instead of logarithmic, which can help to
eliminate R
1
. If less current draw from V
DD
is desired,
then a larger value thermistor should be chosen. The
voltage at the V
IN
pin can also be generated by a volt-
age output temperature sensor device. The key is to
get the desired V
IN
voltage to system (or component)
temperature relationship.
The following equations apply to the circuit in
Figure 7-3.
EQUATION
In order to solve for the values of R
1
and R
2
, the values
for V
IN,
and the temperatures at which they are to
occur, need to be selected. The variables, t1 and t2,
represent the selected temperatures. The value of the
thermistor at these two temperatures can be found in
the thermistor data sheet. With the values for the
thermistor and the values for V
IN
, you now have two
equations from which the values for R
1
and R
2
can be
found.
Example: The following design goals are desired:
• Duty Cycle = 50% (V
IN
= 1.9 V) with Temperature
(t1) = 30°C
• Duty Cycle = 100% (V
IN
= 2.6 V) with Tempera-
ture (t2) = 60°C
Using a 100 k thermistor (25°C value), we look up the
thermistor values at the desired temperatures:
•R
t
= 79428  @ 30°C
•R
t
= 22593  @ 60°C
Substituting these numbers into the given equations,
we come up with the following numbers for R
1
and R
2
.
•R
1
= 34.8 k
•R
2
= 14.7 k
FIGURE 7-4: How Thermistor Resistance,
V
IN
, And R
TEMP
Vary With Temperature.
Figure 7-4 graphs three parameters versus tempera-
ture. They are R
t
, R
1
in parallel with R
t
, and V
IN
. As
described earlier, you can see that the thermistor has a
logarithmic resistance variation. When put in parallel
with R
1
, though, the combined resistance becomes
more linear, which is the desired effect. This gives us
the linear looking curve for V
IN
.
R
2
R
1
R
t
I
DIV
V
IN
V
DD
Vt1
V
DD
R
2

R
TEMP
t1R
2
+
----------------------------------------
=
Vt2
V
DD
R
2

R
TEMP
t2R
2
+
----------------------------------------
=
0
20000
40000
60000
80000
100000
120000
140000
20 30 40 50 60 70 80 90
100
Temperature (ºC)
Network Resistance (
:
)
0.000
0.500
1.000
1.500
2.000
2.500
3.000
3.500
4.000
V
IN
(V)
NTC Thermistor
100K @ 25ºC
V
IN
Voltage
R
TEMP