TC654 View Datasheet(PDF) - Microchip Technology
Part Name
Description
MFG CO.
'TC654' PDF : 36 Pages View PDF
7.3 Temperature Sensor Design
As discussed in previous sections, the VIN analog input
has a range of 1.62V to 2.6V (typical), which represents
a duty cycle range on the VOUT output of 30% to 100%,
respectively. The VIN voltages can be thought of as rep-
resenting temperatures. The 1.62V level is the low tem-
perature at which the system only requires 30% fan
speed for proper cooling. The 2.6V level is the high
temperature, for which the system needs maximum
cooling capability. Therefore, 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.
VDD
IDIV
Rt
R1
VIN
R2
FIGURE 7-3:
Circuit.
Temperature Sensing
Figure 7-3 represents a temperature dependent volt-
age divider circuit. Rt is a conventional NTC thermistor,
R1 and R2 are standard resistors. R1 and Rt form a par-
allel resistor combination that will be referred to as
RTEMP (RTEMP = R1 * Rt/ R1 + Rt). As the temperature
increases, the value of Rt decreases and the value of
RTEMP will decrease with it. Accordingly, the voltage at
VIN increases as temperature increases, giving the
desired relationship for the VIN input. The purpose of
R1 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 which have a lin-
ear resistance instead of logarithmic, which can help to
eliminate R1. If less current draw from VDD is desired,
then a larger value thermistor should be chosen. The
voltage at the VIN pin can also be generated by a volt-
age output temperature sensor device. The key is to
get the desired VIN voltage to system (or component)
temperature relationship.
The following equations apply to the circuit in
Figure 7-3.
TC654/TC655
EQUATION
Vt1 = R----T---E--V--M--D--P-D----t--1----R---+-2----R----2-
Vt2 = R----T---E--V--M--D--P-D----t--2----R---+-2----R----2-
In order to solve for the values of R1 and R2, the values
for VIN 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 thermis-
tor data sheet. With the values for the thermistor and
the values for VIN, you now have two equations from
which the values for R1 and R2 can be found.
Example: The following design goals are desired:
• Duty Cycle = 50% (VIN = 1.9V) with Temperature
(t1) = 30°C
• Duty Cycle = 100% (VIN = 2.6V) with Temperature
(t2) = 60°C
Using a 100 k thermistor (25°C value), we look up the
thermistor values at the desired temperatures:
• Rt = 79428 @ 30°C
• Rt = 22593 @ 60°C
Substituting these numbers into the given equations,
we come up with the following numbers for R1 and R2.
• R1 = 34.8 k
• R2 = 14.7 k
140000
120000
VIN Voltage
100000
80000
60000
40000
20000
0
RTEMP
NTC Thermistor
100K @ 25ºC
20 30 40 50 60 70 80 90 100
Temperature (ºC)
4.000
3.500
3.000
2.500
2.000
1.500
1.000
0.500
0.000
FIGURE 7-4:
How Thermistor Resistance,
VIN, And RTEMP Vary With Temperature.
Figure 7-4 graphs three parameters versus tempera-
ture. They are Rt, R1 in parallel with Rt, and VIN. As
described earlier, you can see that the thermistor has a
logarithmic resistance variation. When put in parallel
with R1, though, the combined resistance becomes
more linear, which is the desired effect. This gives us
the linear looking curve for VIN.
2002-2014 Microchip Technology Inc.
DS20001734C-page 23
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