TC654 datasheet(23/36 Pages) MICROCHIP

23 / 36 page

 2002 Microchip Technology Inc.

DS21734A-page 23

TC654/TC655

7.3

Temperature Sensor Design

As discussed in previous sections, the VIN analog input

has a range of 1.62 V to 2.6 V (typical), which repre-

sents a duty cycle range on the VOUT output of 30% to

100%, respectively. The VIN 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. 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.

FIGURE 7-3:

Temperature Sensing

Circuit.

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.

EQUATION

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 ther-

mistor 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 = 1.9 V) with Temperature

(t1) = 30°C

• Duty Cycle = 100% (VIN = 2.6 V) with Tempera-

ture (t2) = 60°C

Using a 100 k

Ω thermistor (25°C value), we look up the

thermistor values at the desired temperatures:

•Rt = 79428 Ω @ 30°C

•Rt = 22593 Ω @ 60°C

Substituting these numbers into the given equations,

we come up with the following numbers for R1 and R2.

•R1 = 34.8 kΩ

•R2 = 14.7 kΩ

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.

IDIV

VIN

VDD

Vt1

()

VDD R2

RTEMP t1

() R2

----------------------------------------

Vt2

()

VDD R2

RTEMP t2

() R2

----------------------------------------

20000

40000

60000

80000

100000

120000

140000

Temperature (ºC)

0.000

0.500

1.000

1.500

2.000

2.500

3.000

3.500

4.000

NTC Thermistor

100K @ 25ºC

VIN Voltage

RTEMP

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