TC648 datasheet(10/28 Pages) MICROCHIP

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TC648

DS21448C-page 10

 2002 Microchip Technology Inc.

5.1

Temperature Sensor Design

The temperature signal connected to VIN must output a

voltage in the range of 1.25V to 2.65V (typical) for 0%

to 100% of the temperature range of interest. The

circuit in Figure 5-2 illustrates a convenient way to pro-

vide this signal using a temperature dependent voltage

divider circuit.

FIGURE 5-2:

Temperature Sensing

Circuit.

RT1 is a conventional NTC thermistor and R1 and R2

are standard resistors. The supply voltage (VDD) is

divided between R2 and the parallel combination of

RT1 and R1. For convenience, the parallel combination

of RT1 and R1 will be referred to as RTEMP. The resis-

tance of the thermistor at various temperatures is

obtained from the manufacturer’s specifications. Ther-

mistors are often referred to in terms of their resistance

at 25°C.

Generally, the thermistor shown in Figure 5-2 is a non-

linear device with a negative temperature coefficient

(also called an NTC thermistor). In Figure 5-2, R1 is

used to linearize the thermistor temperature response

and R2 is used to produce a positive temperature

coefficient at the VIN node. As an added benefit, this

configuration produces an output voltage delta of 1.4V,

which is well within the range of the VC(SPAN)

specification of the TC648. A 100 k

Ω NTC thermistor is

selected for this application in order to keep IDIV to a

minimum.

For the voltage range at VIN to be equal to 1.25V to

2.65V, the temperature range of this configuration is

0°C to 50°C. If a different temperature range is required

from this circuit, R1 should be chosen to equal the

resistance value of the thermistor at the center of this

new temperature range. It is suggested that a maxi-

mum temperature range of 50°C be used with this cir-

cuit due to thermistor linearity limitations. With this

change, R2 is adjusted according to the following

equations:

EQUATION

These two equations facilitate solving for the two

unknown variables, R1 and R2. More information about

thermistors may be obtained from AN679, “Tempera-

ture Sensing Technologies”, and AN685, “Thermistors

in Single Supply Temperature Sensing Circuits”, which

can be downloaded from Microchip's web site at

www.microchip.com.

5.2

Minimum Speed Mode

The TC648 is configured for minimum speed mode by

grounding VAS and designing the temperature sensor

network such that VIN operates the fan at relatively con-

stant, minimum speed when the thermistor is at

minimum temperature. Figure 5-3 shows operation in

minimum speed mode. The 0% and 100% fan speeds

correspond to VIN values of 1.25V and 2.65V, typical.

Minimum system temperature (TMIN) is defined as the

lowest measured temperature at which proportional fan

speed control is required by the system. The fan

operates at minimum speed for all temperatures below

TMIN and at speeds proportional to the measured

temperature between TMIN and TMAX.

FIGURE 5-3:

Minimum Fan Speed Mode

Operation.

Temperature sensor design consists of a two-point

calculation: one at TMIN and one at TMAX. At TMIN, the

ohmic value of the thermistor must be much higher

than that of R1 so that minimum speed is determined

primarily by the values of R1 and R2. At TMAX, the

ohmic value of the thermistor must result in a VIN of

2.65V nominal. The design procedure consists of ini-

tially choosing R1 to be 10 times smaller than the ther-

R2 = 23.2 kΩ

R1 = 100 kΩ

IDIV

VIN

VDD

NTC Thermistor

100 k

Ω @25˚C

RT1

VDD x R2

RTEMP (T1) + R2

= V(T1)

RTEMP (T2) + R2

= V(T2)

VDD x R2

Where T1 and T2 are the chosen temperatures and

RTEMP is the parallel combination of the thermistor

and R1.

Fan Speed

100%

Minimum

Speed

TMIN

TMAX

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