TC648 datasheet(11/28 Pages) MICROCHIP

11 / 28 page

 2002 Microchip Technology Inc.

DS21448C-page 11

TC648

mistor resistance at TMIN. R2 is then calculated to

deliver the desired speed at TMIN. The values for R1, R2

and RT1 are then checked at TMAX for 2.65V nominal.

It may be necessary to adjust the values of R1 and R2

after the initial calculation to obtain the desired results.

The design equations are:

EQUATION

Because the thermistor characteristics are fixed, it may

not be possible, in certain applications, to obtain the

desired values of VMIN and VMAX using the above

equations. In this case, the circuit in Figure 5-4 can be

used. Diode D1 clamps VIN to the voltage required to

sustain minimum speed. The calculations of R1 and

R2 for the temperature sensor are identical to the

equation on the previous page.

FIGURE 5-4:

Minimum Fan Speed Circuit.

5.3

Auto-Shutdown Temperature

Design

A voltage divider on VAS sets the temperature at which

the part is automatically shut down if the sensed

temperature at VIN drops below the set temperature at

VAS (i.e. VIN < VAS).

As with the VIN input, 1.25V to 2.65V corresponds to

the temperature range of interest from T1 to T2,

respectively. Assuming that the temperature sensor

network designed previously is linearly related to

temperature, the shutdown temperature TAS is related

to T2 and T1 by:

EQUATION

For example, if 1.25V and 2.65V at VIN corresponds to

a temperature range of T1 = 0°C to T2 = 125°C, and the

auto-shutdown temperature desired is 25°C, then the

VAS voltage is:

EQUATION

The VAS voltage may be set using a simple resistor

divider, as shown in Figure 5-5.

FIGURE 5-5:

VAS Circuit.

R1 = (0.1)(RT1MIN)

Where: RT1 = Thermistor resistance at TMIN

R2 =

(RT1MIN)(R1)(VMIN)

(RT1MIN + R1)(VDD - VMIN)

Where VMIN = the value of VIN required for

minimum fan speed. VDD = Power Supply Voltage

VMAX =

(RT1MIN)(R1)(VMIN)

R2 (R1 + RT1MAX )(VDD)

Where RT1MAX = thermistor resistance at TMAX,

VMAX = the value of VIN required for maximum

fan speed.

RT1

VDD

VIN

2.65 - 1.25V

T2 - T1

VAS - 1.25

TAS - T1

VAS =

1.4V

T2 - T1

(TAS - T1) + 1.25

(

)

1.4V

(125 - 0)

VAS =

(25 - 0) + 1.25 = 1.53V

GND

VDD

VAS

IDIV

IIN

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