TC642 datasheet(10/28 Pages) MICROCHIP

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TC642

DS21444C-page 10

 2002 Microchip Technology Inc.

FIGURE 5-2:

Temperature Sensing

Circuit.

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,

while 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 TC642. A 100 k

Ω NTC thermistor is

selected for this application in order to keep IDIV at 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. With this change, R2 is

adjusted according to the formulas below. It is

suggested that a maximum temperature range of 50°C

be used with this circuit due to thermistor linearity

limitations.

The following two equations permit solving for the two

unknown variables, R1 and R2. More information

regarding thermistors can be found in AN679, “Temper-

ature Sensing Technologies”, and AN685, “Thermistors

in Single Supply Temperature Sensing Circuits”, which

can be downloaded from Microchip’s web site at:

www.microchip.com.

EQUATION

5.2

Minimum Fan Speed

A voltage divider on VMIN sets the minimum PWM duty

cycle and, thus, the minimum fan speed. As with the

VIN input, 1.25V to 2.65V typically corresponds to 0%

to 100% duty cycle. Assuming that fan speed is linearly

related to duty cycle, the minimum speed voltage is

given by the equation:

EQUATION

For example, if 2500 RPM equates to 100% fan speed,

and a minimum speed of 1000 RPM is desired, then

the VMIN voltage is:

EQUATION

The VMIN voltage may be set using a simple resistor

divider, as shown in Figure 5-3. Per Section 1.0,

“Electrical Characteristics”, the leakage current at the

VMIN pin is no more than 1 µA. It would be very

conservative to design for a divider current, IDIV, of

100 µA. If VDD = 5.0V then;

EQUATION

FIGURE 5-3:

VIN Circuit.

R2 = 23.2 kΩ

R1 = 100 kΩ

NTC Thermistor

100 k

Ω @ 25˚C

RT1

IDIV

VIN

VDD

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.

Minimum Speed

Full Speed

VMIN =

x (1.4) + 1.25V

1000

2500

VMIN =

x (1.4) + 1.25V = 1.81V

R1+ R2

IDIV = 100µA =

, therefore

5.0V

R1 + R2 =

= 50,000

Ω = 50kΩ

100µA

5.0V

GND

VDD

VMIN

IDIV

IIN

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