TC647EOA View Datasheet(PDF) - Microchip Technology

Part Name

Description

Manufacturer

TC647EOA

PWM Fan Speed Controller with FanSense™ Technology

Microchip Technology 

TC647

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

provide this signal.

VDD

IDIV

RT1

NTC

Thermistor

100 kΩ @ 25ºC

R1 = 100 kΩ

VIN

R2 = 23.2 kΩ

FIGURE 5-2:

Circuit.

Temperature Sensing

Figure 5-2 illustrates a simple temperature dependent

voltage divider circuit. RT1 is a conventional 100 k @

25°C NTC thermistor, while R1 and R2 are standard

resistors. The supply voltage, VDD, is divided between

R2 and the parallel combination of RT1 and R1 (for con-

venience, the parallel combination of RT1 and R1 will

be referred to as RTEMP). The resistance of the therm-

istor at various temperatures is obtained from the man-

ufacturer’s specifications. Thermistors are often

referred to in terms of their resistance at 25°C. Gener-

ally, 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

TC647. A 100 k 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. 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

VDD x R2

= V(T1)

RTEMP (T1) + R2

VDD x R2

= V(T2)

RTEMP (T2) + R2

Where T1 and T2 are the chosen temperatures and

RTEMP is the parallel combination of the thermistor

and R1.

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 website at

www.microchip.com.

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 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

Minimum Speed

VMIN = Full Speed

x (1.4V) + 1.25V

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

VMIN =

1000

2500

x (1.4V) + 1.25V = 1.81V

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

5.0V

IDIV = 1e–4A = R1 + R2 , therefore

R1 + R2 =

5.0V = 50,000  = 50 k

1e–4A

DS21447D-page 10

 2001-2012 Microchip Technology Inc.

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