TC642B datasheet(23/34 Pages) MICROCHIP

23 / 34 page

 2003 Microchip Technology Inc.

DS21756B-page 23

TC642B/TC647B

Step 2: Selecting the Fan Controller.

The requirements for the fan controller are that it have

minimum speed capability at 20ºC and also indicate a

fan fault condition. No over-temperature indication is

necessary. Based on these specifications, the proper

selection is the TC647B device.

Step 3: Setting the PWM Frequency.

The fan is rated at 4200 RPM with a 12V input. Since

the goal is to run to a 40% duty cycle (roughly 40% fan

speed), which equates to approximately 1700 RPM,

we can assume one full fan revolution occurs every

35 msec. The fan being used is a four-pole fan that

gives four current pulses per revolution. Knowing this

and viewing test results at 40% duty cycle, two fan cur-

rent pulses were always seen during the PWM on time

with a PWM frequency of 30 Hz. For this reason, the CF

value is selected to be 1.0

µF.

Step 4: Setting the VIN Voltage.

From the design criteria, the desired duty cycle at 20ºC

is 40%, while full fan speed should be reached at 40ºC.

Based on a VIN voltage range of 1.20V to 2.60V, which

represents 0% to 100% duty cycle, the 40% duty cycle

voltage can be found using the following equation:

EQUATION

Using the above equation, the VIN values are

calculated to be:

-VIN (40%) = 1.76V

-VIN (100%) = 2.60V

Using these values in combination with the thermistor

resistance values calculated earlier, the R1 and R2

resistor values can now be calculated using the

following equation:

EQUATION

RTEMP is the parallel combination of R1 and the ther-

mistor. V(T1) represents the VIN voltage at 20ºC and

V(T2) represents the VIN voltage at 40ºC. Solving the

equations simultaneously yields the following values

(VDD = 5V):

-R1 = 238,455Ω

-R2 = 45,161Ω

Using standard 1% resistor values, the selected R1 and

R2 values are:

-R1 = 237 kΩ

-R2 = 45.3 kΩ

A graph of the VIN voltage, thermistor resistance and

RTEMP resistance versus temperature for this

configuration is shown in Figure 5-10.

FIGURE 5-10:

Thermistor Resistance, VIN,

and RTEMP vs. Temperature.

Step 5: Setting the Minimum Speed Voltage (VMIN).

Setting the voltage for the minimum speed is accom-

plished using a simple resistor voltage divider. The cri-

teria for the voltage divider in this design is that it draw

no more than 100 µA of current. The required minimum

speed voltage was determined earlier in the selection

of the VIN voltage at 40% duty cycle, since this was also

set at the temperature which minimum speed is to

occur (20ºC).

-VMIN = 1.76V

Given this desired setpoint, and knowing the desired

divider current, the following equations can be used to

solve for the resistor values for R3 and R4:

EQUATION

Using the equations above, the resistor values for R3

and R4 are found to be:

-R3 = 32.4 kΩ

-R4 = 17.6 kΩ

VIN = (DC * 1.4V) + 1.20V

DC = Desired Duty Cycle

VT1

()

VDD R2

RTEMP T1

() R2

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

VT2

()

VDD R2

RTEMP T2

() R2

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

100

150

200

250

300

350

400

1020

3040

5060

7080

Temperature (ºC)

0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

4.00

4.50

5.00

VIN

NTC Thermistor

100 k @ 25ºC

RTEMP

VMIN =

5V*R4

R3 + R4

IDIV =

R3 + R4

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