TC649B datasheet(24/36 Pages) MICROCHIP

24 / 36 page

TC646B/TC648B/TC649B

DS21755B-page 24

 2003 Microchip Technology Inc.

Step 2: Selecting the Fan Controller.

The requirements for the fan controller are that it have

auto-shutdown capability at 20ºC and also indicate a

fan fault condition. No over-temperature indication is

necessary. From these specifications, the proper

selection is the TC649B device.

Step 3: Setting the PWM Frequency.

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

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

speed), which equates to approximately 1700 RPM. At

1700 RPM, one full fan revolution occurs every

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

gives four current pulses per revolution. With this infor-

mation, and viewing test results at 40% duty cycle, two

fan current pulses were always seen during the PWM

on time with a PWM frequency of 30 Hz. For this rea-

son, 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% and 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 along 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 = 238,455 Ω

-R2 = 45,161 Ω

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 Auto-Shutdown Voltage (VAS).

Setting the voltage for the auto-shutdown is done using

a simple resistor voltage divider. The criteria for the

voltage divider in this design is that it draw no more

than 100 µA of current. The required auto-shutdown

voltage was determined earlier in the selection of the

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

at the temperature that auto-shutdown is to occur

(20ºC).

-VAS = 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

Using standard 1% resistor values yields the following

values:

-R3 = 32.4 kΩ

-R4 = 17.8 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

IDIV =

R3 + R4

VAS =

5V * R4

R3 + R4

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