TC664 datasheet(10/36 Pages) MICROCHIP

10 / 36 page

TC664/TC665

DS21737A-page 10

 2002 Microchip Technology Inc.

4.1

Fan Speed Control Methods

The speed of a DC brushless fan is proportional to the

voltage across it. For example, if a fan’s rating is

5000 RPM at 12 V, it’s speed would be 2500 RPM at

6 V. This, of course, will not be exact, but should be

close.

There are two main methods for fan speed control. The

first is pulse width modulation (PWM) and the second

is linear. Using either method the total system power

requirement to run the fan is equal. The difference

between the two methods is where the power is con-

sumed.

The following example compares the two methods for

a 12 V, 120 mA fan running at 50% speed. With 6 V

applied across the fan, the fan draws an average cur-

rent of 68 mA. Using a linear control method, there is

6V across the fan and 6V across the drive element.

With 6 V and 68 mA, the drive element is dissipating

410 mW of power. Using the PWM approach, the fan is

modulated at a 50% duty cycle, with most of the 12 V

being dropped across the fan. With 50% duty cycle, the

fan draws an RMS current of 110 mA and an average

current of 72 mA. Using a MOSFET with a 1

Ω RDS(on)

(a fairly typical value for this low current) the power dis-

sipation in the drive element would be: 12 mW (Irms2 *

RDS(on)). Using a standard 2N2222A NPN transistor

(assuming a Vce-sat of 0.8 V), the power dissipation

would be 58 mW (Iavg* Vce-sat).

The PWM approach to fan speed control causes much

less power dissipation in the drive element. This allows

smaller devices to be used and will not require any spe-

cial heatsinking to get rid of the power being dissipated

in the package.

The other advantage to the PWM approach is that the

voltage being applied to the fan is always near 12 V.

This eliminates any concern about not supplying a high

enough voltage to run the internal fan components

which is very relevant in linear fan speed control.

4.2

PWM Fan Speed Control

The TC664/TC665 devices implement PWM fan speed

control by varying the duty cycle of a fixed frequency

pulse train. The duty cycle of a waveform is the on time

divided by the total period of the pulse. For example,

given a 100 Hz waveform (10 msec.) with an on time of

5.0 msec., the duty cycle of this waveform is 50%

(5.0 msec./10.0 msec.). An example of this is shown in

Figure 4-2.

FIGURE 4-2:

Duty Cycle Of A PWM

Waveform.

The TC664/TC665 devices generate a pulse train with

a typical frequency of 30 Hz (CF = 1 µF). The duty cycle

can be varied from 30% to 100%. The pulse train gen-

erated by the TC664/TC665 devices drives the gate of

an external N-channel MOSFET or the base of an NPN

transistor (Figure 4-3). See Section 7.5 for more infor-

mation on output drive device selection.

FIGURE 4-3:

PWM Fan Drive.

By modulating the voltage applied to the gate of the

MOSFET Qdrive, the voltage applied to the fan is also

modulated. When the VOUT pulse is high, the gate of

the MOSFET is turned on, pulling the voltage at the

drain of Qdrive to zero volts. This places the full 12 V

across the fan for the Ton period of the pulse. When the

duty cycle of the drive pulse is 100% (full on, Ton = T),

the fan will run at full speed. As the duty cycle is

decreased (pulse on time “Ton” is lowered), the fan will

slow down proportionally. With the TC664/TC665

devices, the duty cycle can be controlled through the

analog input pin (VIN), or through the SMBus interface,

by using the Duty-Cycle Register. See Section 4.5 for

more details on duty cycle control.

Ton

Toff

T = Period

T = 1/F

F = Frequency

D = Duty Cycle

D = Ton / T

FAN

12 V

Qdrive

TC664

TC665

VDD

GND

VOUT

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