Fan-Speed Regulators and Monitors

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21

The MAX6650 regulates fan speed in the following

manner. The output of an internal 254kHz oscillator is

divided by 128, generating a roughly 2kHz signal. This

signal is divided by 1 plus the value in the speed regis-

ter and is used as a reference frequency. For example,

02h in the speed register will result in a 667Hz [2kHz /

(02h+1)] reference frequency, which is then compared

against the frequency at the tachometer input divided

by the prescaler value. The MAX6650/MAX6651

attempt to keep the tachometer frequency divided by

the prescaler equal to the reference frequency by

adjusting the voltage across the fan. If the tachometer

frequency divided by the prescaler value is less than

the reference frequency, the voltage across the fan is

increased. Remember that the tachometer will give two

pulses per revolution of the fan. The following equations

describe the operation.

When in regulation:

= the value in the speed register, FanSpeed = the

speed of the fan in revolutions per second (Hz),

Solving for all four variables:

can further reduce the equations:

Enter closed-loop mode by entering 10 into bits 5 and 4

of the configuration register.

Note that in equation 3, the fan speed is inversely pro-

speed is a nonlinear function of the value written to the

speed register. Low values written to the speed register

can result in large relative changes in fan speed. For

best results, design the system so that small values

(such as 02h) are not needed. This is easily accom-

plished because an 8-bit speed register is used, and

fan-speed control should rarely need more than 16

speeds. A good compromise is to design the system

(by selecting the appropriate prescaler value) so that

the maximum-rated speed of the fan occurs when the

speed register equals approximately 64 (decimal).

Although 64 is a good target value, values between 20

and 100 will work fine.

The prescaler value also affects the response time and

the stability of the speed-control loop. Adjusting the

prescaler value effectively adjusts the loop gain. A larg-

er prescaler value will slow the response time and

increase stability, while a smaller prescaler value will

yield quicker response time. The optimum prescaler

value for response time and stability depends on the

fan’s mechanical time constant. Small, fast-spinning

fans will tend to have small mechanical time constants

and can benefit from smaller prescaler values. A good

rule of thumb is to try the selected prescaler value in

scale, and watch for overshoot or oscillation in the fan

speed. Also look for overshoot or oscillation when

75% of full-scale speed to 90% of full scale). If there is

unacceptable overshoot or if the fan speeds up and

increase the prescaler value.

Enter the appropriate prescaler value in bits zero to 2 of

the configuration register.

Fan speed is a trade-off between cooling requirements,

noise, power, and fan wear. In general, it is desirable

(within limits) to run the fan at the slowest speed that

will accomplish the cooling goals. This will reduce

power consumption, increase fan life, and minimize

noise. When calculating the desired fan speed, remem-

ber that the above equations are written in rotations per

second (RPS), where most fans are specified in rota-

tions per minute (RPM).

Write the desired fan speed to the speed register.

Example:

Assume the following:

• 12V fan is rated at 2000RPM at 12V.

• Desired fan speed = 1500RPM (25RPS).

First, calculate an appropriate prescaler value

close to 64 as possible for the maximum speed of

2000RPM.

• Set FanSpeed = 33.3RPS (2000RPM/60).