Rainbow Electronics MAX6651 - Manual (Page 21)

MAX6650/MAX6651
Fan-Speed Regulators and Monitors
with SMBus/I 2 C-Compatible Interface
______________________________________________________________________________________ 21
pulses per revolution of the fan. The following equations
describe the operation.
When in regulation:
[f CLK / (128 x (K TACH + 1))] = 2 x FanSpeed / K SCALE
where f CLK = oscillator frequency (either the 254kHz
internal oscillator or the externally applied clock), K TACH
= the value in the speed register, FanSpeed = the
speed of the fan in revolutions per second (Hz),
K
SCALE = the prescaler value (1, 2, 4, 8, or 16).
Solving for all four variables:
K TACH = [(f CLK x K SCALE ) / (256 x FanSpeed)] - 1
K
SCALE
= [256 x FanSpeed x (K
TACH
+ 1)] / f
CLK
FanSpeed = K SCALE x f CLK / [256 x (K TACH + 1)]
f CLK = 256 x FanSpeed x (K TACH + 1) / K SCALE
If the internal oscillator is used, setting f CLK to 254kHz
can further reduce the equations:
Equation 1: K SCALE = FanSpeed x (K TACH + 1) / 992
Equation 2: K
TACH
= (992 x K
SCALE
/ FanSpeed) - 1
Equation 3: FanSpeed = 992 x K SCALE / (K TACH + 1)
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-
portional to (K
TACH + 1). This means the regulated fan
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
the target system. Set K
TACH
to around 75% of full
scale, and watch for overshoot or oscillation in the fan
speed. Also look for overshoot or oscillation when
K
TACH is changed from one value to another (e.g., from
75% of full-scale speed to 90% of full scale). If there is
unacceptable overshoot or if the fan speeds up and
slows down with K
TACH , set it to a constant value;
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.
• Use the internal oscillator (f CLK = 254kHz).
• Desired fan speed = 1500RPM (25RPS).
First, calculate an appropriate prescaler value
(K
SCALE ) using equation 1. Attempt to get K TACH as
close to 64 as possible for the maximum speed of
2000RPM.
• Set FanSpeed = 33.3RPS (2000RPM/60).
• Set K
TACH
= 64.
• Solving equation 1 gives K SCALE = 2.18.
We will start with K SCALE = 2 (to increase stability, a 4
could be tried, or to improve response time, a 1 could
be tried).
Second, calculate the appropriate value for the Speed
Register (K
TACH ) using equation 2.
• Set FanSpeed = 25RPS (1500PRM/60).
• Solving for equation 2 gives K TACH = 78 for K SCALE
= 2, K
TACH
= 39 for K
SCALE
= 1, or K
TACH
= 158 for
K = 4.
Determining the Tachometer Count Time
To monitor the fan speed using the SMBus/I 2 C, the next
step is to determine the tachometer count time. In sys-
tems running in open-loop mode, this is necessary. In
closed-loop or full-speed mode, reading the tachome-
ter can serve as a valuable check to ensure the fan and
the control loop are operating properly.