Rainbow Electronics MAX6641 - Manual (Page 14)

MAX6641
SMBus-Compatible Temperature Monitor with
Automatic PWM Fan-Speed Controller
14 ______________________________________________________________________________________
The duty cycle is limited to the value in the fan maxi-
mum duty-cycle register. If the duty-cycle value is larg-
er than the maximum fan duty cycle, it can be set to the
maximum fan duty cycle as in the fan maximum duty-
cycle register. The temp step is bit D6 of the fan-config-
uration register (0Dh).
If duty cycle is an odd number, the MAX6641 automati-
cally rounds down to the nearest even number.
Duty-Cycle Rate-of-Change Control
To reduce the audibility of changes in fan speed, the
rate of change of the duty cycle is limited by the values
set in the duty-cycle rate-of-change register. Whenever
the target duty cycle is different from the instantaneous
duty cycle, the duty cycle increases or decreases at
the rate determined by the duty-cycle rate-of-change
byte until it reaches the target duty cycle. By setting the
rate of change to the appropriate value, the thermal
requirements of the system can be balanced against
good acoustic performance. Slower rates of change
are less noticeable to the user, while faster rates of
change can help minimize temperature variations.
Remember that the fan controller is part of a complex
control system. Because several of the parameters are
generally not known, some experimentation may be
necessary to arrive at the best settings.
Power-Up Defaults
At power-up, the MAX6641 has the default settings
indicated in
Table 1. Some of these settings are sum-
marized below:
• Temperature conversions are active.
• Remote OT limit = +110
°C.
• Local OT limit = +80 °C.
• Manual fan mode.
• Fan duty cycle = 0.
• PWM Invert bit = 0.
• PWMOUT is high.
When using an nMOS or npn transistor, the fan starts at
full speed on power-up.
Applications Information
Remote-Diode Selection
The MAX6641 can directly measure the die tempera-
ture of CPUs and other ICs that have on-board temper-
ature-sensing diodes (see the Typical Application
Circuit), or they can measure the temperature of a dis-
crete diode-connected transistor.
Effect of Ideality Factor
The accuracy of the remote temperature measurements
depends on the ideality factor (n) of the remote diode
(actually a transistor). The MAX6641 is optimized for n =
1.008, which is the typical value for the Intel Pentium® III
and the AMD Athlon™ MP model 6. If a sense transistor
with a different ideality factor is used, the output data is
different. Fortunately, the difference is predictable.
Assume a remote-diode sensor designed for a nominal
ideality factor n
NOMINAL is used to measure the tem-
perature of a diode with a different ideality factor, n 1 .
The measured temperature T M can be corrected using:
where temperature is measured in Kelvin.
As mentioned above, the nominal ideality factor of the
MAX6641 is 1.008. As an example, assume the MAX6641
is configured with a CPU that has an ideality factor of
1.002. If the diode has no series resistance, the mea-
sured data is related to the real temperature as follows:
For a real temperature of +85°C (358.15K), the mea-
sured temperature is +82.87°C (356.02K), which is an
error of -2.13°C.
Effect of Series Resistance
Series resistance in a sense diode contributes addition-
al errors. For nominal diode currents of 10µA and
100µA, change in the measured voltage is:
∆V
M = R
S
(100µA - 10µA) = 90µA x R S
Since 1°C corresponds to 198.6µV, series resistance
contributes a temperature offset of:
Assume that the diode being measured has a series
resistance of 3
Ω. The series resistance contributes an
offset of:
: 3 0 453 1 36 Ω ×
°
Ω
= + ° . .
C
C
90
198 6
0 453
µ
Ω
µ
°
=
°
Ω
V
V
C
C
.
.
T T
n
n
T T
ACTUAL M M M
NOMINAL =

 

= =
( ) 
 

1
1 008
1 002 1 00599 .
. .
T T
n
n
M ACTUAL
NOMINAL
=

 

1
Pentium is a registered trademark of Intel Corp.
Athlon is a trademark of AMD.