Quick-Start Guide for 8000RPM 4-Pole
(2 Pulses per Revolution) Fan in Automatic
RPM Mode Using the Circuit of Figure 7
1) Write 02h to register 11h to set the PWM output to
drive the n-channel MOSFET.
2) Write 4Bh to register 22h to set the minimum RPM to
3200.
3) Write 5Eh to register 24h to set the pulses per revo-
lution to 2 and to set the maximum RPM speed to
8000RPM.
4) Write 19h to register 28h to set the fan-start temper-
ature to +25°C.
5) Write D2h to register 10h to start automatic
RPM mode.
Remote-Diode Considerations
Temperature accuracy depends upon having a good-
quality, diode-connected, small-signal transistor.
Accuracy has been experimentally verified for all the
devices listed in Table 12. The MAX6639 can also
directly measure the die temperature of CPUs and
other ICs with on-board temperature-sensing diodes.
The transistor must be a small-signal type with a rela-
tively high forward voltage. This ensures that the input
voltage is within the A/D input voltage range. The for-
ward voltage must be greater than 0.25V at 10µA at the
highest expected temperature. The forward voltage
must be less than 0.95V at 100µA at the lowest expect-
ed temperature. The base resistance has to be less
than 100Ω. Tight specification of forward-current gain
(+50 to +150, for example) indicates that the manufac-
turer has good process control and that the devices
have consistent characteristics.
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 MAX6639 is optimized for n
= 1.008, for Intel
®
Pentium
®
II and AMD Athlon
®
MP
compatibility, and the MAX6639F is optimized for n =
1.021 for Penryn compatibiliy. If a sense transistor with
a different ideality factor is used, the output data is dif-
ferent. 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
MAX6639 is 1.008. As an example, assume the
MAX6639 is configured with a CPU that has an ideality
factor of 1.002. If the diode has no series resistance,
the measured data is related to the real temperature
as follows:
For a real temperature of +85°C (358.15K), the mea-
sured temperature is +82.91°C (356.02K), which is an
error of -2.13°C.
.
.
(. )
