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ADT7490
temperature region where the PWM output linearly ramps
from PWM MIN to 100% PWM.
PWM = 100%
PWM MAX
automatically speed up to maximum PWM or 100% PWM,
whichever way the appropriate fan channel is configured.
Automatic Fan Control Overview
The ADT7490 can automatically control the speed of fans
based on the measured temperature. This is done
independently of CPU intervention once the initial
parameters are set up.
The ADT7490 has a local temperature sensor and two
PWM MIN
PWM = 0%
T MIN
T RANGE
remote temperature channels that can be connected to a CPU
on-chip thermal diode (available on Intel ? Pentium ? class
and other CPUs). These three temperature channels can be
used as the basis for automatic fan speed control to drive fans
using pulse-width modulation (PWM).
Figure 47. T RANGE
Programming the Automatic Fan Speed Control Loop
To more efficiently understand the automatic fan speed
control loop, using the ADT7490 evaluation board and
software while reading this section is recommended.
This section provides the system designer with an
understanding of the automatic fan control loop, and
provides step-by-step guidance on effectively evaluating
and selecting critical system parameters. To optimize the
system characteristics, the designer needs to give some
thought to system configuration, including the number of
fans, where they are located, and what temperatures are
being measured in the particular system.
The mechanical or thermal engineer who is tasked with
the system thermal characterization should also be involved
at the beginning of the system development process.
Manual Fan Control Overview
In unusual circumstances, it can be necessary to manually
control the speed of the fans. Because the ADT7490 has an
SMBus interface, a system can read back all necessary
voltage, fan speed, and temperature information, and use
this information to control the speed of the fans by writing
to the current PWM duty cycle register (0x30, 0x31, and
0x32) of the appropriate fan. Bits [7:5] of the PWMx
configuration registers (0x5C, 0x5D, and 0x5E) are used to
set fans up for manual control.
THERM Operation in Manual Mode
In manual mode, if the temperature increases above the
programmed THERM temperature limit, the fans
Automatic fan speed control reduces acoustic noise by
optimizing fan speed according to accurately measured
temperature. Reducing fan speed can also decrease system
current consumption. The automatic fan speed control mode
is very flexible due to the number of programmable
parameters, including T MIN and T RANGE . The T MIN and
T RANGE values for a temperature channel and, therefore, for
a given fan are critical, because they define the thermal
characteristics of the system. The thermal validation of the
system is one of the most important steps in the design
process, so these values should be selected carefully.
Figure 48 gives a top-level overview of the automatic fan
control circuitry on the ADT7490. From a systems-level
perspective, up to three system temperatures can be
monitored and used to control three PWM outputs. The three
PWM outputs can be used to control up to four fans. The
ADT7490 allows the speed of four fans to be monitored.
Each temperature channel has a thermal calibration block,
allowing the designer to individually configure the thermal
characteristics of each temperature channel. For example,
users can decide to run the CPU fan when CPU temperature
increases above 60 ? C and a chassis fan when the local
temperature increases above 45 ? C.
At this stage, the designer has not assigned these thermal
calibration settings to a particular fan drive (PWM) channel.
The right side of Figure 48 shows controls that are
fan-specific. The designer has individual control over
parameters such as minimum PWM duty cycle, fan speed
failure thresholds, and even ramp control of the PWM
outputs. Automatic fan control ultimately allows graceful
fan speed changes that are less perceptible to the system user.
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