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ADT7467
Automatic Fan Control Overview
The ADT7467 can automatically control the speed of fans
based on the measured temperature. This is done
independently of CPU intervention once initial parameters
are set up.
The ADT7467 has a local temperature sensor and two
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).
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. Thermal validation of the
system is one of the most important steps in the design
process; therefore, these values should be selected carefully.
Figure 48 shows a top-level overview of the automatic fan
control circuitry on the ADT7467. 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
ADT7467 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,
one can decide to run the CPU fan when CPU temperature
increases above 60 ? C and to run 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
can individually control parameters such as minimum PWM
duty cycle, fan speed failure thresholds, and even ramp
control of the PWM outputs. Therefore, automatic fan
control ultimately allows gracefully changing fan speed so
that it is less perceptible to the system user.
Thermal Calibration
100%
PWM
MIN
Ramp
Control
(Acoustic
Enhancement)
PWM
CONFIG
PWM
Generator
PWM1
T MIN
T MIN
100%
REMOTE 1
TEMP
LOCAL
TEMP
REMOTE 2
TEMP
0%
T RANGE
Thermal Calibration
100%
0%
T RANGE
Thermal Calibration
0%
T MIN
T RANGE
MUX
PWM
MIN
PWM
MIN
Tachometer 1
Measurement
Ramp
Control
(Acoustic
Enhancement)
Tachometer 2
Measurement
Ramp
Control
(Acoustic
Enhancement)
Tachometer 3
and 4
Measurement
PWM
CONFIG
PWM
Generator
PWM
CONFIG
PWM
Generator
TACH1
PWM2
TACH2
PWM3
TACH3
Figure 48. Automatic Fan Control Block Diagram
Dynamic T MIN Control Mode
In addition to the automatic fan speed control mode, the
ADT7467 has a mode that extends the basic automatic fan
speed control loop. Dynamic T MIN control allows the
ADT7467 to intelligently adapt the system’s cooling
solution to optimize system performance or system
acoustics, depending on user or design requirements. Use of
worst-case conditions, and it significantly reduces the time
required for system design and validation.
Designing for Worst-case Conditions
System design must always allow for worst-case
conditions. In PC design, the worst-case conditions include,
but are not limited to, the following:
dynamic T MIN control alleviates the need to design for
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