ADT7475_13 Datasheet, PDF (28/58 Pages) ON Semiconductor – Remote Thermal Monitor and Fan Controller

ADT7475
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 this process.
Automatic Fan Control Overview
The ADT7475 can automatically control the speed of fans
based upon the measured temperature. This is done
independently of CPU intervention once initial parameters
are set up.
The ADT7475 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
owing 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 select these
values carefully.
Figure 42 gives a top-level overview of the automatic fan
control circuitry on the ADT7475. 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
ADT7475 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, the designer 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 42 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, then, ultimately allows graceful fan speed changes
that are less perceptible to the system user.
REMOTE1
TEMP
LOCAL
TEMP
REMOTE2
TEMP
THERMAL CALIBRATION
100%
T_MIN
0%
T_RANGE
THERMAL CALIBRATION
100%
MUX
T_MIN
0%
T_RANGE
THERMAL CALIBRATION
100%
T_MIN
0%
T_RANGE
PWM
MIN
S
PWM
CONFIG
PWM
GENERATOR
PWM
MIN
S
TACHOMETER 1
MEASUREMENT
PWM
CONFIG
PWM
GENERATOR
PWM
MIN
S
TACHOMETER 2
MEASUREMENT
PWM
CONFIG
PWM
GENERATOR
TACHOMETER 3
AND 4
MEASUREMENT
Figure 42. Automatic Fan Control Block Diagram
RAMP
CONTROL
(ACOUSTIC
ENHANCEMENT)
RAMP
CONTROL
(ACOUSTIC
ENHANCEMENT)
RAMP
CONTROL
(ACOUSTIC
ENHANCEMENT)
PWM1
TACH1
PWM2
TACH2
PWM3
TACH3
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