ADT7475 Datasheet, PDF (27/58 Pages) Analog Devices – dBCool Remote Thermal Monitor and Fan Controller

ADT7475
If V_CC goes high (the system processor power rail is
powered up), a fail−safe timer begins to count down. If the
ADT7475 is not addressed by any valid SMBus transactions
before the fail−safe timeout (4.6 seconds) lapses, the
ADT7475 drives the fans to full speed. If the ADT7475 is
addressed by a valid SMBus transaction after this point, the
fans stop, and the ADT7475 assumes its default settings and
begins normal operation.
If V_CCP goes high (the system processor power rail is
powered up), then a fail−safe timer begins to count down. If
the ADT7475 is addressed by a valid SMBus transaction
before the fail−safe timeout (4.6 seconds) lapses, then the
ADT7475 operates normally, assuming the functionality of
all the default registers. See the flow chart in Figure 41.
Programming the Automatic Fan Speed Control Loop
To more efficiently understand the automatic fan speed
control loop, it is strongly recommended to use the ADT7475
evaluation board and software while reading this section.
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 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 re−
mote 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.
THERMAL CALIBRATION _100%
REMOTE 1
TEMP
T_MIN
0%
T_RANGE
THERMAL CALIBRATION
100%
MUX
LOCAL
TEMP
T_MIN
0%
T_RANGE
THERMAL CALIBRATION
100%
PWM
MIN PWM
CONFIG
PWM
GENERATOR
TACHOMETER 1
MEASUREMENT
PWM
MIN _PWM
CONFIG
PWM
GENERATOR
TACHOMETER 2
MEASUREMENT
PWM
MIN _PWM
CONFIG
PWM
GENERATOR
RAMP
CONTROL
(ACOUSTIC
ENHANCEMENT)
RAMP
CONTROL
(ACOUSTIC
ENHANCEMENT)
RAMP
CONTROL
(ACOUSTIC
ENHANCEMENT)
REMOTE 2
TEMP
T_MIN
0%
T_RANGE
TACHOMETER 3
AND 4
MEASUREMENT
Figure 42. Automatic Fan Control Block Diagram
PWM1
TACH1
PWM2
TACH2
PWM3
TACH3
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