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ADT7473
Power-on Default
When the ADT7473 is powered up, it polls the V CCP
input. By default, the ADT7473 ? 1 powers up with fans
running, eliminating the need for polling of V CCP .
If V CCP stays below 0.75 V (the system CPU power rail
is not powered up), the ADT7473 assumes the functionality
of the default registers after the ADT74731 is addressed via
any valid SMBus transaction.
If V CC goes high (the system processor power rail is
powered up), a fail-safe timer begins to count down. If the
ADT7473 is not addressed by any valid SMBus transactions
before the fail-safe timeout (4.6 seconds) lapses, the
ADT7473 drives the fans to full speed. If the ADT7473 is
addressed by a valid SMBus transaction after this point, the
fans stop, and the ADT7473 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 ADT7473 is addressed by a valid SMBus transaction
before the fail-safe timeout (4.6 seconds) lapses, then the
ADT7473 operates normally, assuming the functionality of
all the default registers. See the flow chart in Figure 46.
ADT7473/ADT7473 ? 1 IS POWERED UP
Programming the Automatic Fan Speed Control Loop
To understand the automatic fan speed control loop, it is
strongly recommended to use the ADT7473/ADT7473 ? 1
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 consider the
system configuration, including the number of fans, where
they are located, and what temperatures are 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 process.
Automatic Fan Control Overview
The ADT7473/ADT7473 ? 1 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 ADT7473/ADT7473 ? 1 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 CPUs and other CPUs). These three
temperature channels can be used as the basis for automatic
Y
HAS THE ADT7473/
ADT7473 ? 1 BEEN
ACCESSED BY A VALID
SMBus TRANSACTION?
N
IS V CCP ABOVE 0.75 V?
Y
START FAIL-SAFE TIMER
N
CHECK V CCP
fan speed control to drive fans using 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. The thermal validation of the
Y
HAS THE ADT7473/
ADT7473 ? 1 BEEN
ACCESSED BY A VALID
SMBus TRANSACTION?
N
FAIL-SAFE TIMER
ELAPSES AFTER THE
FAIL-SAFE TIMEOUT
system is one of the most important steps in the design
process, so these values should be selected carefully.
Figure 47 gives a top-level overview of the automatic fan
control circuitry on the ADT7473/ADT7473 ? 1. 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 ADT7473/ADT7473 ? 1 allows the speed of four
HAS THE ADT7473/
ADT7473 ? 1 BEEN
ACCESSED BY A VALID
SMBus TRANSACTION?
Y
START UP THE ADT7473/
ADT7473 ? 1 NORMALLY
N
RUNS THE FANS
TO FULL SPEED
HAS THE ADT7473/
ADT7473 ? 1 BEEN
ACCESSED BY A VALID
SMBus TRANSACTION?
Y
SWITCH OFF FANS
N
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, a 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 47 shows controls
that are fan-specific. The designer has individual control
Figure 46. Power-on Flow Chart
over parameters such as minimum PWM duty cycle, fan
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