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ADT7473
Bit <6> LT = 1, enables dynamic T MIN control on the local
temperature channel. The chosen T MIN value is dynamically
adjusted based on the current temperature, operating point,
and high and low limits for this zone.
LT = 0, disables dynamic T MIN control. The T MIN value
chosen is not adjusted and the channel behaves as described
in the Automatic Fan Control Overview section.
Bit <5> R1T = 1, enables dynamic T MIN control on the
Remote 1 temperature channel. The chosen T MIN value is
dynamically adjusted based on the current temperature,
operating point, and high and low limits for this zone.
R1T = 0, disables dynamic T MIN control. The T MIN value
chosen is not adjusted, and the channel behaves as described
in the Automatic Fan Control Overview section.
Enhancing System Acoustics
Automatic fan speed control mode reacts instantaneously
to changes in temperature; that is, the PWM duty cycle
responds immediately to temperature change. Any impulses
in temperature can cause an impulse in fan noise. For
psycho-acoustic reasons, the ADT7473/ADT7473 ? 1 can
prevent the PWM output from reacting instantaneously to
temperature changes. Enhanced acoustic mode controls the
maximum change in PWM duty cycle at a given time. The
objective is to prevent the fan from cycling up and down,
annoying the user.
Acoustic Enhancement Mode Overview
Figure 74 gives a top-level overview of the automatic fan
control circuitry on the ADT7473/ADT7473 ? 1 and shows
where acoustic enhancement fits in. Acoustic enhancement
mechanical engineer evaluating best settings for the system.
Having determined the optimal settings for the thermal
solution, the engineer can adjust the system acoustics. The
goal is to implement a system that is acoustically pleasing
without causing user annoyance due to fan cycling. It is
important to realize that although a system might pass an
acoustic noise requirement specification (for example,
36 dB), if the fan is annoying, it fails the consumer test.
Approaches to System Acoustic Enhancement
There are two different approaches to implementing
system acoustic enhancement: temperature-centric and
fan-centric.
The temperature-centric approach involves smoothing
transient temperatures as they are measured by a
temperature source (for example, Remote 1 temperature).
The temperature values used to calculate the PWM duty
cycle values are smoothed, reducing fan speed variation.
However, this approach causes an inherent delay in updating
fan speed and causes the thermal characteristics of the
system to change. It also causes the system fans to stay on
longer than necessary because the fan’s reaction is merely
delayed. The user has no control over noise from different
fans driven by the same temperature source. Consider, for
example, a system in which control of a CPU cooler fan (on
PWM1) and a chassis fan (on PWM2) use Remote 1
temperature. Because the Remote 1 temperature is
smoothed, both fans are updated at exactly the same rate. If
the chassis fan is much louder than the CPU fan, there is no
way to improve its acoustics without changing the thermal
solution of the CPU cooling fan.
is intended as a post design tweak made by a system or
ACOUSTIC
ENHANCEMENT
THERMAL CALIBRATION
100%
PWM
MIN
PWM
CONFIG
RAMP
S
CONTROL
(ACOUSTIC
PWM
GENERATOR
PWM1
ENHANCEMENT)
REMOTE2 =
CPU TEMP
T MIN T RANGE
THERMAL CALIBRATION
0%
100%
PWM
MIN
TACHOMETER1
MEASUREMENT
RAMP
PWM
CONFIG
TACH1
CPU FAN SINK
MUX
S
CONTROL
(ACOUSTIC
ENHANCEMENT)
PWM
GENERATOR
PWM2
T MIN
T RANGE
0%
PWM
TACHOMETER2
MEASUREMENT
PWM
TACH2
LOCAL =
VRM TEMP
THERMAL CALIBRATION
100%
MIN
RAMP
CONFIG
FRONT CHASSIS
S
CONTROL
(ACOUSTIC
ENHANCEMENT)
PWM
GENERATOR
PWM3
T MIN
T RANGE
0%
TACHOMETER3
AND 4
MEASUREMENT
TACH3
REMOTE1 =
AMBIENT TEMP
Figure 74. Acoustic Enhancement Smoothes Fan Speed Variations
Under Automatic Fan Speed Control
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