ADT7476A
www.onsemi.com
34
THERM Operation in Manual Mode
In manual mode, if the temperature increases above the
programmed THERM temperature limit, the fans
automatically speed up to maximum PWM or 100% PWM,
whichever way the appropriate fan channel is configured.
Automatic Fan Control Overview
The ADT7476A 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 ADT7476A 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. The thermal validation of the
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 ADT7476A. 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
ADT7476A 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,
designers 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 fan-specific controls. 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.
Figure 47. Automatic Fan Control Block Diagram
THERMAL CALIBRATION
REMOTE1
TEMP
100%
0%
T
MIN
T
RANGE
THERMAL CALIBRATION
100%
0%
T
MIN
T
RANGE
THERMAL CALIBRATION
100%
0%
T
MIN
T
RANGE
LOCAL
TEMP
REMOTE2
TEMP
PWM
MIN
PWM
MIN
PWM
MIN
MUX
S
S
S
PWM
CONFIG
PWM
GENERATOR
PWM1
PWM2
PWM3
TACH1
TACH2
TACH3
TACHOMETER 1
MEASUREMENT
TACHOMETER 2
MEASUREMENT
RAMP
CONTROL
(ACOUSTIC
ENHANCEMENT)
TACHOMETER 3
AND 4
MEASUREMENT
PWM
CONFIG
PWM
GENERATOR
RAMP
CONTROL
(ACOUSTIC
ENHANCEMENT)
PWM
CONFIG
PWM
GENERATOR
RAMP
CONTROL
(ACOUSTIC
ENHANCEMENT)
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