EVAL-ADT7475EB ON Semiconductor, EVAL-ADT7475EB Datasheet - Page 27

• Manufacturer Part Number: EVAL-ADT7475EB
• Description: BOARD EVALUATION FOR ADT7475
• Manufacturer: ON Semiconductor
• Series: dBCool®r
• Type: Temperature Sensorr

Specifications of EVAL-ADT7475EB

• Contents: Evaluation Board
• For Use With/related Products: ADT7475
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

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.
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
control loop, it is strongly recommended to use the ADT7475
evaluation board and software while reading this section.
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 system thermal characterization should also be involved
at the beginning of this process.
Automatic Fan Control Overview
based upon the measured temperature. This is done
independently of CPU intervention once initial parameters
are set up.
If V
If V
To more efficiently understand the automatic fan speed
This section provides the system designer with an
The mechanical or thermal engineer who is tasked with
The ADT7475 can automatically control the speed of fans
The ADT7475 has a local temperature sensor and two re−
CCP
CC
goes high (the system processor power rail is
goes high (the system processor power rail is
REMOTE 1
REMOTE 2
LOCAL
TEMP
TEMP
TEMP
THERMAL CALIBRATION
THERMAL CALIBRATION
THERMAL CALIBRATION
T
T
T
MIN
MIN
MIN
Figure 42. Automatic Fan Control Block Diagram
T
T
T
RANGE
RANGE
RANGE
100%
100%
100%
0%
0%
0%
http://onsemi.com
MUX
27
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).
optimizing fan speed according to accurately measured
temperature. Reducing fan speed can also decrease system
current consumption.
to the number of programmable parameters, including T
and T
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.
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.
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.
PWM
PWM
PWM
MIN
MIN
MIN
Automatic fan speed control reduces acoustic noise by
The automatic fan speed control mode is very flexible owing
Figure 42 gives a top−level overview of the automatic fan
For example, the designer can decide to run the CPU fan
CONFIG
CONFIG
CONFIG
RANGE
MEASUREMENT
MEASUREMENT
MEASUREMENT
TACHOMETER 1
TACHOMETER 2
TACHOMETER 3
PWM
PWM
PWM
GENERATOR
GENERATOR
GENERATOR
AND 4
PWM
PWM
PWM
. The T
MIN
ENHANCEMENT)
ENHANCEMENT)
ENHANCEMENT)
and T
(ACOUSTIC
(ACOUSTIC
(ACOUSTIC
CONTROL
CONTROL
CONTROL
RAMP
RAMP
RAMP
RANGE
values for a temperature
PWM1
TACH1
PWM2
TACH2
PWM3
TACH3
MIN