Pr Ogramming The Automatic Fan Speed Control Loop; Automatic Fan Control Overview - Analog Devices dBCool ADT7473 Manual

PROGRAMMING THE AUTOMATIC
To understand the automatic fan speed control loop, it is
strongly recommended to use the ADT7473 evaluation board
and software while reading this section.
This section provides the system designer with an understand-
ing f the automatic fan control loop, and provid o
guidance on effectively evaluating and selecting cr
para me rs. To optimize the system te
needs to consider the system configuration, including the
num ber f fans, where they are located, and what temperatures o
are meas ured in the particular
The mechanical or thermal engineer who is tasked with the
syste m thermal characterization should also be involved at th
begi nning of the process.
AUTOM ATIC FAN CONTROL OVERVIEW
T he ADT7473 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 has a local temperature sensor and
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 fan speed control to drive fans using PWM.
REMOTE 1
TEMP
LOCAL
TEMP
REMOTE 2
TEMP
FAN SPEED CO T
es step-by-step
itical system
characteristics, the designer
system.
two remote
THERMAL CALIBRATION
100%
0%
T T
MIN RANGE
THERMAL CALIBRATION
100%
0%
T T
MIN RANGE
THERMAL CALIBRATION
100%
0%
T T
MIN RANGE
Figure 44. Automatic Fan Control Block Diagram
N ROL LOOP
Auto matic fan speed control reduces acoustic noise by optimiz-
ing an speed according to accurately measured temperature. f
Re du cing fan speed can also decrease system current consump-
tion. The a utomatic fan speed control mode is
to th e number of programmable parameters, including T
and T . The T
RANGE
channel and, therefore, for a given fan, are critical because they
def e the thermal characteristics of the system. The therm in
valid ation of t
the d esign process, so these values should be selected car
Figu re 44 gives a top-level overview of the automatic fan control
circu itry on the ADT7473. From a systems-level perspective
e
to th ree system temperatures can be monitored and used to
cont rol three PWM outputs. The three PWM outputs c
used to control up to four fans. The ADT7473 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, a designer can decide to run the CPU fan
when CPU tem
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 44 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
MIN
RAMP
CONTROL
(ACOUSTIC
ENHANCEMENT)
TACHOMETER 1
MEASUREMENT
PWM
MIN
RAMP
CONTROL
MUX
(ACOUSTIC
ENHANCEMENT)
TACHOMETER 2
MEASUREMENT
PWM
MIN
RAMP
CONTROL
(ACOUSTIC
ENHANCEMENT)
TACHOMETER 3
AND 4
MEASUREMENT
Rev. A | Page 33 of 76
and T value s for a temperature
MIN RANGE
he system is one of the most important steps in
perature increases above 60°C, and a chassis fan
PWM
CONFIG
PWM
PWM1
GENERATOR
TACH1
PWM
CONFIG
PWM
PWM2
GENERATOR
TACH2
PWM
CONFIG
PWM
PWM3
GENERATOR
TACH3
ADT7473
very flexible due
MIN
al
efully.
, up
an be