REV. A
ADM1027
27
AUTOMATIC FAN SPEED CONTROL MODE
The ADM1027 has a local temperature sensor and two remote
temperature channels that may be connected to an on-chip
diode-connected transistor on a CPU. These three temperature
channels may be used as the basis for automatic fan speed control
to drive fans using pulsewidth modulation (PWM). In general,
the greater the number of fans in a system, the better the cooling,
but to the detriment of system acoustics. Automatic fan speed
control reduces acoustic noise by optimizing fan speed according
to measured temperature. Reducing fan speed can also decrease
system current consumption. The automatic fan speed control
mode is very flexible, owing to the number of programmable param-
eters, including T
MIN
and T
RANGE
, as discussed in detail later.
The T
MIN
and T
RANGE
values chosen for a given fan are critical,
since these define the thermal characteristics of the system. The
thermal validation of the system is one of the most important
steps of the design process, so these values should be carefully
selected.
The aim of this section is not only to provide the system designer
with an understanding of the automatic fan control loop, but also
to provide step-by-step guidance as to how to most effectively
evaluate and select the critical system parameters. To optimize
the system characteristics, the designer needs to give some fore-
thought to how the system will be configured, e.g., the number
of fans, where they are located, and what temperatures are being
measured in the particular system. The mechanical or thermal
engineer who is tasked with the actual system evaluation should
also be involved at the beginning of the process.
Automatic Fan Control Overview
Figure 27 gives a top-level overview of the automatic fan control
circuitry on the ADM1027. 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 ADM1027 allows the speed
of four fans to be monitored. The right side of the block diagram
shows controls that are fan-specific. The designer has control
over individual parameters such as minimum PWM duty cycle,
fan speed failure thresholds, and even ramp control of the PWM
outputs. This ultimately allows graceful fan speed changes that
are less perceptible to the system user.
TACHOMETER 1
MEASUREMENT
RAMP
CONTROL
(ACOUSTIC
ENHANCEMENT
TACHOMETER 2
MEASUREMENT
RAMP
CONTROL
(ACOUSTIC
ENHANCEMENT
TACHOMETER 3
AND 4
MEASUREMENT
PWM
GENERATOR
PWM
CONFIG
PWM
GENERATOR
PWM
CONFIG
RAMP
CONTROL
(ACOUSTIC
ENHANCEMENT
PWM
GENERATOR
PWM
CONFIG
PWM
MIN
PWM
MIN
PWM
MIN
MUX
THERMAL CALIBRATION
100%
0%
T
MIN
T
RANGE
THERMAL CALIBRATION
100%
0%
T
MIN
T
RANGE
THERMAL CALIBRATION
100%
0%
T
MIN
T
RANGE
PWM1
PWM2
PWM3
REMOTE 1
TEMP
LOCAL
TEMP
REMOTE 2
TEMP
Figure 27. Automatic Fan Control Block Diagram
Rev. 3 | Page 27 of 56 | www.onsemi.com
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