adt7476a Analog Devices, Inc., adt7476a Datasheet - Page 35

• Manufacturer Part Number: adt7476a
• Description: Dbcool Remote Thermal Controller And Voltage Monitor
• Manufacturer: Analog Devices, Inc.

PROGRAMMING THE AUTOMATIC FAN SPEED CONTROL LOOP
To understand the automatic fan speed control loop more
efficiently, it is recommended to use the ADT7476A evaluation
board and software while reading this section.
This section provides the system designer with an 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 tempera-
tures are being measured in the particular system.
The mechanical or thermal engineer who is tasked with the
system thermal characterization should also be involved at
the beginning of the system development process.
MANUAL FAN CONTROL OVERVIEW
In unusual circumstances, it can be necessary to manually
control the speed of the fans. Because the ADT7476A has an
SMBus interface, a system can read back all necessary voltage,
fan speed, and temperature information, and use this information
to control the speed of the fans by writing to the PWM current
duty cycle register (0x30, 0x31, and 0x32) of the appropriate
fan. Bits [7:5] of the PWMx configuration registers (0x5C,
0x5D, 0x5E) are used to set fans up for manual control.
THERM OPERATION IN MANUAL MODE
In manual mode, if the temperature increases above the pro-
grammed THERM temperature limit, the fans automatically
speed up to maximum PWM or100% 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.
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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 optimiz-
ing 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
T
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.
RANGE
. The T
MIN
and T
RANGE
values for a temperature channel
ADT7476A
MIN
and