ADT7476 Datasheet, PDF (37/72 Pages) Analog Devices – dBCool Remote Thermal Controller and Voltage Monitor

ADT7476
PROGRAMMING THE AUTOMATIC FAN SPEED CONTROL LOOP
To understand the automatic fan speed control loop, use the
ADT7476 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 ADT7476 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
current PWM duty cycle registers (0x30, 0x31, and 0x32) of the
appropriate fan. Bits [7:5] of the PWMx configuration registers
(0x5C, 0x5D, 0x5E) are used to set up fans 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 ADT7476 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.
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 owing to the number of programmable parame-
ters, including TMIN and TRANGE. The TMIN and TRANGE 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 ADT7476. 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 ADT7476 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, one 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 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.
The ADT7476 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 CPUs and other
CPUs). These three temperature channels can be used as the
basis for automatic fan speed control to drive fans using PWM.
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