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ADT7467
? Worst-case Altitude
A computer can be operated at different altitudes. The
altitude affects the relative air density, which alters the
effectiveness of the fan cooling solution. For example,
comparing 40 ? C air temperature at 10,000 ft. to 20 ? C
air temperature at sea level, relative air density is
misaligned. Too much or too little thermal grease might
be used, or variations in application pressure for
thermal interface material could affect the efficiency of
the thermal solution. Accounting for manufacturing
variations in every system is difficult; therefore, the
system must be designed for worst-case conditions.
T TIM
q CTIM
?
increased by 40%. This means that at a given
temperature, the fan can spin 40% slower and make less
noise at sea level than it can at 10,000 ft.
Worst-case Fan
Due to manufacturing tolerances, fan speeds in RPM
are normally quoted with a tolerance of ? 20%. The
designer should assume that the fan RPM is 20% below
tolerance. This translates to reduced system airflow and
elevated system temperature. Note that a difference of
20% in the fans’ tolerance can negatively impact
Heat Sink
Thermal
Interface
Material
Integrated
Heat
Spreader
Processor
Substrate
Epoxy
Thermal Interface Material
q SA T S
q TIMS
q TIMC
q JTIM
T A
q CS
T C
T TIM
T J
q CA
q JA
system acoustics because the fans run faster and
Figure 50. Thermal Model
Fan
Fan
Vents
?
generate more noise.
Worst-case Chassis Airflow
The same motherboard can be used in a number of
different chassis configurations. The design of the
chassis and the physical location of fans and
components determine the system thermal
characteristics. Moreover, for a given chassis, the
addition of add-in cards, cables, and other system
configuration options can alter the system airflow and
reduce the effectiveness of the system cooling solution.
The cooling solution can also be inadvertently altered
by the end user. (For example, placing a computer
against a wall can block the air ducts and reduce system
airflow.)
Vents
I/O Cards
I/O Cards Power Power
Supply Supply
Although a design usually accounts for such worst-case
conditions, the system is almost never operated at
worst-case conditions. An alternative to designing for the
worst case is to use the dynamic T MIN control function.
Dynamic T MIN Control Overview
Dynamic T MIN control mode builds on the basic
automatic fan control loop by adjusting the T MIN value
based on system performance and measured temperature.
Therefore, instead of designing for the worst case, the
system thermals can be defined as operating zones.
ADT7467 can self-adjust its fan control loop to maintain
either an operating zone temperature or a system target
temperature. For example, users can specify that the ambient
temperature in a system be maintained at 50 ? C. If the
temperature is below 50 ? C, the fans may not run or may run
very slowly. If the temperature is higher than 50 ? C, the fans
may throttle up.
The challenge presented by any thermal design is finding
Good CPU Airflow
Fan
CPU
Drive
Bays
Poor CPU
Airflow
CPU
Drive
Bays
the right settings to suit the system’s fan control solution.
This can involve designing for the worst case, followed by
weeks of system thermal characterization, and finally fan
acoustic optimization (for psychoacoustic reasons).
Poor Venting =
Vents
Good Venting = Good Air Exchange Poor Air Exchange
Figure 49. Chassis Airflow Issues
? Worst-case Processor Power Consumption
Designing for worst-case CPU power consumption can
result in a processor becoming overcooled, generating
excess system noise.
? Worst-case Peripheral Power Consumption
The tendency is to design to data sheet maximums for
peripheral components (again overcooling the system).
? Worst-case Assembly
Every system is unique because of manufacturing
variations. Heat sinks may be loose fitting or slightly
Optimizing the automatic fan control mode involves
characterizing the system to determine the best T MIN and
T RANGE settings for the control loop and the PWM MIN value
that produces the quietest fan speed setting. Using the
ADT7467 dynamic T MIN control mode, however, shortens
the characterization time and alleviates tweaking the control
loop settings because the device can self-adjust during
system operation.
Dynamic T MIN control mode is operated by specifying the
operating zone temperatures required for the system.
Associated with this control mode are three operating point
registers, one for each temperature channel. This allows the
system thermal solution to be broken down into distinct
thermal zones. For example, CPU operating temperature is
70 ? C, VRM operating temperature is 80 ? C, and ambient
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