Dynamic Tmin Control Mode - Analog Devices dBCool ADT7473 Manual

DYNAMIC T CONTROL MODE
MIN
In addition to th e automatic fan speed control mode described
in the Automatic Fan Control Overview section, the ADT7473
has a mode that extends the basic automatic fan speed contr
loop. Dynamic T control allows the ADT7473 to intelligentl
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adapt the system's cooling solution for best system performance
or lowest possible system acoustics, depending on user or
design requirements. Use of dynamic T
need to design for worst-case conditions and significantly
reduces system design and validation time.
Designing for Worst-Case Conditions
System design must always allow for worst-case conditions.
PC design, the worst-case conditions include, but are not
limited to the following:
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 feet to 20°
relative air density is increased by 40%. This means that the fan
can spin 40% slower and make less noise at sea level than at
10,000 feet while keeping the system at the same temperature at
both locations.
Worst-Case Fan
Due to manufacturing tolerances, fan speeds in RPM are
normally quoted with a tolerance of ±20%. The designer needs
to assume that the fan RPM can be 20% below tolerance. This
translates to reduced system airflow and elevated system
temperature. Note that fans 20% out of tolerance can negatively
impact system acoustics because they run faster and 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, or 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.)
ol
control alleviates the
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In
C air temperature at sea level,
Rev. A | Page 47 of 76
VENTS
I/O CARDS
y
GOOD CPU AIRFLOW
FAN
VENTS
GOOD VENTING =
GOOD AIR EXCHANGE
W orst-Case Processor Power Consumption
This data sheet maximum does not necessarily reflect the true
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 dat
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 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 the worst case.
HEAT
SINK
THERMAL
INTERFACE
MATERIAL
INTEGRATED
HEAT
SPREADER
SUBSTRATE
EPOXY
THERMAL INTERFACE MATERIAL
Although a design usually accounts for worst-case conditions in
all these cases, the actual system is almost never operated at
worst-case conditions. The alternative to designing for the
worst case is to use the dynamic T
FAN
I/O CARDS
VENTS
POWER
SUPPLY
CPU
POOR CPU
AIRFLOW
DRIVE
BAYS
POOR VENTING =
POOR AIR EXCHANGE
Figure 61. Chassis Airflow Issues
a sheet maximums for
T
A
θ
T
SA
S
θ
TIMS
T
TIM
θ
CTIM
T
C
θ
TIMC
T
TIM
PROCESSOR
θ
JTIM
T
J
Figure 62. Thermal Model
control function.
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ADT7473
FAN
POWER
SUPPLY
CPU
DRIVE
BAYS
θ
CA
θ
CS
θ
JA