Table of Contents
Advertisement
DYNAMIC T
CONTROL MODE
MIN
In addition to the 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 control
loop. Dynamic T
control allows the ADT7473 to intelligently
MIN
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. In
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 ft. to 20°C air
temperature at sea level, 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 ft. 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.)
control alleviates the
MIN
Rev. 0 | Page 45 of 76
VENTS
FAN
POWER
I/O CARDS
SUPPLY
CPU
GOOD CPU AIRFLOW
DRIVE
FAN
BAYS
VENTS
GOOD VENTING =
GOOD AIR EXCHANGE
Figure 61. Chassis Airflow Issues
•
Worst-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 data sheet maximums for
peripheral components (again overcooling the system).
•
Worst-Case Assembly
Every system manufactured 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
PROCESSOR
SUBSTRATE
EPOXY
THERMAL INTERFACE MATERIAL
Figure 62. Thermal Model
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
ADT7473
FAN
I/O CARDS
VENTS
POWER
SUPPLY
CPU
POOR CPU
AIRFLOW
DRIVE
BAYS
POOR VENTING =
POOR AIR EXCHANGE
T
A
θ
T
SA
S
θ
θ
CA
TIMS
T
θ
TIM
CS
θ
CTIM
T
C
θ
TIMC
T
TIM
θ
JTIM
T
J
control function.
MIN
θ
JA
Advertisement
Table of Contents
