ADT7467ARQZ-R7 datasheet(36/77 Pages) ONSEMI

36 / 77 page

ADT7467

Rev. 3 | Page 36 of 77 | www.onsemi.com

DYNAMIC TMIN CONTROL MODE

In addition to the automatic fan speed control mode, the

ADT7467 has a mode that extends the basic automatic fan

speed control loop. Dynamic TMIN control allows the ADT7467

to intelligently adapt the system’s cooling solution to optimize

system performance or system acoustics, depending on user or

design requirements. Use of dynamic TMIN control alleviates the

need to design for worst-case conditions, and it significantly

reduces the time required for system design and validation.

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 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 system acoustics

because the fans 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, 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.)

FAN

I/O CARDS

POOR CPU

AIRFLOW

VENTS

POWER

SUPPLY

CPU

DRIVE

BAYS

GOOD VENTING =

GOOD AIR EXCHANGE

POOR VENTING =

POOR AIR EXCHANGE

VENTS

FAN

I/O CARDS

GOOD CPU AIRFLOW

FAN

VENTS

POWER

SUPPLY

CPU

DRIVE

BAYS

Figure 50. 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

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.

SUBSTRATE

HEAT

SINK

THERMAL

INTERFACE

MATERIAL

INTEGRATED

HEAT

SPREADER

EPOXY

THERMAL INTERFACE MATERIAL

PROCESSOR

θCA

θSA

θTIMS

θCTIM

θTIMC

θJTIM

θCS

TTIM

θJA

Figure 51. Thermal Model

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 TMIN control function.

ADT7467ARQZ-R7 Datasheet(PDF) 36 Page - ON Semiconductor

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