Intel 5400 Series manual Processor Thermal Characterization Parameter Relationships

Thermal/Mechanical Reference Design

Table 2-6. Fan Speed Control, TCONTROL and DTS Relationship

Condition	FSC Scheme
DTS ≤ TCONTROL	FSC can adjust fan speed to maintain DTS ≤ TCONTROL (low acoustic region).
DTS >TCONTROL	FSC should adjust fan speed to keep TCASE at or below the Thermal Profile
	specification (increased acoustic region).

There are many different ways of implementing fan speed control, including FSC based on processor ambient temperature, FSC based on processor Digital Thermal Sensor (DTS) temperature or a combination of the two. If FSC is based only on the processor ambient temperature, low acoustic targets can be achieved under low ambient temperature conditions. However, the acoustics cannot be optimized based on the behavior of the processor temperature. If FSC is based only on the Digital Thermal

Sensor, sustained temperatures above TCONTROL drives fans to maximum RPM. If FSC is based both on ambient and Digital Thermal Sensor, ambient temperature can be used

to scale the fan RPM controlled by the Digital Thermal Sensor. This would result in an optimal acoustic performance. Regardless of which scheme is employed, system designers must ensure that the Thermal Profile specification is met when the processor Digital Thermal Sensor temperature exceeds the TCONTOL value for a given processor.

2.4.2Processor Thermal Characterization Parameter Relationships

The idea of a “thermal characterization parameter”, Ψ (psi), is a convenient way to characterize the performance needed for the thermal solution and to compare thermal solutions in identical conditions (heating source, local ambient conditions). A thermal characterization parameter is convenient in that it is calculated using total package power, whereas actual thermal resistance, θ (theta), is calculated using actual power dissipated between two points. Measuring actual power dissipated into the heatsink is difficult, since some of the power is dissipated via heat transfer into the socket and board. Be aware, however, of the limitations of lumped parameters such as Ψ when it comes to a real design. Heat transfer is a three-dimensional phenomenon that can rarely be accurately and easily modeled by lump values.

The case-to-local ambient thermal characterization parameter value (ΨCA) is used as a measure of the thermal performance of the overall thermal solution that is attached to the processor package. It is defined by the following equation, and measured in units of °C/W:

Equation 2-3.ΨCA= (TCASE - TLA) / TDP

Where:
ΨCA	= Case-to-local ambient thermal characterization parameter (°C/W).
TCASE	= Processor case temperature (°C).
TLA	= Local ambient temperature in chassis at processor (°C).
TDP	= TDP dissipation (W) (assumes all power dissipates through the
	integrated heat spreader (IHS)).

34 Quad-Core Intel® Xeon® Processor 5400 Series TMDG