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System integrators should note that there is no defined correlation between the on-die thermal
diode and the processor case temperature. The temperature distribution across the die is affected by
the power being dissipated, type of activity the processor is performing e.g., integer or floating
point intensive and the leakage current. The dynamic and independent nature of these effects
makes it difficult to provide a meaningful correlation for the processor population.
System integrators planning to use the thermal diode for system or component level fan control to
optimize acoustics need to refer to
4.2.7.1
Reading the On-Die Thermal Diode Interface
The on-die thermal diode is accessible from a pair of pins on the processor. The fan speed
controller remote thermal sense signals should be connected to these pins per the vendor's
recommended layout guidelines.
Table 3.
Thermal Diode Interface
Pin Name
THERMDA
THERMDC
4.2.7.2
Correction Factors for the On-Die Thermal Diode
A number of issues can affect the accuracy of the temperature reported by thermal diode sensors.
These include the diode ideality and the series resistance, which are characteristics of the processor
on-die thermal diode. The processor datasheet provides specifications for these parameters. The
trace layout recommendations between the thermal diode sensors and the processor socket should
be followed as listed in the vendor datasheets. Design characteristics and usage models of the
thermal diode sensors should be reviewed in the datasheets available from the manufacturers.
The choice of a remote diode sensor measurement component has a significant impact on the
accuracy of the reported on-die diode temperature. Component vendors offer components that have
stated accuracy of ± 3 °C to ± 1 °C. The improved accuracy generally comes from the number of
times a current is passed through the diode and the ratio of the currents. Consult the vendor
datasheet for details on their measurement process and stated accuracy.
The ideality factor, n, represents the deviation from ideal diode behavior as exemplified by the diode
equation:
Equation 3.
Where I
FW
the diode, k = Boltzmann Constant and T = absolute temperature (Kelvin).
Equation 4.
This equation determines the ideality factor of an individual diode.
For the purpose of determining a correction factor to use with the thermal sensor, the ideality
equation can be simplified to the following:
®
®
Intel
Celeron
D Processor in the 775-Land LGA Package Thermal Design Guide27
303730
Order #
Section 4.3, "Acoustic Fan Speed Control" on page
Pin Number
B3
C4
qVD/nkT
I
= I
* (E
FW
S
= forward bias current, I
S
Thermal Management Logic and Thermal Monitor
Pin Description
Diode anode
Diode anode
-1)
= saturation current, q = electronic charge, V = voltage across
29.
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