Intel Pentium 4 Processor on 90 nm Process Thermal and Mechanical Design Guidelines

Thermal Requirements
R
Intel
®
Pentium
®
4 on 90 nm Process Thermal Design Guide 35
3.8 Impacts to Accuracy
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 that are characteristics of the processor.
The processor datasheet provides the specification for these parameters. The trace layout
recommendations between the thermal diode sensors and the processor socket should be followed
as listed the vendor datasheets. The 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 to the
accuracy of the reported on-die diode temperature. The component vendors offer components that
have stated accuracy of ± 3 °C to ± 1 °C. The improved accuracy generally comes from the
number times a current is passed through the diode and the difference in 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 5
I
FW
= I
S
* (E
qVD/nkT
-1)
Where:
I
FW
= Forward bias bias current
I
S
= saturation current
q = electronic charge
V = voltage across the diode
k = Boltzmann Constant
T = absolute temperature (Kelvin).
The series resistance, R
T
, is provided to allow for a more accurate measurement of the on-die
thermal diode temperature. R
T
, as defined, includes the pins of the processor but does not include
any socket resistance or board trace resistance between the socket and the external remote diode
thermal sensor. R
T
can be used by remote diode thermal sensors with automatic series resistance
cancellation to calibrate out this error term. Another application is that a temperature offset can be
manually calculated and programmed into an offset register in the remote diode thermal sensors
as exemplified by the equation:
Equation 6
T
error
= (R
T
* (N – 1) * I
FWmin
) / (nk/q * I
N
ln N)
Where:
T
ERROR
= sensor temperature error
N = sensor current ratio
k = Boltzmann Constant
q = electronic charge