Correction Factors For The On-Die Thermal Diode - Intel EM64T - Celeron D 336 Boxed Ena User Manual

Thermal Management Logic and Thermal Monitor Feature
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 that are characteristics of the processor
on-die thermal diode. 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 in 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 of times a current is passed through the diode and the ratios 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:
Where I
FW
the diode, k = Boltzmann Constant and T = absolute temperature (Kelvin). 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:
Where T
ERROR
reported by the thermal sensor (Kelvin), N
= the assumed ideality used by the thermal sensor. For the range of temperatures where the
thermal diode is being measured, 30–80 °C this error term is nearly constant.
The value of N
thermal diode. N
The series resistance, R
thermal diode temperature. R
any socket resistance or board trace resistance between the socket and the external remote diode
thermal sensor. R
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:
Where T
ERROR
q = electronic charge.
36
= forward bias current, I = saturation current, q = electronic charge, V = voltage across
S
T = T
ERROR
= correction factor to add to the reported temperature, T
is available from the datasheet of the device measuring the processor on die
TRIM
can be assumed to be typical for this equation.
ACTUAL
, is provided to allow for a more accurate measurement of the on-die
T
, as defined, includes the pads of the processor but does not include
T
can be used by remote diode thermal sensors with automatic series resistance
T
T = (R * (N – 1) * I
error T
= sensor temperature error, N = sensor current ratio, k = Boltzmann Constant,
qVD/nkT
I = I * (E –1)
FW S
* (1 – N / N
MEASURED ACTUAL
= the ideality of the on-die thermal diode, N
ACTUAL
) / (nk/q * I
FWmin
Thermal/Mechanical Design Guide
)
TRIM
= temperature
MEASURED
ln N)
N
R
TRIM