Thermal Management Logic and Thermal Monitor Feature
R
36 Thermal/Mechanical Design Guide
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:
I
FW
= I
S
* (E
qVD/nkT
–1)
Where I
FW
= forward bias current,
I
S
= saturation current, q = electronic charge, V = voltage across
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:
T
ERROR
= T
MEASURED
* (1 – N
ACTUAL
/ N
TRIM
)
Where T
ERROR
= correction factor to add to the reported temperature, T
MEASURED
= temperature
reported by the thermal sensor (Kelvin), N
ACTUAL
= the ideality of the on-die thermal diode, N
TRIM
= 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
TRIM
is available from the datasheet of the device measuring the processor on die
thermal diode. N
ACTUAL
can be assumed to be typical for this equation.
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 pads 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:
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.
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