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84 Appendix A - Thermal Diode Calculations
AMD Athlon™ XP Processor Model 8 Data Sheet 25175H—March 2003
Preliminary Information
Equation (1) shows the ideal diode calculation.
Sourcing two currents and using Equation (1) derives the
difference in the base-to-emitter voltage that leads to finding
the diode temperature as shown in Equation (2). The use of
dual sourcing currents allows the measurement of the thermal
diode temperature to be more accurate and less susceptible to
die and process revisions. Temperature sensors that utilize
series resistance cancellation can use more than two sourcing
currents and are suitable to be used with the AMD thermal
diode. Equation (2) is the formula for calculating the
temperature of a thermal diode.
Temperature Offset Correction
A temperature offset may be required to correct the value
measured by a temperature sensor. An offset is necessary if a
difference exists between the lumped ideality factor of the
processor and the ideality factor assumed by the temperature
sensor. The lumped ideality factor can be calculated using the
equations in this section to find the temperature offset that
should be used with the temperature sensor.
Table 30 shows the constants and variables used to calculate the
temperature offset correction.
(1)
V
BE
n
flumped,
k
q
-- -
T
I
C
I
S
----
ln⋅⋅⋅=
(2)
T
V
BE high,
V
BE low,
–
n
f lumped,
k
q
-- -
I
high
I
low
--------
ln⋅⋅
-------------------------------------------------------------- -
=
Table 30. Constants and Variables Used in Temperature Offset Equations
Equation Symbol Variable, Constant Description
n
f, actual
Actual ideality factor
n
f, lumped
Lumped ideality factor
n
f, TS
Ideality factor assumed by temperature sensor
I
high
High sourcing current
I
low
Low sourcing current
T
die, spec
Die temperature specification
T
offset
Temperature offset