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© Andigilog, Inc. 2006
www.andigilog.com
October 2006 - 70A06010
aSC7621
Applications Information
Remote Diodes
The aSC7621 is designed to work with a variety of
remote sensors in the form of the substrate thermal diode
of a CPU or graphics controller or a diode-connected
transistor. Actual diodes are not suited for these
measurements.
There is some variation in the performance of these
diodes, described in terms of its departure from the ideal
diode equation. This factor is called diode non-
ideality,
.
nf
The equation relating diode temperature to a change in
thermal diode voltage with two driving currents is:
ΔV
BE
= (nf )
KT
q
ln( N )
where:
nf = diode non-ideality factor, (nominal 1.009).
K = Boltzman’s constant, (1.38 x 10
-23).
T = diode junction temperature in Kelvins.
q = electron charge (1.6 x 10
-19 Coulombs).
N = ratio of the two driving currents (16).
The aSC7621 is designed and trimmed for an expected
nf value of 1.009, based on the typical value for the Intel
Pentium™ III and AMD Athlon™. There is also a
tolerance on the value provided. The values for other
CPUs and the 2N3904 may have different nominal values
and tolerances. Consult the CPU or GPU manufacturer’s
data sheet for the
nf factor.
Table 31 gives a
representative sample of what one may expect in the
range of non-ideality. The trend with CPUs is for a lower
value with a larger spread.
When thermal diode has a non-ideality factor other than
1.009 the difference in temperature reading at a particular
temperature may be interpreted with the following
equation:
T
actual = Treported
1.009
n
actual
⎛
⎝
⎜⎜
⎞
⎠
⎟⎟
where:
reported
T
= reported temperature in temperature register.
actual
T
= actual remote diode temperature.
actual
n
= selected diode’s non-ideality factor, nf .
Temperatures are in Kelvins or °C + 273.15.
This equation assumes that the series resistance of the
remote diode is the same for each. This resistance is
given in the data sheet for the CPU and may vary from
2.5Ω to 4.5Ω.
Although the temperature error caused by non-ideality
difference is directly proportional to the difference from
1.008, but a small difference in non-ideality results in a
relatively large difference in temperature reading. For
example, if there were a ±1% tolerance in the non-Ideality
of a diode it would result in a ±2.7 degree difference (at
0°C) in the result (0.01 x 273.15).
This difference varies with temperature such that a fixed
offset value may only be used over a very narrow range.
Typical correction method required when measuring a
wide range of temperature values is to scale the
temperature reading in the host firmware.
Part
nf Min
nf Nom
nf Max
Series
Res
Pentium™ III
(CPUID 68h)
1.0057
1.008
1.0125
Pentium 4,
130nM
1.001
1.002
1.003
3.64
Pentium 4, 90nM
1.011
3.33
Pentium 4, 65nM
1.009
4.52
Intel Pentium M
1.0015
1.0022
1.0029
3.06
AMD Athlon™
Model 6
1.002
1.008
1.016
AMD Duron™
Models 7 and 8
1.002
1.008
1.016
AMD Athlon
Models 8 and 10
1.0000
1.0037
1.0090
2N3904
1.003
1.0046
1.005
Table 31 Representative CPU Thermal Diode
and Transistor Non-Ideality Factors
CPU or ASIC Substrate Remote Diodes
A substrate diode is a parasitic PNP transistor that has its
collector tied to ground through the substrate and the
base (Remote -) and emitter (Remote +) brought out to
pins. Connection to these pins is shown in Figure 13. The
non-ideality figures in Table 31 include the effects of any
package resistance and represent the value seen from
the CPU socket. The temperature indicated will need to
be compensated for the departure from a non-ideality of
1.008.
Remote +
Remote -
CPU
aSC7621
Substrate
Figure 13 CPU Remote Diode Connection
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