ASC7512 ETC2 [List of Unclassifed Manufacturers], ASC7512 Datasheet - Page 30

• Manufacturer Part Number: ASC7512
• Description: DIGITAL TEMPERATURE SENSOR WITH INTEGRATED FAN CONTROL
• Manufacturer: ETC2 [List of Unclassifed Manufacturers]

Applications Information
Remote Diodes
The aSC7512 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,
The equation relating diode temperature to a change in
thermal diode voltage with two driving currents is:
where:
The aSC7512 is designed and trimmed for an expected
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
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
1.008 the difference in temperature reading at a particular
temperature may be interpreted with the following
equation:
where:
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.
© Andigilog, Inc. 2006
T = diode junction temperature in Kelvins.
T
T
nf = diode non-ideality factor, (nominal 1.008).
K = Boltzman’s constant, (1.38 x 10
q = electron charge (1.6 x 10
N = ratio of the two driving currents (16).
nf value of 1.008, based on the typical value for the Intel
n
reported
actual
actual
= actual remote diode temperature.
= selected diode’s non-ideality factor, nf .
nf
= reported temperature in temperature register.
.
T
actual
Δ
V
BE
=
= (nf )
nf factor. Table 23 gives a
T
reported
-19
KT
q
Coulombs).
⎛
⎜
⎜
⎝
ln( N )
n
. 1
actual
-23
008
).
⎞
⎟
⎟
⎠
www.andigilog.com
- 30 -
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.
Pentium 4, 90nM
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 (D-) and emitter (D+) brought out to pins.
Connection to these pins is shown in Figure 11. The non-
ideality figures in Table 23 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.
Models 8 and 10
Intel Pentium M
Models 7 and 8
AMD Athlon™
AMD Duron™
Pentium™ III
(CPUID 68h)
Table 23 Representative CPU Thermal Diode
Figure 11 CPU Remote Diode Connection
AMD Athlon
Pentium 4,
Model 6
2N3904
130nM
Part
and Transistor Non-Ideality Factors
CPU
Substrate
1.0057
1.0015
1.0000
nf Min
1.001
1.002
1.002
1.003
D+
D-
nf Nom
1.0022
1.0037
1.0046
1.008
1.002
1.011
1.008
1.008
August 2006 - 70A05003
aSC7512
aSC7512
1.0125
1.0029
1.0090
nf Max
1.003
1.016
1.016
1.005
Series
3.64
3.33
3.06
Res