ASC7531A ETC2 [List of Unclassifed Manufacturers], ASC7531A Datasheet - Page 14

• Manufacturer Part Number: ASC7531A
• Description: LOW-VOLTAGE 1-WIRE DIGITAL TEMPERATURE SENSOR AND VOLTAGE MONITOR
• Manufacturer: ETC2 [List of Unclassifed Manufacturers]

Applications Information
Remote Diodes
The aSC7531 is designed to work with a variety of remote
sensors in the form of a diode-connected transistor or the
substrate thermal diode of a CPU or graphics controller.
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:
factor.
The aSC7531A is designed and trimmed for an expected
value of 1.0046, based on the typical value for the 2N3904.
There is also a tolerance on the value provided.
gives a representative sample of what one may expect in the
range of non-ideality.
For the aSC7531A, when thermal diode has a non-ideality
factor other than 1.0046 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 0.6Ω.
Although the temperature error caused by non-ideality
difference is directly proportional to the difference from
1.0046, 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).
© Andigilog, Inc. 2006
T = diode junction temperature in Kelvins.
T
T
nf = diode-connected 2N3904 or CPU substrate non-ideality
K = Boltzman’s constant, (1.38 x 10
q = electron charge (1.6 x 10
N = ratio of the two driving currents (10).
n
reported
actual
actual
= actual remote diode temperature.
= selected diode’s non-ideality factor,
= reported temperature in temperature register.
T
actual
Δ
V
BE
=
= (nf )
T
reported
-19
Coulombs).
KT
q
⎛
⎜
⎜
⎝
. 1
ln( N )
-23
n
actual
0046
).
⎞
⎟
⎟
⎠
nf
.
nf
.
Table 6
www.andigilog.com
nf
- 14 -
The aSC7531B is designed and trimmed for an expected
value of 1.009, based on the typical value for the 65nM
Pentium CPU. There is also a tolerance on the value
provided. The values for CPUs may have different nominal
values
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
1.009 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 4.52Ω.
Although the temperature error caused by non-ideality
difference is directly proportional to the difference from 1.009,
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.
T
T
n
reported
actual
actual
Pentium 4, 90nM
Pentium 4, 65nM
Table 6 Representative CPU Thermal Diode and
Intel Pentium M
Pentium™ III
(CPUID 68h)
Pentium 4,
= actual remote diode temperature.
= selected diode’s non-ideality factor,
2N3904
130nM
= reported temperature in temperature register.
Part
and
Transistor Non-Ideality Factors
tolerances.
T
actual
1.0057
1.0015
nf Min
1.001
1.000
1.003
= T
reported
Consult
nf Nom
1.0022
1.0046
1.008
1.002
1.011
1.009
⎛
⎜ ⎜
⎝
1.009
n
December 2006 - 70A05012
nf
actual
aSC7531A
factor.
the
⎞
⎟ ⎟
⎠
1.0125
1.0029
nf Max
1.003
1.050
1.005
nf
CPU
Table 6 gives a
.
or
Series
3.64
4.52
3.06
3.33
Res
0.6
GPU
nf