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參數(shù)資料
| 型號： | LM90CIMM |
| 廠商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類： | 溫度/濕度傳感器 |
| 英文描述： | 3 Degree Celcious Accurate, Remote Diode and Local Digital Temperature Sensor with Two-Wire Interface |
| 中文描述： | DIGITAL TEMP SENSOR-SERIAL, 11BIT(s), 4Cel, SQUARE, SURFACE MOUNT |
| 封裝： | PLASTIC, MSOP-8 |
| 文件頁數(shù)： | 17/20頁 |
| 文件大?。?/td> | 384K |
| 代理商： | LM90CIMM |
4.0 Application Hints
(Continued)
be used to sense the temperature of external objects or
ambient air. Remember that a discrete diode’s temperature
will be affected, and often dominated, by the temperature of
its leads.
Most silicon diodes do not lend themselves well to this
application. It is recommended that a 2N3904 transistor
base emitter junction be used with the collector tied to the
base.
A diode connected 2N3904 approximates the junction avail-
able on a Pentium III microprocessor for temperature mea-
surement. Therefore, the LM90 can sense the temperature
of this diode effectively.
4.1 DIODE NON-IDEALITY
4.1.1 Diode Non-Ideality Factor Effect on Accuracy
When a transistor is connected as a diode, the following
relationship holds for variables V
BE
, T and I
f
:
where:
q = 1.6x10
19
Coulombs (the electron charge),
T = Absolute Temperature in Kelvin
k = 1.38x10
23
joules/K (Boltzmann’s constant),
η
is the non-ideality factor of the process the diode is
manufactured on,
I
S
= Saturation Current and is process dependent,
I
f
= Forward Current through the base emitter junction
V
BE
= Base Emitter Voltage drop
In the active region, the -1 term is negligible and may be
eliminated, yielding the following equation
In the above equation,
η
and I
are dependant upon the
process that was used in the fabrication of the particular
diode. By forcing two currents with a very controlled ration
(N) and measuring the resulting voltage difference, it is
possible to eliminate the I
S
term. Solving for the forward
voltage difference yields the relationship:
The non-ideality factor,
η
, is the only other parameter not
accounted for and depends on the diode that is used for
measurement. Since
V
is proportional to both
η
and T,
the variations in
η
cannot be distinguished from variations in
temperature. Since the non-ideality factor is not controlled by
the temperature sensor, it will directly add to the inaccuracy
of the sensor. For the Pentium III Intel specifies a
±
1%
variation in
η
from part to part. As an example, assume a
temperature sensor has an accuracy specification of
±
3C at
room temperature of 25 C and the process used to manu-
facture the diode has a non-ideality variation of
±
1%. The
resulting accuracy of the temperature sensor at room tem-
perature will be:
T
ACC
=
±
3C + (
±
1% of 298 K) =
±
6 C
The additional inaccuracy in the temperature measurement
caused by
η
, can be eliminated if each temperature sensor is
calibrated with the remote diode that it will be paired with.
The following table shows the variations in non-ideality for a
variety of processors.
Processor Family
η
, non-ideality
min
1
1.0065
1
1.0065
1.0057
1.008
typ
max
1.0173
1.0125
1.0125
Pentium II
Pentium III CPUID 67h
Pentium III CPUID
68h/PGA370Socket/Celeron
Pentium 4, 423 pin
Pentium 4, 478 pin
MMBT3904
AMD Athlon MP model 6
0.9933
0.9933
1.0045
1.0045
1.003
1.008
1.0368
1.0368
1.002
1.016
4.1.2 Compensating for Diode Non-Ideality
In order to compensate for the errors introduced by
non-ideality, the temperature sensor is calibrated for a par-
ticular processor. National Semiconductor temperature sen-
sors are always calibrated to the typical non-ideality of a
given processor type. The LM90 is calibrated for the
non-ideality of a mobile Pentium III processor, 1.008. When
a temperature sensor calibrated for a particular processor
type is used with a different processor type or a given
processor type has a non-ideality that strays from the typical,
errors are introduced. Figure 12 shows the minimum and
maximum errors introduced to a temperature sensor cali-
brated specifically to the typical value of the processor type
it is connected to. The errors in this figure are attributed only
to the variation in non-ideality from the typical value. In
Figure 13 is a plot of the errors that result from using a
temperature sensor calibrated for a Pentium II, the LM84,
with a typical Pentium 4 or AMD Athlon MP Model 6.
20033715
Mobile Pentium III or 3904 Temperature vs LM90
Temperature Reading
FIGURE 11.
L
www.national.com
17
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