Rainbow Electronics LM86 - Manual (Page 17)

3.0 Application Hints
(Continued)
pins, its temperature will effectively be that of the printed
circuit board lands and traces soldered to the LM86’s pins.
This presumes that the ambient air temperature is almost the
same as the surface temperature of the printed circuit board;
if the air temperature is much higher or lower than the
surface temperature, the actual temperature of the of the
LM86 die will be at an intermediate temperature between the
surface and air temperatures. Again, the primary thermal
conduction path is through the leads, so the circuit board
temperature will contribute to the die temperature much
more strongly than will the air temperature.
To measure temperature external to the LM86’s die, use a
remote diode. This diode can be located on the die of a
target IC, allowing measurement of the IC’s temperature,
independent of the LM86’s temperature. The LM86 has been
optimized to measure the remote diode of a Pentium III
processor as shown in Figure 11. A discrete diode can also
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 LM86 can sense the temperature
of this diode effectively.
3.1 DIODE NON-IDEALITY
3.1.1 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
S
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
BE
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 ± 1˚C 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
= ± 1˚C + ( ± 1% of 298 ˚K) = ± 4 ˚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 typ max
Pentium II 1 1.0065 1.0173
Pentium III CPUID 67h 1 1.0065 1.0125
Pentium III CPUID
68h/PGA370Socket/Celeron
1.0057 1.008 1.0125
Pentium 4, 423 pin 0.9933 1.0045 1.0368
Pentium 4, 478 pin 0.9933 1.0045 1.0368
MMBT3904 1.003
AMD Athlon MP model 6 1.002 1.008 1.016
3.1.2 Compensating for Diode Non-Ideality
In order to compensate for the errors introduced by non-
ideality, the temperature sensor is calibrated for a particular
processor. National Semiconductor temperature sensors are
always calibrated to the typical non-ideality of a given pro-
cessor type. The LM86 is calibrated for the non-ideality of a
10130315
Mobile Pentium III or 3904 Temperature vs LM86
Temperature Reading
FIGURE 11.
LM86
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