Rainbow Electronics LM84 - Manual (Page 14)

3.0 Application Hints
The LM84 can be applied easily in the same way as other
integrated-circuit temperature sensors, and its remote diode
sensing capability allows it to be used in new ways as well.
It can be soldered to a printed circuit board, and because the
path of best thermal conductivity is between the die and the
pins, its temperature will effectively be that of the printed
circuit board lands and traces soldered to the LM84’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
LM84 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 LM84’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 LM84’s temperature. The LM84 has been
optimized to measure the remote diode of a Pentium II
processor as shown in
Figure 5. 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 microprocessor for temperature measure-
ment. Therefore, the LM84 can sense the temperature of this
diode effectively.
3.1 ACCURACY EFFECTS OF DIODE NON-IDEALITY
FACTOR
The technique used in today’s remote temperature sensors
is to measure the change in V
BE
at two different operating
points of a diode. For a bias current ratio of N:1, this differ-
ence is given as:
where:
• η is the non-ideality factor of the process the diode is
manufactured on,
• q is the electron charge,
• k is the Boltzmann’s constant,
• N is the current ratio,
• T is the absolute temperature in ˚K.
The temperature sensor then measures
∆
V
BE
and converts
to digital data. In this equation, k and q are well defined
universal constants, and N is a parameter controlled by the
temperature sensor. The only other parameter is
η
, which
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 tempera-
ture sensor, it will directly add to the inaccuracy of the
sensor. For the Pentium II Intel specifies a
± 1% variation in
η
from part to part. As an example, assume a temperature
sensor has an accuracy specification of ± 3˚C at room tem-
perature of 25˚C and the process used to manufacture the
diode has a non-ideality variation of
± 1%. The resulting
accuracy of the temperature sensor at room temperature will
be:
T
ACC
= ± 3˚C + ( ± 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.
3.2 PCB LAYOUT for MINIMIZING NOISE
In a noisy environment, such as a processor mother board,
layout considerations are very critical. Noise induced on
traces running between the remote temperature diode sen-
sor and the LM84 can cause temperature conversion errors.
The following guidelines should be followed:
1. Place a 0.1 µF power supply bypass capacitor as close
as possible to the V
CC
pin and the recommended 2.2 nF
capacitor as close as possible to the D+ and D− pins.
Make sure the traces to the 2.2 nF capacitor are
matched.
2. Ideally, the LM84 should be placed within 10 cm of the
Processor diode pins with the traces being as straight,
short and identical as possible.
3. Diode traces should be surrounded by a GND guard ring
to either side, above and below if possible. This GND
guard should not be between the D+ and D− lines. In the
event that noise does couple to the diode lines it would
be ideal if it is coupled common mode. That is equally to
the D+ and D− lines.(See
Figure 6)
4. Avoid routing diode traces in close proximity to power
supply switching or filtering inductors.
5. Avoid running diode traces close to or parallel to high
speed digital and bus lines. Diode traces should be kept
at least 2 cm. apart from the high speed digital traces.
6. If it is necessary to cross high speed digital traces, the
diode traces and the high speed digital traces should
cross at a 90 degree angle.
DS100961-16
Pentium Temperature vs LM84 Temperature Reading
LM84
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