Rainbow Electronics LM83 - Manual (Page 17)

4.0 Application Hints
The LM83 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 cir-
cuit board lands and traces soldered to the LM83’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 sur-
face temperature, the actual temperature of the of the LM83
die will be at an intermediate temperature between the sur-
face and air temperatures. Again, the primary thermal con-
duction path is through the leads, so the circuit board tem-
perature will contribute to the die temperature much more
strongly than will the air temperature.
To measure temperature external to the LM83’s die, use a
remote diode. This diode can be located on the die of a tar-
get IC, allowing measurement of the IC’s temperature, inde-
pendent of the LM83’s temperature. The LM83 has been op-
timized to measure the remote diode of a Pentium II
processor as shown in
Figure 9. 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 af-
fected, and often dominated, by the temperature of its leads.
Most silicon diodes do not lend themselves well to this appli-
cation. 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 LM83 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 uni-
versal constants, and N is a parameter controlled by the tem-
perature sensor. The only other parameter is
η
, which de-
pends on the diode that is used for measurement. Since
∆
V
BE
is proportional to both
η
and T, the variations in
η
can-
not 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 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 temperature 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 LM83 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 2.2nF capacitor are
matched.
2. The recommended 2.2nF diode bypass capacitor actu-
ally has a range of 200pF to 3.3nF. The average tem-
perature accuracy will not degrade. Increasing the ca-
pacitance will lower the corner frequency where
differential noise error affects the temperature reading
thus producing a reading that is more stable. Con-
versely, lowering the capacitance will increase the cor-
ner frequency where differential noise error affects the
temperature reading thus producing a reading that is
less stable.
3. Ideally, the LM83 should be placed within 10cm of the
Processor diode pins with the traces being as straight,
short and identical as possible. Trace resistance of 1
Ω
can cause as much as 1˚C of error.
4. 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 10)
5. Avoid routing diode traces in close proximity to power
supply switching or filtering inductors.
DS101058-15
Pentium or 3904 Temperature vs LM83 Temperature
Reading
LM83
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