LM82CIMQA National Semiconductor, LM82CIMQA Datasheet - Page 15

• Manufacturer Part Number: LM82CIMQA
• Description: IC,SSOP,16PIN
• Manufacturer: National Semiconductor

Specifications of LM82CIMQA

• Rohs Compliant: NO

4.0 Application Hints
The LM82 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 LM82’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
LM82 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 LM82’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 LM82’s temperature. The LM82 has been
optimized 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
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 LM82 can sense the temperature of this
diode effectively.
FIGURE 9. Pentium or 3904 Temperature vs LM82
Temperature Reading
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4.1 ACCURACY EFFECTS OF DIODE NON-IDEALITY
FACTOR
The technique used in today’s remote temperature sensors
is to measure the change in V
points of a diode. For a bias current ratio of N:1, this differ-
ence is given as:
where:
The temperature sensor then measures ∆V
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
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
η from part to part. As an example, assume a temperature
sensor has an accuracy specification of
temperature of 25 ˚C and the process used to manufacture
the diode has a non-ideality variation of
accuracy of the temperature sensor at room temperature will
be:
.
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.
4.2 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 LM82 can cause temperature conversion errors.
The following guidelines should be followed:
1. Place a 0.1 µF power supply bypass capacitor as close
2. The recommended 2.2nF diode bypass capacitor actu-
• η is the non-ideality factor of the process the diode is
• q is the electron charge,
• k is the Boltzmann’s constant,
• N is the current ratio,
• T is the absolute temperature in ˚K.
BE
manufactured on,
as possible to the V
capacitor as close as possible to the D+ and D− pins.
Make sure the traces to the 2.2nF capacitor are
matched.
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.
is proportional to both η and T, the variations in η
T
ACC
=
±
3˚C + (
CC
±
pin and the recommended 2.2 nF
1% of 298 ˚K) =
BE
at two different operating
±
1%. The resulting
±
BE
±
1% variation in
±
3 ˚C at room
6 ˚C
and converts
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