Applications
The LM45 can be applied easily in the same way as other
integrated-circuit temperature sensors It can be glued or
cemented to a surface and its temperature will be within
about 0 2 C of the surface temperature
This presumes that the ambient air temperature is almost
the same as the surface temperature if the air temperature
were much higher or lower than the surface temperature
the actual temperature of the LM45 die would be at an inter-
mediate temperature between the surface temperature and
the air temperature
To ensure good thermal conductivity the backside of the
LM45 die is directly attached to the GND pin The lands and
traces to the LM45 will of course be part of the printed
circuit board which is the object whose temperature is be-
ing measured These printed circuit board lands and traces
will not cause the LM45s temperature to deviate from the
desired temperature
Alternatively the LM45 can be mounted inside a sealed-end
metal tube and can then be dipped into a bath or screwed
into a threaded hole in a tank As with any IC the LM45 and
accompanying wiring and circuits must be kept insulated
and dry to avoid leakage and corrosion This is especially
true if the circuit may operate at cold temperatures where
condensation can occur Printed-circuit coatings and var-
nishes such as Humiseal and epoxy paints or dips are often
used to insure that moisture cannot corrode the LM45 or its
connections
Temperature Rise of LM45 Due to Self-Heating
(Thermal Resistance)
SOT-23
SOT-23
no heat sink
small heat fin
Still air
450 C W
260 C W
Moving air
180 C W
Heat sink used is
square printed circuit board with 2 oz foil with part
attached as shown in
Figure 3
Part soldered to 30 gauge wire
Typical Applications
CAPACITIVE LOADS
Like most micropower circuits the LM45 has a limited ability
to drive heavy capacitive loads The LM45 by itself is able to
drive 500 pF without special precautions If heavier loads
are anticipated it is easy to isolate or decouple the load with
a resistor see
Figure 4
Or you can improve the tolerance of
capacitance with a series R-C damper from output to
ground see
Figure 5
Any linear circuit connected to wires in a hostile environ-
ment can have its performance affected adversely by in-
tense electromagnetic sources such as relays radio trans-
mitters motors with arcing brushes SCR transients etc as
its wiring can act as a receiving antenna and its internal
junctions can act as rectifiers For best results in such cas-
es a bypass capacitor from V
IN
to ground and a series R-C
damper such as 75X in series with 0 2 or 1
mF
from output
to ground as shown in
Figure 5
are often useful
TL H 11754 – 9
FIGURE 5 LM45 with R-C Damper
TL H 11754 – 12
TL H 11754–8
FIGURE 4 LM45 with Decoupling from Capacitive Load
FIGURE 6 Temperature Sensor
Single Supply
b
20 C to
a
100 C
4
NSC [ NATIONAL SEMICONDUCTOR ]
