10 Mounting

The LM50 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 02 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 LM50 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

LM50 die is directly attached to the GND pin The lands and

traces to the LM50 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 LM50s temperature to deviate from the

desired temperature

Alternatively the LM50 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 LM50 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 ensure that moisture cannot corrode the LM50 or its

connections

Temperature Rise of LM50 Due to Self-Heating

SOT-23

SOT-23

no heat sink

small heat fin

Still air

450 CW

260 CW

Moving air

180 CW

Heat sink used is

square printed circuit board with 2 oz foil with part

attached as shown in

Figure 2

Part soldered to 30 gauge wire

20 Capacitive Loads

TLH12030 – 7

FIGURE 3 LM50 No Decoupling Required

for Capacitive Load

TLH12030 – 8

FIGURE 4 LM50C with Filter for Noisy Environment

The LM50 handles capacitive loading very well Without any

special precautions the LM50 can drive any capacitive load

The LM50 has a nominal 2 kX output impedance (as can be

seen in the block diagram) The temperature coefficient of

the output resistors is around 1300 ppm C Taking into ac-

count this temperature coefficient and the initial tolerance of

the resistors the output impedance of the LM50 will not ex-

ceed 4 kX In an extremely noisy environment it may be

necessary to add some filtering to minimize noise pickup It

bypass the power supply voltage as shown in

Figure 4 Ina

noisy environment it may be necessary to add a capacitor

from the output to ground A 1 mF output capacitor with the

4kX output impedance will form a 40 Hz lowpass filter

Since the thermal time constant of the LM50 is much slower

than the 25 ms time constant formed by the RC the overall

response time of the LM50 will not be significantly affected

For much larger capacitors this additional time lag will in-

crease the overall response time of the LM50

TLH12030 – 17

R2

2k with a typical 1300 ppm C drift

FIGURE 5 Block Diagram

http

www nationalcom

4