Printed Circuit Board

1.0 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 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 LM50 die would be at an interme-

diate 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 being

measured. These printed circuit board lands and traces will

not cause the LM50s temperature to deviate from the de-

sired 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 conden-

sation can occur. Printed-circuit coatings and varnishes such

as Humiseal and epoxy paints or dips are often used to

ensure that moisture cannot corrode the LM50 or its connec-

tions.

Temperature Rise of LM50 Due to Self-Heating

(Thermal Resistance,

θ

SOT-23

SOT-23

no heat sink*

small heat fin**

Still air

450˚C/W

260˚C/W

Moving air

180˚C/W

* Part soldered to 30 gauge wire.

attached as shown in Figure 2.

2.0 Capacitive Loads

The LM50 handles capacitive loading very well. Without any

special precautions, the LM50 can drive any capacitive load.

The LM50 has a nominal 2 k

Ω output impedance (as can be

seen in the block diagram). The temperature coefficient of

the output resistors is around 1300 ppm/˚C. Taking into

account this temperature coefficient and the initial tolerance

of the resistors the output impedance of the LM50 will not

exceed 4 k

Ω. In an extremely noisy environment it may be

necessary to add some filtering to minimize noise pickup. It

is recommended that 0.1 µF be added from V

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 µF output capacitor with the

4k

Ω 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.

01203019

FIGURE 2. Printed Circuit Board Used

for Heat Sink to Generate All Curves.

1

with 2 oz. Foil or Similar

01203007

FIGURE 3. LM50 No Decoupling Required

for Capacitive Load

01203008

FIGURE 4. LM50C with Filter for Noisy Environment

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