LM35

www.ti.com

SNIS159F – AUGUST 1999 – REVISED JANUARY 2016

9 Power Supply Recommendations

The LM35 device has a very wide 4-V to 5.5-V power supply voltage range, which makes it ideal for many

applications. In noisy environments, TI recommends adding a 0.1

μF from V+ to GND to bypass the power

supply voltage. Larger capacitances maybe required and are dependent on the power-supply noise.

10 Layout

10.1 Layout Guidelines

The LM35 is easily applied in the same way as other integrated-circuit temperature sensors. Glue or cement the

device to a surface and the temperature should be within about 0.01°C of the surface temperature.

The 0.01°C proximity 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

LM35 die would be at an intermediate temperature between the surface temperature and the air temperature;

this is especially true for the TO-92 plastic package. The copper leads in the TO-92 package are the principal

thermal path to carry heat into the device, so its temperature might be closer to the air temperature than to the

surface temperature.

Ensure that the wiring leaving the LM35 device is held at the same temperature as the surface of interest to

minimize the temperature problem. The easiest fix is to cover up these wires with a bead of epoxy. The epoxy

bead will ensure that the leads and wires are all at the same temperature as the surface, and that the

temperature of the LM35 die is not affected by the air temperature.

The TO-46 metal package can also be soldered to a metal surface or pipe without damage. Of course, in that

case the V

− terminal of the circuit will be grounded to that metal. Alternatively, mount the LM35 inside a sealed-

end metal tube, and then dip into a bath or screw into a threaded hole in a tank. As with any IC, the LM35 device

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 varnishes such as a conformal coating and epoxy paints or dips are often used to insure that

moisture cannot corrode the LM35 device or its connections.

These devices are sometimes soldered to a small light-weight heat fin to decrease the thermal time constant and

speed up the response in slowly-moving air. On the other hand, a small thermal mass may be added to the

sensor, to give the steadiest reading despite small deviations in the air temperature.

TO, no heat

TO-92, no heat

SOIC-8, no

TO-220, no

small heat

sink

heat fin

sink

heat fin

heat sink

heat sink

fin

Still air

400°C/W

100°C/W

180°C/W

140°C/W

220°C/W

110°C/W

90°C/W

Moving air

100°C/W

40°C/W

90°C/W

70°C/W

105°C/W

90°C/W

26°C/W

Still oil

100°C/W

40°C/W

90°C/W

70°C/W

—

—

—

Stirred oil

50°C/W

30°C/W

45°C/W

40°C/W

—

—

—

(Clamped to

metal, Infinite

(24°C/W)

—

—

(55°C/W)

—

heat sink)

(1)

Wakefield type 201, or 1-in disc of 0.02-in sheet brass, soldered to case, or similar.

(2)

TO-92 and SOIC-8 packages glued and leads soldered to 1-in square of 1/16-in printed circuit board with 2-oz foil or similar.

Copyright © 1999–2016, Texas Instruments Incorporated

Submit Documentation Feedback

19

Product Folder Links: LM35