1.0 Mounting

The LM61 can be applied easily in the same way as other

integrated-circuit temperature sensors. It can be glued or

cemented to a surface. The temperature that the LM61 is

sensing will be within about +0.2˚C of the surface tempera-

ture that LM61’s leads are attached to.

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 measured would be at an intermediate

temperature between the surface temperature and the air

temperature.

To ensure good thermal conductivity the backside of the

LM61 die is directly attached to the GND pin. The lands and

traces to the LM61 will, of course, be part of the printed

circuit board, which is the object whose temperature is being

measured.

Alternatively, the LM61 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 LM61 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 LM61 or its connec-

tions.

The thermal resistance junction to ambient (

θ

rameter used to calculate the rise of a device junction tem-

perature due to its power dissipation. For the LM61 the

equation used to calculate the rise in the die temperature is

as follows:

T

θ

where I

the output. Since the LM61’s junction temperature is the

actual temperature being measured care should be taken to

minimize the load current that the LM61 is required to drive.

The table shown in Figure 3 summarizes the rise in die

temperature of the LM61 without any loading with a 3.3V

supply, and the thermal resistance for different conditions.

2.0 Capacitive Loads

The LM61 handles capacitive loading well. Without any spe-

cial precautions, the LM61 can drive any capacitive load as

shown in Figure 4. Over the specified temperature range the

LM61 has a maximum output impedance of 5 k

Ω.Inan

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

supply voltage, as shown in Figure 5. In a noisy environment

it may be necessary to add a capacitor from the output to

ground. A 1 µF output capacitor with the 5 k

Ω maximum

output impedance will form a 32 Hz lowpass filter. Since the

thermal time constant of the LM61 is much slower than the 5

ms time constant formed by the RC, the overall response

time of the LM61 will not be significantly affected. For much

larger capacitors this additional time lag will increase the

overall response time of the LM61.

SOT-23*

SOT-23**

TO-92*

TO-92***

no heat sink

small heat fin

no heat sink

small heat fin

θ

JA

T

θ

JA

T

θ

JA

T

θ

JA

T

(˚C/W)

(˚C)

(˚C/W)

(˚C)

(˚C/W)

(˚C)

(˚C/W)

(˚C)

Still air

450

0.26

260

0.13

180

0.09

140

0.07

Moving air

180

0.09

90

0.05

70

0.03

*Part soldered to 30 gauge wire.

***Part glued and leads soldered to 1" square of 1/16" printed circuit board with 2oz. foil or similar.

FIGURE 3. Temperature Rise of LM61 Due to

Self-Heating and Thermal Resistance (

θ

01289715

FIGURE 4. LM61 No Decoupling Required for

Capacitive Load

01289716

FIGURE 5. LM61 with Filter for Noisy Environment

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