Rainbow Electronics LM61 - Manual (Page 5)

Typical Performance Characteristics
The LM61 in the SOT-23 package mounted to a printed circuit
board as shown in Figure 2 was used to generate the following thermal curves. (Continued)
Start-Up Response
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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
JA
) is the pa-
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
J
= T
A
+ θ
JA
[(+V
S
I
Q
) + (+V
S
− V
O
) I
L
]
where I
Q
is the quiescent current and I
L
is the load current on
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.
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FIGURE 2. Printed Circuit Board Used
for Heat Sink to Generate All Curves.
1 ⁄
2
" Square Printed Circuit Board
with 2 oz. Copper Foil or Similar.
LM61
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