Rainbow Electronics LM26 - Manual (Page 6)

Applications Hints
(Continued)
TABLE 1. Resistive compensation for capacitive
loading of V
TEMP
C
LOAD
R ( Ω)
≤100pF 0
1nF 8200
10nF 3000
100nF 1000
≥1µF 430
NOISE CONSIDERATIONS
The LM26 has excellent power supply noise rejection. Listed
below is a variety of signals used to test the LM26 power
supply rejection. False triggering of the output was not ob-
served when these signals where coupled into the V+ pin of
the LM26.
• square wave 400kHz, 1Vp-p
• square wave 2kHz, 200mVp-p
• sine wave 100Hz to 1MHz, 200mVp-p
Testing was done while maintaining the temperature of the
LM26 one degree centigrade way from the trip point with the
output not activated.
MOUNTING CONSIDERATIONS
The LM26 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 LM26 is
sensing will be within about +0.06˚C of the surface tempera-
ture to which the LM26’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
LM26 die is directly attached to the GND pin (pin 2). The
temperatures of the lands and traces to the other leads of the
LM26 will also affect the temperature that is being sensed.
Alternatively, the LM26 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 LM26 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 LM26 or its connec-
tions.
The junction to ambient thermal resistance ( θ
JA
) is the pa-
rameter used to calculate the rise of a part’s junction tem-
perature due to its power dissipation. For the LM26 the
equation used to calculate the rise in the die junction tem-
perature is as follows:
(3)
where T
A
is the ambient temperature, V + is the power supply
voltage, I
Q
is the quiescent current, I
L_TEMP
is the load
current on the V
TEMP
output, V
DO
is the voltage on the digital
output, and I
DO
is the load current on the digital output. Since
the LM26’s junction temperature is the actual temperature
being measured, care should be taken to minimize the load
current that the LM26 is required to drive.
The tables shown in Figure 3 summarize the thermal resis-
tance for different conditions and the rise in die temperature
of the LM26 without any loading on V
TEMP
and a 10k pull-up
resistor on an open-drain digital output with a 5.5V power
supply.
10132317
a) R in series with capacitor
10132318
b) R in series with signal path
FIGURE 2. Resistor placement for capacitive loading
compensation of V
TEMP
SOT23-5
no heat sink
SOT23-5
small heat sink
θ
JA
(˚C/W)
T
J
−T
A
(˚C)
θ
JA
(˚C/W)
T
J
−T
A
(˚C)
Still Air 250 0.11 TBD TBD
Moving Air TBD TBD TBD TBD
FIGURE 3. Thermal resistance ( θ
JA
) and temperature
rise due to self heating (T
J
−T
A
)
LM26
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