2.0 Mounting
The LM20 can be applied easily in the same way as other
integrated-circuit temperature sensors. It can be glued or ce-
mented to a surface. The temperature that the LM20 is sens-
ing will be within about +0.02°C of the surface temperature to
which the LM20'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 temper-
ature between the surface temperature and the air tempera-
ture.
To ensure good thermal conductivity the backside of the
LM20 die is directly attached to the pin 2 GND pin. The tem-
pertures of the lands and traces to the other leads of the LM20
will also affect the temperature that is being sensed.
Alternatively, the LM20 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 LM20 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 LM20 or its connections.
The thermal resistance junction to ambient (θJA) is the pa-
rameter used to calculate the rise of a device junction tem-
perature due to its power dissipation. For the LM20 the
equation used to calculate the rise in the die temperature is
as follows:
TJ = TA + θJA [(V+ IQ) + (V+ − VO) IL]
where IQ is the quiescent current and ILis the load current on
the output. Since the LM20's junction temperature is the ac-
tual temperature being measured care should be taken to
minimize the load current that the LM20 is required to drive.
The tables shown in Figure 3 summarize the rise in die tem-
perature of the LM20 without any loading, and the thermal
resistance for different conditions.
SC70-5 SC70-5
no heat sink small heat sink
θJA TJ − TAθJA TJ − TA
(°C/W) (°C) (°C/W) (°C)
Still air 412 0.2 350 0.19
Moving air 312 0.17 266 0.15
See Figure 1 for PCB layout samples.
micro SMD micro SMD
no heat sink small heat fin
θJA TJ − TAθJA TJ − TA
(°C/W) (°C) (°C/W) (°C)
Still air 340 0.18 TBD TBD
Moving air TBD TBD TBD TBD
FIGURE 3. Temperature Rise of LM20 Due to
Self-Heating and Thermal Resistance (θJA)
3.0 Capacitive Loads
The LM20 handles capacitive loading well. Without any pre-
cautions, the LM20 can drive any capacitive load less than
300 pF as shown in Figure 4. Over the specified temperature
range the LM20 has a maximum output impedance of 160
Ω. In an extremely noisy environment it may be necessary to
add some filtering to minimize noise pickup. It is recommend-
ed that 0.1 μF be added from V+ to GND to bypass the power
supply voltage, as shown in Figure 5. In a noisy environment
it may even be necessary to add a capacitor from the output
to ground with a series resistor as shown in Figure 5. A 1 μF
output capacitor with the 160 Ω maximum output impedance
and a 200 Ω series resistor will form a 442 Hz lowpass filter.
Since the thermal time constant of the LM20 is much slower,
the overall response time of the LM20 will not be significantly
affected.
10090815
FIGURE 4. LM20 No Decoupling Required for Capacitive
Loads Less than 300 pF.
R (Ω) C (µF)
200 1
470 0.1
680 0.01
1 k 0.001
10090816
10090833
FIGURE 5. LM20 with Filter for Noisy Environment and
Capacitive Loading greater than 300 pF. Either placement
of resistor as shown above is just as effective.
www.national.com 6
LM20