Rainbow Electronics LM20 - Manual (Page 6)

2.0 Mounting
(Continued)
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 recom-
mended 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.
4.0 LM20 micro SMD Light
Sensitivity
Exposing the LM20 micro SMD package to bright sunlight
may cause the output reading of the LM20 to drop by 1.5V. In
a normal office environment of fluorescent lighting the output
voltage is minimally affected (less than a millivolt drop). In
either case it is recommended that the LM20 micro SMD be
placed inside an enclosure of some type that minimizes its
light exposure. Most chassis provide more than ample pro-
tection. The LM20 does not sustain permanent damage from
light exposure. Removing the light source will cause LM20’s
output voltage to recover to the proper value.
SC70-5 SC70-5
no heat sink small heat sink
θ
JA
T
J
− T
A
θ
JA
T
J
− T
A
(˚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
θ
JA
T
J
− T
A
θ
JA
T
J
− T
A
(˚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 ( θ
JA
)
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.
LM20
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