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

power supply voltage, as shown in

Figure 5. In a noisy envi-

ronment 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 ei-

ther 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

θ

JA

T

(˚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

θ

JA

T

(˚C/W)

(˚C)

(˚C/W)

(˚C)

Still air

TBD

TBD

TBD

TBD

Moving

air

TBD

TBD

TBD

TBD

FIGURE 3. Temperature Rise of LM20 Due to

Self-Heating and Thermal Resistance (

θ

DS100908-15

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

DS100908-16

DS100908-33

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.

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