CD00000448 datasheet(17/31 Pages) STMICROELECTRONICS

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L78MxxAB, L78MxxAC

Applications information

Doc ID 2147 Rev 13

17/31

Applications information

7.1

Design considerations

The L78MxxA series of fixed voltage regulators are designed with thermal overload

protection that shuts down the circuit when subjected to an excessive power overload

condition, internal short-circuit protection that limits the maximum current the circuit will

pass, and output transistor safe-area compensation that reduces the output short-circuit as

the voltage across the pass transistor is increased. In many low current applications,

compensation capacitors are not required. However, it is recommended that the regulator

input be bypassed with a capacitor if the regulator is connected to the power supply filter

with long wire lengths, or if the output load capacitance is large. An input bypass capacitor

should be selected to provide good high-frequency characteristics to insure stable operation

under all load conditions. A 0.33 µF or larger tantalum, mylar, or other capacitor having low

internal impedance at high frequencies should be chosen. The bypass capacitor should be

mounted with the shortest possible leads directly across the regulators input terminals.

Normally good construction techniques should be used to minimize ground loops and lead

resistance drops since the regulator has no external sense lead.

Figure 17.

Current regulator

IO = VXX/R1+Id

Figure 18.

Adjustable output regulator

VO = 7.0 V to 20 V

VI - VO ≥ 2.0 V

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