L78M datasheet(18/45 Pages) STMICROELECTRONICS

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Applications information

7.1

Design considerations

The L78M 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. Fixed output regulator

AMG080320171807MT

Note:

Although no output capacitor is need for stability, Co improve transient response if present. CI is required if

regulator is located an appreciable distance from power supply filter.

Figure 18. Constant current regulator

AMG080320171808MT

IO = VXX/R1+Id

L78M

Applications information

DS0425 - Rev 24

page 18/45

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