L78 datasheet(22/55 Pages) STMICROELECTRONICS

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Application information

6.1

Design consideration

The L78 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 current 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

capacitor if the regulator is connected to the power supply filter with long 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.

The addition of an operational amplifier allows adjustment to higher or intermediate values while retaining

regulation characteristics. The minimum voltage obtained with the arrangement is 2 V greater than the regulator

voltage.

The circuit of Figure 14. High current voltage regulator can be modified to provide supply protection against short

circuit by adding a short circuit sense resistor, RSC, and an additional PNP transistor. The current sensing PNP

must be able to handle the short circuit current of the three terminal regulator Therefore a four ampere plastic

power transistor is specified.

Figure 9. Fixed output regulator

GAMG220920161007MT

Although no output capacitor is need for stability, it does improve transient response.

Required if regulator is located an appreciable distance from power supply filter.

L78

Application information

DS0422 - Rev 36

page 22/55

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