L78 datasheet(23/54 Pages) STMICROELECTRONICS

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L78

Application information

DocID2143 Rev 34

23/54

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 13: "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 8: Fixed output regulator

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.

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