Application information

L78xxAB - L78xxAC

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6

Application information

6.1

Design consideration

The L78xxA 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 6 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.

DC parameter