Application information
L78xx, L78xxC, L78xxAB, L78xxAC
6
Application information
6.1
Design consideration
The L78xx 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 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
1. To specify an output voltage, substitute voltage value for "XX".
2. Although no output capacitor is need for stability, it does improve transient response.
3. Required if regulator is locate an appreciable distance from power supply filter.
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