Application information
L78xxAB - L78xxAC
6
6.1
Application information
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
I
O
= V
XX
/R
1
+I
d
14/26
STMICROELECTRONICS [ STMICROELECTRONICS ]
