APPLICATIONS INFORMATION
DESIGNCONSIDERATIONS
The L7800ASeries 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 a capacitor if the regulator is
connected to the power supply filter with long
wire 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 0.33µF 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 regulatorhas no external sense lead.
Figure 4 :
Current Regulator.
Figure 5 :
Adjustable Output Regulator.
Figure 6 : Current Boost Regulator. Figure 7 : Short-circuit Protection.
V
O
, 7.0V to 20V
V
i
–V
O
≥2.0V
The addition of an operational amplifier allows
adjustment to higher or intermediate values while
retaining regulation characteristics. The minimum
voltage obtainable with this arrangement is 2.0V
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, R
sc
, 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 plasticpower transistor is specified.
R
1
=
V
BEQ
1
I
REQ
−
I
Q
1
β
Q
1
I
O
=
I
REG
+
Q
1
(
I
REG
−
V
BEQ
1
R
1
)
I
O
=
V
XX
R
1
+
I
d
L7800AB/AC
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