
L7800AB/AC SERIES
14/18
APPLICATION INFORMATION
DESIGN CONSIDERATION
The L7800A 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 voltageacross 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 theoutput loadcapacitance
is large. An input bypass capacitor should be
selected
to
provide
characteristics to insure stable operation under all
load conditions. A 0.33mF 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
regulators
input
terminals.
construction
techniques
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 2V
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,
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.
good
high
frequency
directly
across
the
Normally
be
good
should
used
to
RSC,
and
an
Figure 4 :
DC Parameter
Figure 5 :
DC Parameter
Figure 6 :
DC Parameter
Figure 7 :
DC Parameter
1
I
O
=
+ I
d
V
O
, 7.0 to 20V
V
I
- V
O
≥
2.0V
R
1
=
I
REQ
-
β
Q1
1
I
O
= I
REG
+Q
1
(I
REG
)