Design Considerations
The LM79XX fixed voltage regulator series has thermal
overload protection from excessive power dissipation inter-
nal short circuit protection which limits the circuit’s maxi-
mum current and output transistor safe-area compensation
for reducing the output current as the voltage across the
pass transistor is increased
Although the internal power dissipation is limited the junc-
tion temperature must be kept below the maximum speci-
fied temperature (125 C) in order to meet data sheet specifi-
cations To calculate the maximum junction temperature or
heat sink required the following thermal resistance values
should be used
Typ
i
JC
C W
30
Max
i
JC
C W
50
Typ
i
JA
C W
60
Max
i
JA
C W
40
Where
T
J
T
A
P
D
e
Junction Temperature
e
Ambient Temperature
e
Power Dissipation
i
JA
e
Junction-to-Ambient Thermal Resistance
i
JC
e
Junction-to-Case Thermal Resistance
i
CA
e
Case-to-Ambient Thermal Resistance
i
CS
e
Case-to-Heat Sink Thermal Resistance
i
SA
e
Heat Sink-to-Ambient Thermal Resistance
Package
TO-220
T
J Max
b
T
A
T
J Max
T
A
P
D MAX
e
or
i
JC
a
i
CA
i
JA
e
i
CS
a
i
SA
(without heat sink)
i
CA
Solving for T
J
T
J
e
T
A
a
P
D
(i
JC
a
i
CA
) or
e
T
A
a
P
D
i
JA
(without heat sink)
Typical Applications
(Continued)
Bypass capacitors are necessary for stable operation of the
LM79XX series of regulators over the input voltage and out-
put current ranges Output bypass capacitors will improve
the transient response by the regulator
The bypass capacitors (2 2
mF
on the input 1 0
mF
on the
output) should be ceramic or solid tantalum which have
good high frequency characteristics If aluminum electrolyt-
ics are used their values should be 10
mF
or larger The
bypass capacitors should be mounted with the shortest
leads and if possible directly across the regulator termi-
nals
High Stability 1 Amp Regulator
TL H 7340 – 5
Load and line regulation
k
0 01% temperature stability
s
0 2%
Determine Zener current
Solid tantalum
Select resistors to set output voltage 2 ppm C tracking suggested
4
NSC [ NATIONAL SEMICONDUCTOR ]
