SNOSBQ7C – JUNE 1999 – REVISED MAY 2013
DESIGN CONSIDERATIONS
The LM79XX fixed voltage regulator series has thermal overload protection from excessive power dissipation,
internal short circuit protection which limits the circuit's maximum 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 junction temperature must be kept below the maximum
specified temperature (125°C) in order to meet data sheet specifications. To calculate the maximum junction
temperature or heat sink required, the following thermal resistance values should be used:
Typ
Package
TO-220
θ
JC
°C/W
3.0
Max
θ
JC
°C/W
5.0
Typ
θ
JA
°C/W
60
Max
θ
JA
°C/W
40
(1)
Solving for T
J
:
T
J
= T
A
+ P
D
(θ
JC
+
θ
CA
)
or
= T
A
+ P
D
θ
JA
(without heat sink)
where
•
•
•
•
•
•
•
•
T
J
= Junction Temperature
T
A
= Ambient Temperature
P
D
= Power Dissipation
θ
JA
= Junction-to-Ambient Thermal Resistance
θ
JC
= Junction-to-Case Thermal Resistance
θ
CA
= Case-to-Ambient Thermal Resistance
θ
CS
= Case-to-Heat Sink Thermal Resistance
θ
SA
= Heat Sink-to-Ambient Thermal Resistance
Typical Applications
Bypass capacitors are necessary for stable operation of the LM79XX series of regulators over the input voltage
and output current ranges. Output bypass capacitors will improve the transient response by the regulator.
The bypass capacitors, (2.2μF on the input, 1.0μF on the output) should be ceramic or solid tantalum which have
good high frequency characteristics. If aluminum electrolytics are used, their values should be 10μF or larger.
The bypass capacitors should be mounted with the shortest leads, and if possible, directly across the regulator
terminals.
4
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