APPLICATION INFORMATION
Design Considerations
The GM78L00 series regulators have thermal overload
protection from excessive power, internal short-circuit
protection limits each circuit's maximum current, and
output transistor safe-area protection for reducing the
output current as the voltage across each pass transis-
tor 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.
Thermal Considerations
The TO-92 molded package is capable of unusual high
power dissipation due to the lead frame design.
However, its thermal capabilities are generally over-
looked because of a lack of understanding of the ther-
mal paths from the semiconductor junction to ambient
temperature.
While thermal resistance is normally specified for the
device mounted 1cm above an infinite heat sink, very
little has been mentioned of the options available to im-
prove the conservatively rated thermal capability. An ex-
planation of the thermal paths of the TO-92 will allow
the designer to determine the thermal stress which is
applying in any given application.
The TO-92 Package
The TO-92 package thermal paths are complex. In addi-
tion to the path through the molding compound to ambi-
ent temperature, there is another path through the
leads, in parallel with the case path, to ambient tem-
Figure 5. TO-92 Thermal Equivalent Circuit
JC
JL
Where:
q
JC
= thermal resistance of the case between the reg-
ulator die and a point on the case directly above the
die location.
q
CA
= thermal resistance between the case and air at
ambient temperature.
q
JL
= thermal resistance from regulator die through
the input lead to a point 1/16 inch below the regula-
tor case.
q
LA
= total thermal resistance of the input/output
ground leads to ambient temperature.
q
JA
= junction to ambient thermal resistance.
T
J
P
E
(W)
CA
LA
T
A
GM78L00 SERIES
perature, as shown in Figure 5.
The total thermal resistance in this model is then:
(
JA
+
CA
)(
JL
+
JA
=
JA
+
LA
)
CA
+
JL
+
LA
10
GAMMA [ GAMMA MICROELECTRONICS INC. ]
