GM78L00
100 mA POSITIVE VOLTAGE REGULATORS
Revision 1, May 2002
www.gammamicro.com
PRELIMINARY
6
APPLICATION INFORMATION
Design Considerations
The GM78L series regulators have thermal overload
protection from excessive power, internal short-circuit
protection which limits each circuit’s maximum current,
and output transistor safe-area protection for reducing
the output current as the voltage across each 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.
Thermal Considerations
The TO-92 molded package is capable of unusually
high power dissipation due to the lead frame design.
However, its thermal capabilities are generally
overlooked because of a lack of understanding of the
thermal paths from the semiconductor junction to
ambient temperature.
While thermal resistance is normally specified for the
device mounted 1 cm above an infinite heat sink, very
little has been mentioned of the options available to
improve on the conservatively rated thermal capability.
An explanation of the thermal paths of the TO-92 will
allow the designer to determine the thermal stress he
is applying in any given application.
The TO-92 Package
The TO-92 package thermal paths are complex. In
addition to the path through the molding compound to
ambient temperature, there is another path through the
leads, in parallel with the case path, to ambient
temperature, as shown in Figure 1.
The total thermal resistance in this model is then:
Where:
θJC = thermal resistance of the case between the
regulator die and a point on the case directly above the
die location.
θCA = thermal resistance between the case and air at
ambient temperature.
θJL= thermal resistance from regulator die through the
input lead to a point 1/16 inch below the regulator case.
θLA= total thermal resistance of the input/output ground
leads to ambient temperature.
θJA = junction to ambient thermal resistance.
θJA=
(
θJA + θCA) ( θJL + θLA)
θJA + θCA + θJL + θLA
PE (W)
θJL
θLA
θJC
θCA
TJ
TA
FIGURE 1. TO-92 Thermal Equivalent Circuit
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ORDERING INFORMATION
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