Advanced Analog Technology, Inc.
AAT1204
THERMAL INFORMATION
To ensure reliable operation of the device, the junction temperature of the output device must be within the
safe operating area（SOA）. This is achieved by having a means to dissipate the heat generated from the
junction of the output structure. There are two components that contribute to thermal resistance. They
consist of two paths in series. The first is the junction to case thermal resistance, RJC; the second is the case
to ambient thermal resistance, RCA. The overall junction to ambient thermal resistance, RJA, is determined
by：
RJA＝RJC＋RCA
The ability to efficiently dissipate the heat from the junction is a function of the package style and board
layout incorporated in the application. The operating junction temperature is determined by the operation
ambient temperature, TA, and the junction power dissipation, PJ.
The junction temperature, TJ, is equal to the following thermal equation：
TJ＝TA＋PJ（RJC）＋PJ（RCA）
TJ＝TA＋PJ（RJA）
This particular application uses the 8-pin SO package with standard lead frame with a dedicated ground
terminal. Hence, the maximum power dissipation allowable for an operating ambient temperature of 70℃,
and a maximum junction temperature of 150℃ is determined as：
PJ＝（TJ－TA）/ RJA
PJ＝（150－70）/70＝1.1W
Worst case maximum power dissipation is determined by：
PD＝（5.5－3.135）×0.4＝0.946W
Normal operating maximum power dissipation is：
PD＝（5－3.3）×0.4＝0.68W
Note: The thermal characteristics of the AAT1204 were measured using a double-sided board with two
square inches of copper area connected to the GND pins for "heat spreading". The use of multi-layer
board construction with power planes will further enhance the thermal performance of the package.
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Advanced Analog Technology, Inc.
V 1.0
AAT [ ADVANCED ANALOG TECHNOLOGY, INC. ]
