Advanced Analog Technology, Inc.
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 path is the junction to case thermal resistance, RJC; the second path is
the case to ambient thermal resistance, RCA. The overall junction to ambient thermal resistance, RJA, is
determined by：
RJA＝RJC＋RCA
Package and board layout incorporated in the application determines the ability to efficiently dissipate the
heat from the junction. The operating junction temperature is determined by the operation ambient
temperature, TC, and the junction power dissipation, PJ.
The junction temperature, TJ, is equal to the following thermal equation：
TJ＝TC＋PJ（RJC）＋PJ（RCA）
TJ＝TC＋PJ（RJA）
The AAT1205 is housed in a thermal enhanced package where the power pad is located in the bottom of the
device. When the power pad is soldered on a double-sided printed circuit board with two square inches of
copper allocated for "heat spreading", the resulting θJA is 50℃/W. 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－TC）/ RJA
PJ＝（150－70）/50＝1.6W
Worst case maximum power dissipation is determined by：
Pd＝（5.5－2.375）×0.3+(5.5-3.135) ×0.1＝0.0.9375+0.2365=1.174W
Normal operating maximum power dissipation is：
Pd＝（5－2.5）×0.3+(5-3.3) ×0.1＝0.75+0.17=0.92W
Note: All thermal characteristics of the AAT1205 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.
– 台灣類比科技股份有限公司 –
– Advanced Analog Technology, Inc. –
Page 15 of 21
V 1.0
AAT [ ADVANCED ANALOG TECHNOLOGY, INC. ]
