I can't find THETA(ja) for ADRF6755, could you please provide it to me ?
Power dissipation numbers are in the datasheet. Nominal current is 380mA at 5V = 1.9W. You should calculate for maximum which is 420mA x 5.25V = 2.2W.
Junction temperature is harder to calculate as it depends on a number of factors such as environment, PCB stack-up and layout. See diagram below.
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