Ray,
You can find the thermal information on page 3 of the TS3DV642 datasheet.
Here is a link to the Thermal Metrics application report.
The power dissipation will be calculated by Pd= Vcc*Icc + n(Iio^2*Ron) where n is the number of channels used.
For example operating the TS3DV642 at 3V with all 12 channels active would be Pd= (3V)(50uA) + 12(40mA^2*6.5 ohms) = 125mW
The junction temperature can be calculated by Tj= Ambient Tempmax + Pmax(θJA)
Tj = (25°C) + (125mW)(31°C/W)
Tj = 28.8°C
To get the Tj max fill in the above equation with the maximum operating values of the TS3DV642.
Tjmax = (85°C) + (1.9W)(31°C/W)
Tjmax = 143.9°C
Here is another thread on signal switch thermal metrics you can reference.
Thank you,
Adam
