Question
We are about to decide an A/D converter for one of our applications. One of the
strongest alternatives is AD7763, but I have a question about this component:
In the data-sheet junction-ambient thermal impedance is shown as 92.7 C/W. Also
the part has industrial operating temperature range of -40/+85. The power
dissipation of the device is neraly 1 watt. Then 85C (ambient temperature) +
92.7C/W x 1W (junction to ambient temperature at 1 watt) ~=177C which is higher
than the maximum allowable junction temperature 150C. How can we solve this
problem? (Is the effect of thermal paddle included in these numbers? Or
otherwise do we need external cooling techniques?)
Answer
θja for the AD7763 is indeed almost 100ºC/W but this is the value with the
paddle unconnected. Since the device should never be used without its paddle
being soldered, or otherwise thermally connected, to a PCB plane or some other
heat sink this high value of θja is unimportant.
θjc, however, is only 5.1ºC/W so if you have a reasonable value of θ
(paddle-ambient) you will easily be able to use the device at 85ºC.