Thanks for your quick response! In the data sheet Junction to case thermal resistance mentioned as 0.9 degrees C/W. Considering its generating 5W, can I assume the heat it will produce is 0.9 * 5, i.e 4.5 degrees centigrade? is it correct? or I have to multiply with junction to ambient thermal resistance i.e, 50 degrees C/W? in this case the heat its going to produce is 250 degrees centigrade.