Heatsink calculator
This calculator will indicate if a heatsink is required and the maximum thermal resistance required. When choosing the heatsink you must select one which has a better (lower) thermal resistance than the value given.
Tamb °C   Maximum ambient temperature
Pd Watts   The maximum power to be dissipated by the device
Tj max °C   Maximum junction temperature of the device
Rth(ja) °C/Watt   Thermal resistance between the junction of the device and Ambient
Rth(jc) °C/Watt   Thermal resistance between the junction and case of the device
Rth(ch) °C/Watt   Thermal resistance between the case of the device and the Heatsink
 
Rth(ha)