Question

Do I need a heatsink on my linear regulator?

Answer

Looking at the excerpt from the datasheet the Junction-to-Ambient Thermal Resistance for the MC33269 in a TO-220 package is 65°C per Watt. This thermal resistance specifies the temperature rise, per Watt of power dissipated, when the device is operated without heatsinking or forced cooling. For every watt dissipated the junction temperature will rise 65°C.

The next number of interest is the Maximum Operating Die Junction Temperature. For the MC33269 (any package) this number is 150°C

For a 3.3V output the power dissipated in the linear regulator is

substituting the values for your input voltage and output current will yield the power dissipated in the linear regulator. To calculate the junction temperature substitute the power dissipation in the following equation --

If your system is operating at an elevated temperature you need to change the 25°C value in the previous equation. If you calculate a junction temperature that is below 80% of the Maximum Operating Die Junction Temperature you do not need a heatsink. However, a heatsink will increase the reliability of your system by reducing the operating temperature of the regulator.