The thermal performance of the ATL431LI will depend on the power dissipation, thermal resistance, and ambient temperature. The ability to remove heat from the die is different for each package type, presenting different considerations in the printed-circuit-board (PCB) layout. The PCB area around the device that is free of other components moves the heat from the device to the ambient air. Performance data for JEDEC thermal metrics are given in the Specifications table.
Power dissipation depends on input voltage and load conditions. Power dissipation (PD) is equal to the product of the output current and the voltage drop across the output pass element. For a ATL431LI, the pass element voltage drop will be across the cathode and anode as shown in Equation 5. In certain packages, like the DQN package, the thermal metrics will vary with how the thermal pad is connected. The DQN package is designed to be soldered to a thermal pad on the board. It is reccomended that the DQN thermal pad is conencted to a thermal dissipative section of the PCB if the ATL431LI in DQN package is expected to dissipate a significant amount of power. It is reccomended to use large biasing resistors to keep the cathode current low on the ATL431LI for better thermal performance. For more information on designing and manufacturing with DQN, see Texas Instruments literature number SLUA271 and SCEA055. For reliable operation, limit junction temperature to 125°C (maximum) or its respective temperature maximum from the Reccomended Operating Conditions.