The junction-to-ambient thermal resistance of the LM5576-Q1 varies with the application. The most significant variables are the area of copper in the PCB, the number of vias under the IC exposed pad and the amount of forced air cooling provided. Referring to the evaluation board artwork, the area under the LM5576-Q1 (component side) is covered with copper and there are five connection vias to the solder side ground plane. Additional vias under the IC have diminishing value as more vias are added. The integrity of the solder connection from the IC exposed pad to the PCB is critical. Excessive voids greatly diminish the thermal dissipation capacity. The junction-to-ambient thermal resistance of the LM5576-Q1 mounted in the evaluation board varies from 45°C/W with no airflow to 25°C/W with 900 LFM (Linear Feet per Minute). With a 25°C ambient temperature and no airflow, the predicted junction temperature for the LM5576-Q1 is 25 + (45 × 2.5) = 137.5°C. If the evaluation board is operated at 3A output current and 70V input voltage for a prolonged period of time, the thermal shutdown protection within the IC activates. The IC turns off allowing the junction to cool, followed by restart with the soft-start capacitor reset to zero.

One or more of the following modifications prevent the thermal shutdown from being activated: apply forced air cooling, reduce the maximum input voltage, lower the maximum output current, reduce the operating frequency, add more heat sinking to the PCB. For example, applying forced air cooling of 225 LFM reduces the LM5576-Q1 thermal resistance to approximately 30°C/W. The junction temperature reduces to 25 + (2.5 × 30) = 100°C. If the maximum input voltage for the application is 48V, then the IC power dissipation reduces to 2W (at 3A output current). With the same forced air cooling the junction temperature reduces to 25 + (2 × 30) = 85°C.