10.3 Thermal Considerations

The junction-to-ambient thermal resistance of the LM5575-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. As shown in the evaluation board artwork, the area under the LM5575-Q1 (component side) is covered with copper and there are 5 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 will greatly diminish the thermal dissipation capacity. The junction-to-ambient thermal resistance of the LM5575-Q1 mounted in the evaluation board varies from 50°C/W with no airflow to 28°C/W with 900 LFM (linear feet per minute). With a 25°C ambient temperature and no airflow, the predicted junction temperature for the LM5575-Q1 is 25 + (50 × 1.25) = 88°C. If the evaluation board operates at 1.5-A output current, 70-V input voltage, and a high ambient temperature for a prolonged period of time, the thermal shutdown protection within the IC may activate. The IC turns off to allow the junction to cool, followed by restart with the soft-start capacitor reset to zero.