10.3 Estimate Power Dissipation and Thermal Considerations

Use the current derating curves in the Typical Characteristics section to obtain an estimate of power loss (P_{IC_LOSS}). For the design case of V_IN = 5 V, V_OUT = 2.5 V, I_OUT = 3 A, T_A(MAX) = 85°C , and T_J(MAX) = 125°C, the device must see a thermal resistance from case to ambient (θ_CA) of less than:

Equation 21.

Equation 22.

Given the typical thermal resistance from junction to case (θ_JC) to be 1.9°C/W (typical). Continuously operating at a T_J greater than 125°C will have a shorten lifespan.

To reach θ_CA = 69.5°C/W, the PCB is required to dissipate heat effectively. With no airflow and no external heat, a good estimate of the required board area covered by 1-oz. copper on both the top and bottom metal layers is:

Equation 23.

Equation 24.

As a result, approximately 7.2 square cm of 1-oz. copper on top and bottom layers is required for the PCB design.

The PCB copper heat sink must be connected to the exposed pad (EP). Approximately thirty six, 8 mils thermal vias spaced 59 mils (1.5 mm) apart must connect the top copper to the bottom copper. For an extended discussion and formulations of thermal rules of thumb, refer to AN-2020 Thermal Design By Insight, Not Hindsight (SNVA419). For an example of a high thermal performance PCB layout with θ_JA of 20°C/W, refer to the evaluation board application note AN-2074 LMZ1050xEXT Evaluation Board (SNVA450) and for results of a study of the effects of the PCB designs, refer to AN-2026 Effect of PCB Design on Thermal Performance of SIMPLE SWITCHER Power Modules (SNVA424).