As with any power conversion device, the device dissipates internal power while operating. The effect of this power dissipation is to raise the internal temperature of the converter above ambient. The internal die temperature (T_J) is a function of the ambient temperature, the power loss, and the effective thermal resistance, R_θJA, of the device and PCB combination. The maximum junction temperature for the LMR38020 must be limited to 150°C. This limit establishes a limit on the maximum device power dissipation and, therefore, the load current. Equation 15 shows the relationships between the important parameters. Seeing that larger ambient temperatures (T_A) and larger values of R_θJA reduce the maximum available output current is easy. The converter efficiency can be estimated by using the curves provided in this data sheet. If the desired operating conditions cannot be found in one of the curves, interpolation can be used to estimate the efficiency. Alternatively, the EVM can be adjusted to match the desired application requirements and the efficiency can be measured directly. The correct value of R_θJA is more difficult to estimate. As stated in the Semiconductor and IC Package Thermal Metrics , the values given in the Thermal Information are not valid for design purposes and must not be used to estimate the thermal performance of the application. The values reported in that table were measured under a specific set of conditions that are rarely obtained in an actual application.

where

• η = efficiency

The effective R_θJA is a critical parameter and depends on many factors such as the following:

• Power dissipation
• Air temperature, flow
• PCB area
• Copper heat-sink area
• Number of thermal vias under the package
• Adjacent component placement

Use the following resources as guides to optimal thermal PCB design and estimating R_θJA for a given application environment:

• AN-2020 Thermal Design By Insight, Not Hindsight application report
• AN-1520 A Guide to Board Layout for Best Thermal Resistance for Exposed Packages application report
• Semiconductor and IC Package Thermal Metrics application report
• How to Properly Evaluate Junction Temperature with Thermal Metrics application report
• Using New Thermal Metrics application report