As with any power conversion device, the LMR3650x 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 LMR3650x must be limited to 150°C. This limit establishes a limit on the maximum device power dissipation and, therefore, the load current. Equation 14 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. For more information, refer to the Semiconductor and IC Package Thermal Metrics application report.

where

• η is the efficiency.

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

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

The IC junction temperature can be estimated for a given operating condition using Equation 15.

where

• T_J is the IC junction temperature (°C).
• T_A is the ambient temperature (°C).
• R_θJA is the thermal resistance (°C/W)
• IC Power Loss is the power loss for the IC (W).

The IC Power loss mentioned above is the overall power loss minus the loss that comes from the inductor DC Resistance. The overall power loss can be approximated by using WEBENCH for a specific operating condition and temperature.

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

• Thermal Design by Insight not Hindsight application report
• A Guide to Board Layout for Best Thermal Resistance for Exposed Pad Packages application report
• Semiconductor and IC Package Thermal Metrics application report
• Thermal Design Made Simple with LM43603 and LM43602 application report
• PowerPAD™ Thermally Enhanced Package application report
• PowerPAD^™ Made Easy application report
• Using New Thermal Metrics application report
• PCB Thermal Calculator