9.2.1.2.10 Maximum Ambient Temperature

As with any power conversion device, the LMR36520 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 internal die temperature for the LMR36520 must be limited to 150°C. This establishes a limit on the maximum device power dissipation and, therefore, the load current. Equation 11 shows the relationships between the important parameters. It is easy to see that larger ambient temperatures (T_A) and larger values of R_θJA reduce the maximum available output current. The converter efficiency can be estimated using the curves provided in this data sheet. If the desired operating conditions cannot be found in one of the curves, then 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 Application Report, the values given in 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.

Equation 11.

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
• Cooper heat-sink area
• Number of thermal vias under the package
• Adjacent component placement

A typical example of R_θJA versus copper board area can be found in Figure 15. Note that the data given in this graph is for illustration purposes only, and the actual performance in any given application depends on all of the factors mentioned above.

Figure 15. R_θJA versus Copper Board Area

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

• Thermal Design by Insight not Hindsight Application Report
• Semiconductor and IC Package Thermal Metrics Application Report
• Thermal Design Made Simple with LM43603 and LM43602 Application Report
• Using New Thermal Metrics Application Report