As with any power conversion device, the TPS563300 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 following:

• Ambient temperature
• Power loss
• Effective thermal resistance, R_θJA, of the device
• PCB combination

The maximum internal die temperature for the TPS563300 must be limited to 150°C. This establishes a limit on the maximum device power dissipation and, therefore, the load current. Equation 22 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 by using the curves provided in this data sheet. Note that these curves include the power loss in the inductor. 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 value of R_θJA given in the Thermal Information table is 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. The data given for R_θJC(bott) and Ψ_JT can be useful when determining thermal performance. See the Semiconductor and IC Package Thermal Metrics Application Report for more information and the resources given at the end of this section.

Equation 22.

where

• ŋ is the 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