As with any power conversion device, the regulator 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 LM656x5-Q1 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. Higher 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. 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 application note, the values given in the Thermal Information table 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. 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 note for more information and the resources given at the end of this section.

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

The advanced package used for this regulator features a die attach paddle, or "thermal pad" (DAP), to provide a place to solder down to the PCB heat-sinking copper. This feature provides a good heat conduction path from the regulator junction to the heat sink and must be properly soldered to the PCB heat sink copper. A typical curve of R_θJA versus copper board area can be found in Figure 8-4. The copper area given in the graph is for each layer. The top and bottom layers are 2-oz. copper each, while the inner layers are 1 oz. Remember 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 previously mentioned factors. As one data point, the LM65645EVM exhibits an approximate R_θJA of about 25ºC/W for a copper area of about 58cm².

Figure 8-4 Thermal Resistance vs. Copper Area

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
• How to Properly Evaluate Junction Temperature with Thermal Metrics application report