As with any power conversion device, the LMZ20501 will dissipate 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 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 LMZ20501 is 125°C, thus establishing a limit on the maximum device power dissipation and, therefore, load current at high ambient temperatures. Equation 3 shows the relationships between the important parameters.

It is easy to see that larger ambient temperatures and larger values of R_θJA will reduce the maximum available output current. As stated in the Semiconductor and IC Package Thermal Metrics Application Report, 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 never obtain in an actual application. The effective R_θJA is a critical parameter and depends on many factors such as the following:

• Power dissipation
• Air temperature
• PCB area
• Copper heatsink area
• Air flow
• Adjacent component placement

The resources found in Table 11-1 can be used as a guide to estimate the R_θJA for a given application environment. A typical example of R_θJA versus copper board area is shown in Figure 8-3. The copper area in this graph is that for each layer; the inner layers are 1 oz (35µm). An R_θJA of 44°C/W is the approximate value for the LMZ20501 evaluation board. The efficiency found in Equation 3, η, should be taken at the elevated ambient temperature. For the LMZ20501, the efficiency is approximately two to three percent lower at high temperatures. Therefore, a slightly lower value than the typical efficiency can be used in the calculation. In this way, Equation 3 can be used to estimate the maximum output current for a given ambient, or to estimate the maximum ambient for a given load current.

A typical curve of maximum load current versus ambient temperature is shown in Figure 8-4. This graph assumes a R_θJA of 44°C/W and an input voltage of 5 V.