As with any power conversion device, the LM708x0-Q1 dissipates internal power while operating. The effect of this power dissipation is to raise the internal temperature of the converter above ambient temperature. 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 layout

The maximum internal die temperature for the LM708x0-Q1 must be limited to 150°C. This limit establishes a limit on the maximum device power dissipation and, therefore, the load current. Equation 30 shows the relationships between the important parameters. 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 LM708x0-Q1 Quickstart Calculator tool. Alternatively, the EVM can be adjusted to match the desired application requirements and the efficiency can be measured directly.

where

• η = efficiency
• T_A = ambient temperature
• T_J = junction temperature
• R_θJA = the effective thermal resistance of the IC junction to the air, mainly through the PCB

The correct value of R_θJA is more difficult to estimate. As stated in the Semiconductor and IC Package Thermal Metrics Application Report, the JESD 51-7 value of R_θJA given in Thermal Information is not valid for design purposes and must not be used to estimate the thermal performance of the device in a real application. The JESD 51-7 values reported in Thermal Information were measured under a specific set of conditions that are rarely obtained in an actual application.

The effective R_θJA is a critical parameter and depends on many factors. The following are the most critical parameters:

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

Typical curves of maximum output current versus ambient temperature are shown in Derating Curves for a good thermal layout.

Use Thermal Design Resources as a guide for thermal PCB design and estimating R_θJA for a given application environment.