As with any power conversion device, the LMR33620-Q1 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 LMR33620-Q1 must be limited to 125°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 by 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 value of R_θJA given in Section 6.4 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.

Equation 11.

where

• η = efficiency

The effective R_θJA is a critical parameter and depends on many factors such as power dissipation, air temperature/flow, PCB area, copper heat-sink area, number of thermal vias under the package, and adjacent component placement, just to mention just a few. Due to the ultra-miniature size of the VQFN (RNX) package, a DAP is not available. This means that this package exhibits a somewhat large value R_θJA.A typical example of R_θJA vs copper board area can be found in Figure 8-7. 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. A typical curve of maximum output current vs. ambient temperature is shown in Figure 8-8. This data was taken with a device/PCB combination giving an R_θJA of about 50°C/W. It must be remembered that the data given in these graphs are for illustration purposes only, and the actual performance in any given application depends on all of the previously mentioned factors.

Figure 8-7 R_θJA versus Copper Board Area for the VQFN (RNX) Package

VIN = 12 V		VOUT = 5 V
?SW = 400 kHz		RθJA = 50°C/W

Figure 8-8 Maximum Output Current versus Ambient Temperature

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