SNVSBZ3 June 2021 LM5168-Q1
ADVANCE INFORMATION
As with any power conversion device, the LM516x-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 following:
The maximum internal die temperature for the LM516x-Q1 must be limited to 150°C. This establishes a limit on the maximum device power dissipation and, therefore, the load current. Equation 32 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 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.
where
The effective R_{θJA} is a critical parameter and depends on many factors such as the following:
The LM516x-Q1 features a die attach paddle, or "thermal pad" (EP), to provide a place to solder down to the PCB heat-sinking copper. This 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. Typical examples of R_{ΘJA} can be found in Figure 9-2. 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.
To continue with the design example, assume that the user has an ambient temperature of 70ºC and wishes to estimate the required copper area to keep the device junction temperature below 125ºC, at full load. From the curves in Section 9.2.3, an efficiency of about 81% was found at an input voltage of 24 V and 0.3 A from both primary and secondary outputs. The efficiency will be somewhat less at high junction temperatures, so an efficiency of approximately 79% is assumed. This gives a total loss of about 1.59 W. Subtract out the copper loss of the inductor, and arrive at a device dissipation of about 1.37 W. With this information, the user can calculate the required R_{θJA} of about 40ºC/W. Based on Figure 9-2 the required copper area is about 18 cm^{2} to 20 cm^{2}, for a two-layer PCB. For details of this calculation, please see the PCB Thermal Design Tips for Automotive DC/DC Converters Application Note.
The following resources can be used as a guide to optimal thermal PCB design and estimating R_{θJA} for a given application environment: