For a design case of V_IN = 24 V, V_OUT = 12 V, I_OUT = 1 A, T_A (MAX) = 85°C , and T_JUNCTION = 125°C, the device must see a maximum junction-to-ambient thermal resistance of:

This R_θJA-MAX will ensure that the junction temperature of the regulator does not exceed T_J-MAX in the particular application ambient temperature.

To calculate the required R_θJA-MAX we need to get an estimate for the power losses in the IC. Figure 10-4 is taken from the Typical Characteristics section and shows the power dissipation of the LMZ14201H for V_OUT = 12 V at 85°C T_A.

Figure 10-4 Power Dissipation V_OUT = 12 V, T_A = 85°C

Using the 85°C T_A power dissipation data as a conservative starting point, the power dissipation P_D for V_IN = 24 V and V_OUT = 12 V is estimated to be 0.75 W. The necessary R_θJA-MAX can now be calculated.

To achieve this thermal resistance the PCB is required to dissipate the heat effectively. The area of the PCB will have a direct effect on the overall junction-to-ambient thermal resistance. In order to estimate the necessary copper area we can refer to Figure 10-5. This graph is taken from the Typical Characteristics section and shows how the R_θJA varies with the PCB area.

Figure 10-5 Package Thermal Resistance R_θJA 4-Layer PCB With 1-oz Copper

For R_θJA-MAX< 53.3°C/W and only natural convection (that is. no air flow), the PCB area can be smaller than 9 cm². This corresponds to a square board with 3 cm × 3 cm (1.18 in × 1.18 in) copper area, 4 layers, and 1 oz copper thickness. Higher copper thickness will further improve the overall thermal performance. Note that thermal vias should be placed under the IC package to easily transfer heat from the top layer of the PCB to the inner layers and the bottom layer.

For more guidelines and insight on PCB copper area, thermal vias placement, and general thermal design practices, refer to Application Note AN-2020 (SNVA419).