SNVS690I January 2011 – August 2021 LMZ14201H
PRODUCTION DATA
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}.
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.
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 cm^{2}. 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).