SNVS690I January   2011  – August 2021

PRODUCTION DATA

1. Features
2. Applications
3. Description
4. Revision History
5. Pin Configuration and Functions
6. Specifications
7. Detailed Description
1. 7.1 Overview
2. 7.2 Functional Block Diagram
3. 7.3 Feature Description
4. 7.4 Device Functional Modes
8. Application and Implementation
1. 8.1 Application Information
2. 8.2 Typical Application
1. 8.2.1 Design Requirements
2. 8.2.2 Detailed Design Procedure
1. 8.2.2.1 Design Steps for the LMZ14201H Application
3. 8.2.3 Application Curve
9. Power Supply Recommendations
10. 10Layout
1. 10.1 Layout Guidelines
2. 10.2 Layout Example
11. 11Device and Documentation Support
12. 12Mechanical, Packaging, and Orderable Information

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### 10.2.1 Power Dissipation and Board Thermal Requirements

For a design case of VIN = 24 V, VOUT = 12 V, IOUT = 1 A, TA (MAX) = 85°C , and TJUNCTION = 125°C, the device must see a maximum junction-to-ambient thermal resistance of:

Equation 20. RθJA-MAX < (TJ-MAX – TA(MAX)) / PD

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

To calculate the required RθJA-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 VOUT = 12 V at 85°C TA.

Figure 10-4 Power Dissipation VOUT = 12 V, TA = 85°C

Using the 85°C TA power dissipation data as a conservative starting point, the power dissipation PD for VIN = 24 V and VOUT = 12 V is estimated to be 0.75 W. The necessary RθJA-MAX can now be calculated.

Equation 21. RθJA-MAX < (125°C - 85°C) / 0.75 W
Equation 22. RθJA-MAX < 53.3°C/W

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 RθJA varies with the PCB area.

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

For RθJA-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).