LMZ14202HTZXSLASHNOPB datasheet(19/28 Pages) TI

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BOARD AREA (cm

)

0LFM (0m/s) air

225LFM (1.14m/s) air

500LFM (2.54m/s) air

Evaluation Board Area

0.0

0.5

1.0

1.5

2.0

0.0

0.5

1.0

1.5

2.0

2.5

3.0

OUTPUT CURRENT (A)

VIN = 15V

VIN = 24V

VIN = 30V

VIN = 36V

VIN = 42V

LMZ14202H

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SNVS691D – JANUARY 2011 – REVISED FEBRUARY 2013

To calculate the required

θJA-MAX we need to get an estimate for the power losses in the IC. The following graph

is taken form the Typical Performance Characteristics section and shows the power dissipation of the

LMZ14202H for VOUT = 12V at 85°C TAMB.

Figure 48. Power Dissipation VOUT = 12V TAMB = 85°C

Using the 85°C TAMB power dissipation data PD for VIN = 24V and VOUT = 12V is estimated to be 1.8W. The

necessary

θJA-MAX can now be calculated.

θJA-MAX < (125°C - 85°C) / 1.8W

θJA-MAX < 22.2°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 the following Package Thermal Resistance graph. This graph is taken from the

Typical Performance Characteristics section and shows how the

θJA varies with the PCB area.

Figure 49. Package Thermal Resistance

θJA 4 Layer Printed Circuit Board with 1oz Copper

For

θJA-MAX< 22.2°C/W and only natural convection (i.e. no air flow), the PCB area will have to be at least 30cm

This corresponds to a square board with approximately 5.5cm x 5.5cm (2.17in x 2.17in) copper area, 4 layers,

and 1oz 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 please refer to Application Note AN-2020 (http://www.ti.com/lit/an/snva419b/snva419b.pdf).

PC BOARD LAYOUT GUIDELINES

PC board layout is an important part of DC-DC converter design. Poor board layout can disrupt the performance

of a DC-DC converter and surrounding circuitry by contributing to EMI, ground bounce and resistive voltage drop

in the traces. These can send erroneous signals to the DC-DC converter resulting in poor regulation or instability.

Good layout can be implemented by following a few simple design rules.

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