0

10

20

30

40

50

60

0

5

10

15

20

25

30

35

40

BOARD AREA (cm

2

)

0LFM (0m/s) air

225LFM (1.14m/s) air

500LFM (2.54m/s) air

Evaluation Board Area

LMZ14203H

SNVS692G – JANUARY 2011 – REVISED OCTOBER 2015

www.ti.com

Power Dissipation and Board Thermal Requirements (continued)

Using the 85°C TA power dissipation data as a conservative starting point, the power dissipation PD for VIN = 24

V and VOUT = 12V is estimated to be 3.5W. The necessary RθJA-MAX can now be calculated.

RθJA-MAX < (125°C - 65°C) / 3.5W

(25)

RθJA-MAX < 17.1°C/W

(26)

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 52. This graph is taken from the Typical Characteristics section (Figure 31)

and shows how the RθJA varies with the PCB area.

Figure 52. Package Thermal Resistance RθJA 4-Layer PCB With 1-oz Copper

For RθJA-MAX< 17.1°C/W and only natural convection (that is., no air flow), the PCB area will have to be at least

52 cm2. This corresponds to a square board with 7.25 cm x 7.25 cm (2.85 in x 2.85 in) copper area, 4 layers, and

1oz copper thickness. Higher copper thickness will further improve the overall thermal performance. As a

reference, the evaluation board has 2-oz copper on the top and bottom layers, achieving RθJA of 14.9°C/W for

the same board area. Note 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 see the application note, AN-2020 Thermal Design By Insight, Not Hindsight (SNVA419).

24

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