Datasheet
0 10 20 30 40 50 60
0
5
10
15
20
25
30
35
40
THERMAL RESISTANCE
JA
(°C/W)
BOARD AREA (cm
2
)
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
POWER DISSIPATION (W)
OUTPUT CURRENT (A)
VIN = 15V
VIN = 24V
VIN = 30V
VIN = 36V
VIN = 42V
LMZ14202H
www.ti.com
SNVS691E –JANUARY 2011–REVISED OCTOBER 2013
To calculate the required θ
JA-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 V
OUT
= 12V at 85°C T
AMB
.
Figure 48. Power Dissipation V
OUT
= 12V T
AMB
= 85°C
Using the 85°C T
AMB
power dissipation data P
D
for V
IN
= 24V and V
OUT
= 12V is estimated to be 1.8W. The
necessary θ
JA-MAX
can now be calculated.
θ
JA-MAX
< (125°C - 85°C) / 1.8W
θ
JA-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 θ
JA
varies with the PCB area.
Figure 49. Package Thermal Resistance θ
JA
4 Layer Printed Circuit Board with 1oz Copper
For θ
JA-MAX
< 22.2°C/W and only natural convection (i.e. no air flow), the PCB area will have to be at least 30cm
2
.
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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