6JA <

TJ-MAX - TA-MAX

PIC_LOSS

6JA <

(125 - 50) °C

3.9 W

< 19.23

°C

W

500

6

CA

Board Area_cm

2 &

°C x cm

2

W

.

6

CA <

125°C - 50°C

3.9 W

- 1.0 °C

W

< 18.23

°C

W

6

CA <

TJ-MAX ± TA-MAX

PIC_LOSS

- 6JC

LMZ22010

www.ti.com

SNVS687H – MARCH 2011 – REVISED AUGUST 2015

10.3 Power Dissipation and Thermal Considerations

When calculating module dissipation use the maximum input voltage and the average output current for the

application. Many common operating conditions are provided in the characteristic curves such that less common

applications can be derived through interpolation. In all designs, the junction temperature must be kept below the

rated maximum of 125°C.

For the design case of VIN = 12 V, VOUT = 3.3 V, IOUT = 10 A, and TA-MAX = 50°C, the module must see a thermal

resistance from case to ambient (

θCA) of less than:

(15)

Given the typical thermal resistance from junction to case (

θJC) to be 1.0°C/W. Use the 85°C power dissipation

curves in the Typical Characteristics section to estimate the PIC-LOSS for the application being designed. In this

application it is 5.3W.

(16)

To reach

θCA = 13.15, the PCB is required to dissipate heat effectively. With no airflow and no external heat-sink,

a good estimate of the required board area covered by 2-oz. copper on both the top and bottom metal layers is:

(17)

As a result, approximately 38.02 square cm of 2-oz. copper on top and bottom layers is the minimum required

area for the example PCB design. This is 6.16 × 6.16 cm (2.42 x 2.42 in) square. The PCB copper heat sink

must be connected to the exposed pad. For best performance, use approximately 100, 8 mil thermal vias spaced

59 mil (1.5 mm) apart connect the top copper to the bottom copper.

Another way to estimate the temperature rise of a design is using

θJA. An estimate of θJA for varying heat sinking

copper areas and airflows can be found in the typical applications curves. If our design required the same

operating conditions as before but had 225 LFPM of airflow. We locate the required

θJA of

(18)

On the

θJA vs copper heatsinking curve, the copper area required for this application is now only 2 square

inches. The airflow reduced the required heat sinking area by a factor of three.

To reduce the heat sinking copper area further, this package is compatable with D3-PAK surface mount heat

sinks.

For an example of a high thermal performance PCB layout for SIMPLE SWITCHER© power modules, refer to

AN-2093 SNVA460, AN-2084 SNVA456, AN-2125 SNVA473, AN-2020 SNVA419 and AN-2026 SNVA424.

10.4 Power Module SMT Guidelines

The recommendations below are for a standard module surface mount assembly

•

Land Pattern — Follow the PCB land pattern with either soldermask defined or non-soldermask defined pads

•

Stencil Aperture

–

For the exposed die attach pad (DAP), adjust the stencil for approximately 80% coverage of the PCB land

pattern

–

For all other I/O pads use a 1:1 ratio between the aperture and the land pattern recommendation

•

Solder Paste — Use a standard SAC Alloy such as SAC 305, type 3 or higher

•

Stencil Thickness — 0.125 to 0.15mm

•

Reflow — Refer to solder paste supplier recommendation and optimized per board size and density

•

Refer to Design Summary LMZ1xxx and LMZ2xxx Power Modules Family (SNAA214) for reflow information

Copyright © 2011–2015, Texas Instruments Incorporated

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