LMZ22005TZXSLASHNOPB datasheet(25/34 Pages) TI

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LMZ22005

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SNVS686J – MARCH 2011 – REVISED AUGUST 2015

Layout Examples (continued)

Figure 55. Bottom View Demonstration Board

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, VO = 3.3 V, IO = 5 A, and TAMB(MAX) = 85°C, the module must see a thermal

resistance from case to ambient of less than:

RθCA< (TJ-MAX – TA-MAX) / PIC-LOSS – RθJC

(15)

Given the typical thermal resistance from junction to case to be 1.9°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 4.3W.

RθCA = (125 – 85) / 4.3 W – 1.9 = 7.4

(16)

To reach RθCA = 7.4, 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:

Board_Area_cm

2 = 500°C x cm2/W / R

θCA

(17)

As a result, approximately 67 square cm of 2-oz. copper on top and bottom layers is required for the PCB

design. The PCB copper heat sink must be connected to the exposed pad. Approximately sixty, 8 mils thermal

vias spaced 39 mils (1.0 mm) apart connect the top copper to the bottom copper. For an example of a high

thermal performance PCB layout for SIMPLE SWITCHER power modules, refer to AN-2085 (SNVA457), AN-

2125 (SNVA437), AN-2020 (SNVA419) and AN-2026 (SNVA424).

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