SNVS658I March 2011 – August 2015 LMZ22003
PCB 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. A good example layout is shown in Figure 54.
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 = 3 A, and TAMB(MAX) = 85°C, the module must see a thermal resistance from case to ambient of less than:
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 2W.
To reach RθCA = 18.1, 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:
As a result, approximately 28 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 mil 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).
|PROBE||MAX TEMP (°C)||REACHED MAX TEMP||TIME ABOVE 235°C||REACHED 235°C||TIME ABOVE 245°C||REACHED 245°C||TIME ABOVE 260°C||REACHED 260°C|