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开关电源PCB布局指南英文版

SIMPLE SWITCHER ®PCB Layout Guidelines

开关电源PCB布局指南英文版

INTRODUCTION

One problem with writing an Application Note on PCB layout is that the people who read it are usually not the ones who are going to use it.Even if the designer has struggled through electromagnetic fields,EMC,EMI,board parasitics,transmission line effects,grounding,etc.,he will in all prob-ability then go on with his primary design task,leaving the layout to the CAD/layout person.Unfortunately,especially when it comes to switching regulators,it is not enough to be concerned with just basic routing/connectivity and mechani-cal issues.Both the designer and the CAD person need to be aware that the design of a switching power con-verter is only as good as its layout.Which probably ex-plains why a great many of customer calls received,con-cerning switcher applications,are ultimately traced to poor layout practices.Sadly,these could and should have been avoided on the very first prototype board,saving time and money on all sides.

The overall subject of PCB design is an extremely wide one,embracing several test/mechanical/production issues and also in some cases compliance/regulatory issues.There is also a certain amount of physics/electromagnetics involved,if a clearer understanding is sought.But the purpose of this Application Note is to reach the audience most likely to use it.Though there is enough design information for the more

experienced designer/CAD person,the Application Note in-cludes a quick-set of clear and concise basic rules that should be scrupulously followed to avoid a majority of prob-lems.In particular,we have provided recommended start-ing points for layout when using the popular LM267x,LM259x and LM257x families (Figure 2)The focus is on the step-down (Buck)Simple Switcher ICs from National,but the same principles hold for any topology and switching power application.

Most of the issues discussed in this Note revolve around simply assuring the desired performance in terms of basic electrical functionality.Though luckily,as the beleaguered switcher designer will be happy to know,in general all the electrical aspects involved are related and point in the same general ‘direction’.So for example,an ‘ideal’layout,i.e.one which helps the IC function properly,also leads to reduced electromagnetic emissions,and vice-versa.For example,reducing the area of loops with switching currents will help in terms of EMI and performance.However the designer is cautioned that there are some exceptions to this general ’trend’.One which is brought out in some detail here is the practice of ’copper-filling’,which may help reduce parasitic inductances and reduce noise-induced IC problems,but can also increase EMI.

Quick-Set of Rules for SIMPLE SWITCHER PCB Layout (Buck)

a)Place the catch diode and input capacitor as shown in Figure 2.

b)For high-speed devices (e.g.LM267x)do not omit placing input decoupling/bypass ceramic capacitor (0.1µF–0.47µF)as in Figure 2.

c)Connect vias to a Ground plane if available (optional,marked ‘X’in Figure 2)

d)If vias fall under tab of SMT power device,these are considered ‘thermal vias’.Use correct dimensions as discussed to avoid production issues.Or place the vias close to but not directly under the tab.

e)Route feedback trace correctly as discussed,away from noise sources such as the inductor and the diode.f)Do not increase width of copper on switching node injudiciously.

g)If very large heatsink area is required for catch diode (having estimated the heatsink requirement correctly)use isolation as discussed.

h)

For higher power SMT applications,use 2oz board for better thermal management with less copper area.

SIMPLE SWITCHER is a Registered Trademark of National Semiconductor.

National Semiconductor Application Note 1229Sanjaya Maniktala July 2002

SIMPLE SWITCHER PCB Layout Guidelines

AN-1229

©2002National Semiconductor Corporation http://www.wendangku.net/doc/b69fafc75fbfc77da269b177.html