1、1. 外部输入电源电压的范围,输出电流的大小。2. DC-DC输出的电压,电流,系统的功率最大值。二基于以上两点选择PWM IC要考虑: PWM IC的最大输入电压。 PWM开关的频率,这一点的选择关系到系统的效率。对储能电感,电容的大小的选择也有一定影响。3. MOS管的所能够承受的最大额定电流及其额定功率,如果DC-DC IC内部自带MOS,只需要考虑IC输出的额定电流。4. MOS的开关电压Vgs大小及最大承受电压。三电感(L1),二极管(CR1),电容(C2)的选择1. 电感量:大小选择主要由开关频率决定,大小会影响电源纹波;额定电流,电感的内阻选择由系统功耗决定。2. 二极管:通常都
2、用肖特基二极管。选择时要考滤反向电压,前向电流,一般情况反向电压为输入电源电压的二倍,前向电流为输出电流的两倍。3. 电容:电容的选择基于开关的频率,系统纹波的要求及输出电压的要求。容量和电容内部的等效电阻决定纹波大小(当然和电感也有关)。如何得到一个电源纹波相对较小、对系统其他电路干扰相对较小,而且相对稳定可靠的DC-DC电路,需要对以上电路的原理做如下修改: 输入部分:电源输入端需要加电感电容滤波。目的:由于MOS管的开关及电感在瞬间的变化会造成输入电源的波动,尤其是在系统耗电波动较大时,影响更为明显。 输出部分:(1)假定C2的选择的100uF是正确的,我们想得到更小的纹波,可以将100
3、uF的电容改成两颗47uF的电容(基于相同类型的电容);如果100uF电容采用的是铝电解,可以在原来的基础上加一颗10uF的磁片电容或钽电容。(2) 在输出端再加一颗电容和一颗电容对原来的电源做一个LC滤波,会得到一个纹波更小的电源。PCB布线时,应注意几点: 输入电源与MOS的连线要尽可能的粗。 Vgs也要粗一点,千万不要以为粗细没关系,(注:一般系统功率相对较低时,输出电流不大,粗细的影响不明显)关键时刻会影响电源的稳定性。 CR1, L1尽量靠近Q1。C2尽量靠近L1。 反馈电阻的线尽量远离电感L1。5. 反馈电压的地与系统的地尽量的近,保持在一个电位上。6. CR1的地线千万要粗,在M
4、OS的打开的时间里,L的电流是由的通路提供,即由地流向。附实例:Lm系列电源芯片布线指导1.Both the designer and the CAD person need to be aware that the design of a switching power converter is only as good as its layout设计者和CAD布局者都要明白:开关转换器的设计仅仅是布局良好的一方面。2. The overall subject of PCB design is an extremely wide one,embracing several test/mech
5、anical/production issues and also in some cases compliance/regulatory issues. There is also a certain amount of physics/electromagnetics involved,PCB设计的总体具体是一个非常广泛,包括几个测试/机械/生产问题,并在某些情况下遵守/管理问题。也有涉及到一些物理学和电磁学的问题。3. we have provided recommended starting points for layout when using the popular LM267x
6、,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我们对布局提供建议的出发点是使用比较流行的LM267x、LM259x和LM257x系列,主要集中在降压(Buck)简单开关电源转换芯片,但是同样的原则在其它的任何拓补结构和开关电源应用中依然适用。4. In switchin
7、g regulator layout, it is the AC paths that are considered critical, whereas the DC paths are not. That is the only basic rule to be kept in mind, and from which all the others follow. This is also true for any topology。在开关稳压器的布局中,考虑交流路径是非常必要的,而直流路径则显得不是很重要,这是唯一值得牢牢记住的基本原则,并且,同其它原则一样,这也适用于任何布局拓补结构。5
8、. Perform an analysis of the current flow for any topology in the same manner as we did for the Buck, to find the difference traces: and these traces are by definition the critical ones for layout.对执行中的任何拓补结构进行电流路径,采用我们分析Buck降压器时同样的分析方法,来找出不同路径,这些不同路径的确定是布局的关键。6. every inch of trace length has an in
9、ductance of about 20 nH。Like the trace resistance, that too doesnt seem much at first sight. But it is this rather minute inductance which is in fact responsible for a great many customer calls in SIMPLE SWITCHER applications!每英寸长度的走线会产生大约20 nH的电感效应。初次看起来,这电感大小和电阻一样小得可以忽略,但是,这正是对许多客户在简单电源转换应用中要负责任的(
10、要值得关注的)。7. the change in current in the AC sections of a typical buck converter is about 1.2 times the load current during the switch turn-off transition and is about 0.8 times the load current during the switch turn-on transition在典型的降压转换器的交流变化电流部分,在开关关断过程中,大约有1.2倍的负载电流,约0.8倍的负载电流在开关开通的过程中。8. The AC
11、 and DC Current Paths,注意直流和交流电流路径9. Placing Components acap (as Close as Possible) the input capacitors, especially the bypass capacitor CBYPASS should be placed very close to the IC, even for a Buck IC放置输入电容特别是旁路电容尽可能的靠近IC芯片。即使是降压IC。10. Therefore a high frequency bypass or decoupling capacitor with
12、 small or no leads, is to be placed very close to the VIN and GND pins of the IC. This is usually a 0.1 F0.47 F (monolithic) multilayer ceramic (.因此,在高频中,旁路电容或者去耦电容很小,并且放置在离芯片的VIN和GND管脚越近越好,这个电容通常是1.00.47uF的多层陶瓷电容。11. For lighter loads, and if it is possible to place the input bulk capacitor very cl
13、ose to the IC, the high frequency bypass capacitor may sometimes be omitted. But for high-speed switchers like the LM267x, the input ceramic bypass capacitor is considered almost mandatory for any application.对于轻微的负载,尽可能的放置大容量的电容器接近IC芯片,高频的旁路电容有时候可以忽略,但是在像LM267x等高速开关转换应用中,输入的旁路陶瓷电容被认为几乎在所有的应用中石必须的。1
14、2. It is essential to place the catch diode very close to the IC and connect it directly to the SW pin and GND pins of the IC, with traces that are very short and fairly wide .放置集流二极管距离IC很近、并且直接连接到SW脚和GND脚是很有必要的,这样的放置会使导线很短和很宽。13. several vias in parallel will yield better results than a single via.
15、 And larger via diameters would help further (unless they are being used as thermal vias 用几个并行的过孔将会得到比使用一个过孔的效果更好,并且大的过孔直径还可以进一步增大。14. a common intuitive mistake is to assume that inductance is inversely proportional to the width of the trace.一个普遍的错误是认为电感感性和导线的宽度成反比。15. 注意正确选定单板坐标原点的位置,原点的设置原则: A. 单板左边和下边的延长线交汇点。B. 单板左下角的第一个焊盘。板框四周倒圆角,倒角半径5mm,特殊情况参考结构设计要求。16. 按工艺设计规范的要求进行尺寸标注。17. 布局操作的基本原则 A. 遵照“先大后小,先难后易”的布置原则,即重要
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