📄 tl431特性及应用.htm
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<TD align=middle width=868 colSpan=2><B><SPAN lang=EN-US
style="FONT-FAMILY: 黑体"><FONT size=2>1 TL431的简介</FONT></SPAN><SPAN
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<P style="MARGIN-LEFT: 36pt; MARGIN-RIGHT: 36pt">德州仪器公司（<SPAN
lang=EN-US style="FONT-FAMILY: System">TI</SPAN>）生产的<SPAN lang=EN-US
style="FONT-FAMILY: System">TL431</SPAN>是一是一个有良好的热稳定性能的三端可调分流基准源。它的输出电压用两个电阻就可以任意地设置到从<SPAN
lang=EN-US style="FONT-FAMILY: System">Vref</SPAN>（<SPAN lang=EN-US
style="FONT-FAMILY: System">2.5V</SPAN>）到<SPAN lang=EN-US
style="FONT-FAMILY: System">36V</SPAN>范围内的任何值（如图<SPAN lang=EN-US
style="FONT-FAMILY: System">2</SPAN>）。该器件的典型动态阻抗为<SPAN lang=EN-US
style="FONT-FAMILY: System">0.2</SPAN><SPAN lang=EN-US
style="mso-ascii-font-family: System; mso-fareast-font-family: System">Ω</SPAN>，在很多应用中可以用它代替齐纳二极管，例如，数字电压表，运放电路、可调压电源，开关电源等等。<V:SHAPETYPE
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<P style="MARGIN-LEFT: 36pt; MARGIN-RIGHT: 36pt">左图是该器件的符号。<SPAN
lang=EN-US style="FONT-FAMILY: System">3</SPAN>个引脚分别为：阴极（<SPAN
lang=EN-US style="FONT-FAMILY: System">CATHODE</SPAN>）、阳极（<SPAN
lang=EN-US style="FONT-FAMILY: System">ANODE</SPAN>）和参考端（<SPAN
lang=EN-US style="FONT-FAMILY: System">REF</SPAN>）。<SPAN lang=EN-US
style="FONT-FAMILY: System">TL431</SPAN>的具体功能可以用如图<SPAN lang=EN-US
style="FONT-FAMILY: System">1</SPAN>的功能模块示意。<V:SHAPE id=_x0000_s1027
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v:shapes="_x0000_s1027"><SPAN lang=EN-US><O:P> </O:P></SPAN></P>
<P style="MARGIN-LEFT: 36pt; MARGIN-RIGHT: 36pt">由图可以看到，<SPAN
lang=EN-US style="FONT-FAMILY: System">VI</SPAN>是一个内部的<SPAN
lang=EN-US
style="FONT-FAMILY: System">2.5V</SPAN>基准源，接在运放的反相输入端。由运放的特性可知，只有当<SPAN
lang=EN-US style="FONT-FAMILY: System">REF</SPAN>端（同相端）的电压非常接近<SPAN
lang=EN-US style="FONT-FAMILY: System">VI</SPAN>（<SPAN lang=EN-US
style="FONT-FAMILY: System">2.5V</SPAN>）时，三极管中才会有一个稳定的非饱和电流通过，而且随着<SPAN
lang=EN-US style="FONT-FAMILY: System">REF</SPAN>端电压的微小变化，通过三极管
图<SPAN lang=EN-US style="FONT-FAMILY: System">1 </SPAN>的电流将从<SPAN
lang=EN-US style="FONT-FAMILY: System">1</SPAN>到<SPAN lang=EN-US
style="FONT-FAMILY: System">100mA</SPAN>变化。当然，该图绝不是<SPAN lang=EN-US
style="FONT-FAMILY: System">TL431</SPAN>的实际内部结构，所以不能简单地用这种组合来代替它。但如果在设计、分析应用<SPAN
lang=EN-US
style="FONT-FAMILY: System">TL431</SPAN>的电路时，这个模块图对开启思路，理解电路都是很有帮助的，本文的一些分析也将基于此模块而展开。</P> </TD></TR>
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<TD align=middle width=868 colSpan=2><B><SPAN lang=EN-US><FONT
size=2><SPAN style="FONT-FAMILY: 黑体">2.
恒压电路应用</SPAN></FONT></SPAN></B></TD></TR>
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<P style="MARGIN-LEFT: 72pt; MARGIN-RIGHT: 36pt"><IMG height=203
alt="wpe7.jpg (6688 字节)" src="TL431特性及应用.files/tl431_7.jpg"
width=288 align=left v:shapes="_x0000_s1028">前面提到<SPAN lang=EN-US
style="FONT-FAMILY: System">TL431</SPAN>的内部含有一个<SPAN lang=EN-US
style="FONT-FAMILY: System">2.5V</SPAN>的基准电压，所以当在<SPAN lang=EN-US
style="FONT-FAMILY: System">REF</SPAN>端引入输出反馈时，器件可以通过从阴极到阳极很宽范围的分流，控制输出电压。如图<SPAN
lang=EN-US style="FONT-FAMILY: System">2</SPAN>所示的电路，当<SPAN
lang=EN-US style="FONT-FAMILY: System">R1</SPAN>和<SPAN lang=EN-US
style="FONT-FAMILY: System">R2</SPAN>的阻值确定时，两者对<SPAN lang=EN-US
style="FONT-FAMILY: System">Vo</SPAN>的分压引入反馈，若<SPAN lang=EN-US
style="FONT-FAMILY: System">V o</SPAN>增大，反馈量增大，<SPAN lang=EN-US
style="FONT-FAMILY: System">TL431</SPAN>的分流也就增加，从而又导致<SPAN
lang=EN-US
style="FONT-FAMILY: System">Vo</SPAN>下降。显见，这个深度的负反馈电路必然在<SPAN
lang=EN-US style="FONT-FAMILY: System">VI</SPAN>等于基准电压处稳定，此时<SPAN
lang=EN-US
style="FONT-FAMILY: System">Vo=(1+R1/R2)Vref</SPAN>。选择不同的<SPAN
lang=EN-US style="FONT-FAMILY: System">R1</SPAN>和<SPAN lang=EN-US
style="FONT-FAMILY: System">R2</SPAN>的值可以得到从<SPAN lang=EN-US
style="FONT-FAMILY: System">2.5V</SPAN>到<SPAN lang=EN-US
style="FONT-FAMILY: System">36V</SPAN>范围内的任意电压输出，特别地，当<SPAN
lang=EN-US style="FONT-FAMILY: System">R1=R2</SPAN>时，<SPAN
lang=EN-US
style="FONT-FAMILY: System">Vo=5V</SPAN>。需要注意的是，在选择电阻时必须保证<SPAN
lang=EN-US
style="FONT-FAMILY: System">TL431</SPAN>工作的必要条件，就是通过阴极的电流要大于<SPAN
lang=EN-US style="FONT-FAMILY: System">1 mA </SPAN>。<SPAN
lang=EN-US><O:P> </O:P></SPAN></P>
<P
style="MARGIN-LEFT: 72pt; MARGIN-RIGHT: 36pt">当然，这个电路并不太实用，但它很清晰地展示了该器件的工作原理在应用中的方法。将这个电路稍加改动，就可以得到在很多实用的电源电路，如图<SPAN
lang=EN-US style="FONT-FAMILY: System">3</SPAN>，<SPAN lang=EN-US
style="FONT-FAMILY: System">4</SPAN>。<SPAN lang=EN-US><O:P>
</SPAN></P>
<P style="MARGIN-LEFT: 72pt; MARGIN-RIGHT: 36pt"><SPAN
lang=EN-US></O:P></SPAN></P>
<P style="MARGIN-LEFT: 72pt; MARGIN-RIGHT: 36pt"><V:SHAPE
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<P style="MARGIN-LEFT: 72pt; MARGIN-RIGHT: 36pt">图<SPAN lang=EN-US
style="FONT-FAMILY: System">3 </SPAN>大电流的分流稳压电路<SPAN
lang=EN-US>
图</SPAN><SPAN lang=EN-US
style="FONT-FAMILY: System">4 </SPAN>精密<SPAN lang=EN-US
style="FONT-FAMILY: System">5V</SPAN>稳压器<B><SPAN
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