iC921 发表于 2008-3-12 00:23

电路精选：曾经有人说不明白的驱动电路

在模拟技术，如图是曾经有人跟帖说不明白如图的电-光驱动电路。那些电阻，那些二极管为什么要加？如何加？<br /><br />时间好长了，记得当时没有明确的答案，我也答这不上。偶然碰到一个器件手册内的部分内容，这就有了明确的答案了。前面部分我译出来了，感觉还可以，可是后面部分有的有点难，看看哪位来？<br /><br /><b>Common-Mode&nbsp;Rejection&nbsp;&&nbsp;LED&nbsp;Driver&nbsp;Circuits</b><br /><br />［1］With&nbsp;the&nbsp;combination&nbsp;of&nbsp;a&nbsp;high&nbsp;efficiency&nbsp;AlGaAs&nbsp;LED&nbsp;and&nbsp;a&nbsp;high-gain&nbsp;amplifier&nbsp;in&nbsp;the&nbsp;HCPL-&nbsp;47XX&nbsp;optocoupler,&nbsp;a&nbsp;few&nbsp;circuit&nbsp;techniques&nbsp;can&nbsp;enhance&nbsp;the&nbsp;common-mode&nbsp;rejection&nbsp;(CMR)&nbsp;of&nbsp;this&nbsp;optocoupler.&nbsp;First,&nbsp;use&nbsp;good&nbsp;high-frequency&nbsp;circuit&nbsp;layout&nbsp;practices&nbsp;to&nbsp;minimize&nbsp;coupling&nbsp;of&nbsp;common-mode&nbsp;signals&nbsp;between&nbsp;input&nbsp;and&nbsp;output&nbsp;circuits.&nbsp;Keep&nbsp;input&nbsp;traces&nbsp;away&nbsp;from&nbsp;output&nbsp;traces&nbsp;to&nbsp;minimize&nbsp;capacitive&nbsp;coupling&nbsp;of&nbsp;interference&nbsp;between&nbsp;input&nbsp;and&nbsp;output&nbsp;sections.&nbsp;If&nbsp;possible,&nbsp;parallel,&nbsp;or&nbsp;shunt&nbsp;switch&nbsp;the&nbsp;LED&nbsp;current&nbsp;as&nbsp;shown&nbsp;in&nbsp;Figure&nbsp;13,&nbsp;rather&nbsp;than&nbsp;series&nbsp;switch&nbsp;the&nbsp;LED&nbsp;current&nbsp;as&nbsp;illustrated&nbsp;in&nbsp;Figure&nbsp;15.&nbsp;Not&nbsp;only&nbsp;will&nbsp;CMR&nbsp;be&nbsp;enhanced&nbsp;with&nbsp;these&nbsp;circuits&nbsp;(Figures&nbsp;13&nbsp;and&nbsp;14),&nbsp;but&nbsp;the&nbsp;switching&nbsp;speed&nbsp;of&nbsp;the&nbsp;optocoupler&nbsp;will&nbsp;be&nbsp;improved&nbsp;as&nbsp;well.&nbsp;This&nbsp;is&nbsp;because&nbsp;in&nbsp;the&nbsp;parallel&nbsp;switched&nbsp;case&nbsp;the&nbsp;LED&nbsp;current&nbsp;is&nbsp;current-steered&nbsp;into&nbsp;or&nbsp;away&nbsp;from&nbsp;the&nbsp;LED,&nbsp;rather&nbsp;than&nbsp;being&nbsp;fully&nbsp;turned&nbsp;off&nbsp;as&nbsp;in&nbsp;the&nbsp;series&nbsp;switched&nbsp;case.<br /><br />一些电路技术能增强集高效率和高增益放大器于一体的HCPL-&nbsp;47XX光电耦合器的共模抑制（CMR）［效果］。好的高频布线习惯能让输入输出端的共模信号耦合最小化。布线时，让输入线路和输出线路尽可能地远离，可让输入输出侧的偶合电容最小化。如果可以，象图13那样并联或串联开关，其效果将优于图15那样的开关串联在LED二极管回路中的方法。这样的电路(Figures&nbsp;13&nbsp;and&nbsp;14)不但共模抑制CMR增强，光耦的开关速度也将得到改善。这是因为并联的开关可以直接引起LED二极管电流的流入或流出，其效果将优于完全依靠串联开关关断（控制通断）的情况。<br /><br />［2］&nbsp;Figure&nbsp;13&nbsp;illustrates&nbsp;this&nbsp;type&nbsp;of&nbsp;circuit.&nbsp;The&nbsp;Schottky&nbsp;diode&nbsp;helps&nbsp;quickly&nbsp;to&nbsp;discharge&nbsp;and&nbsp;pre-bias&nbsp;the&nbsp;LED&nbsp;in&nbsp;the&nbsp;off&nbsp;state.&nbsp;If&nbsp;a&nbsp;common-mode&nbsp;voltage&nbsp;across&nbsp;the&nbsp;optocoupler&nbsp;suddenly&nbsp;attempts&nbsp;to&nbsp;inject&nbsp;a&nbsp;current&nbsp;into&nbsp;the&nbsp;off&nbsp;LED&nbsp;anode,&nbsp;the&nbsp;Schottky&nbsp;diode&nbsp;would&nbsp;divert&nbsp;the&nbsp;interfering&nbsp;current&nbsp;to&nbsp;ground.&nbsp;The&nbsp;combination&nbsp;of&nbsp;the&nbsp;Schottky&nbsp;diode&nbsp;forward&nbsp;voltage&nbsp;and&nbsp;the&nbsp;Vol&nbsp;saturation&nbsp;voltage&nbsp;of&nbsp;the&nbsp;driver&nbsp;output&nbsp;stage&nbsp;(on-condition)&nbsp;will&nbsp;keep&nbsp;the&nbsp;LED&nbsp;voltage&nbsp;at&nbsp;or&nbsp;below&nbsp;0.8&nbsp;V.&nbsp;This&nbsp;will&nbsp;prevent&nbsp;the&nbsp;LED&nbsp;(off-condition)&nbsp;from&nbsp;conducting&nbsp;any&nbsp;significant&nbsp;forward&nbsp;current&nbsp;that&nbsp;might&nbsp;cause&nbsp;the&nbsp;HCPL-47XX&nbsp;to&nbsp;turn&nbsp;on.&nbsp;Also,&nbsp;if&nbsp;the&nbsp;driver&nbsp;stage&nbsp;is&nbsp;an&nbsp;active&nbsp;totem-pole&nbsp;output,&nbsp;the&nbsp;Schottky&nbsp;diode&nbsp;allows&nbsp;the&nbsp;active&nbsp;output&nbsp;pull-up&nbsp;section&nbsp;to&nbsp;disconnect&nbsp;from&nbsp;the&nbsp;LED&nbsp;and&nbsp;pull&nbsp;high.<br /><br />如图13所示的电路型式，肖特基二极管可以帮助关断状态的LED快速充电和预偏置。如果光耦的共模电压地试图注入关断中的LED阳极，则肖特基二极管会帝路此电流到地GND。肖特基二极管的正向电压和驱动器输出级的饱和导通电压(on-condition)将一起保持LED在/低于0.8&nbsp;V，也就防止了能引起HCPL-47XX导通的任何正向电流进入LED二极管(off-condition)。同样，如果驱动级是有源的图腾柱输出（totem-pole&nbsp;output），肖特基二极管允许有源输出的上拉但却截断了LED的联系。<br /><br />&nbsp;<br />&nbsp;<br />［3］As&nbsp;shown&nbsp;in&nbsp;Figure&nbsp;14,&nbsp;most&nbsp;active&nbsp;output&nbsp;driver&nbsp;integrated&nbsp;circuits&nbsp;can&nbsp;source&nbsp;directly&nbsp;the&nbsp;forward&nbsp;current&nbsp;needed&nbsp;to&nbsp;operate&nbsp;the&nbsp;LED&nbsp;of&nbsp;the&nbsp;HCPL-47XX&nbsp;optocoupler.&nbsp;The&nbsp;advantage&nbsp;of&nbsp;using&nbsp;the&nbsp;silicon&nbsp;diode&nbsp;in&nbsp;this&nbsp;circuit&nbsp;is&nbsp;to&nbsp;conduct&nbsp;charge&nbsp;out&nbsp;of&nbsp;the&nbsp;LED&nbsp;quickly&nbsp;when&nbsp;the&nbsp;LED&nbsp;is&nbsp;turned&nbsp;off.&nbsp;Upon&nbsp;turn-on&nbsp;of&nbsp;the&nbsp;LED,&nbsp;the&nbsp;silicon&nbsp;diode&nbsp;capacitance&nbsp;will&nbsp;provide&nbsp;a&nbsp;rapid&nbsp;charging&nbsp;path&nbsp;(peaking&nbsp;current)&nbsp;for&nbsp;the&nbsp;LED.&nbsp;In&nbsp;addition,&nbsp;this&nbsp;silicon&nbsp;diode&nbsp;prevents&nbsp;commonmode&nbsp;current&nbsp;from&nbsp;entering&nbsp;the&nbsp;LED&nbsp;anode&nbsp;when&nbsp;the&nbsp;driver&nbsp;IC&nbsp;is&nbsp;on&nbsp;and&nbsp;no&nbsp;operating&nbsp;LED&nbsp;current&nbsp;exists.<br /><br />如图14所示，大多数集成电路有源驱动器能泵出HCPL-47XX光电耦合器的LED所需的正向工作电流。该电路用硅二极管的优势在于在关闭LED二极管时能快速抽出它的电荷。而要点亮该LED二极管时，硅二极管的电容又能为它提供快速充电的路径（即峰值电流）。另外，在驱动器IC导通（ON）和LED不存在工作电流时，该硅二极管还可以防止共模电流注入发光二极管的阳极。<br /><br /><img src="http://blog.21ic.com/uploadfiles/2008-5/3311478008.gif"><br />原图不能显示，改由BLOG上传补上https://bbs.21ic.com/upfiles/img/20083/200831201718650.gif<br /><br /><b>以下部分怎么也译不好，看看哪位来？！</b><br /><br />［4］&nbsp;In&nbsp;general,&nbsp;series&nbsp;switching&nbsp;the&nbsp;low&nbsp;input&nbsp;current&nbsp;of&nbsp;the&nbsp;HCPL-47XX&nbsp;LED&nbsp;is&nbsp;not&nbsp;recommended.&nbsp;This&nbsp;is&nbsp;particularly&nbsp;valid&nbsp;when&nbsp;in&nbsp;a&nbsp;high&nbsp;common-mode&nbsp;interference&nbsp;environment.&nbsp;However,&nbsp;if&nbsp;series&nbsp;switching&nbsp;of&nbsp;the&nbsp;LED&nbsp;current&nbsp;must&nbsp;be&nbsp;done,&nbsp;use&nbsp;an&nbsp;additional&nbsp;pull-up&nbsp;resistor&nbsp;from&nbsp;the&nbsp;cathode&nbsp;of&nbsp;the&nbsp;LED&nbsp;to&nbsp;the&nbsp;input&nbsp;VCC&nbsp;as&nbsp;shown&nbsp;in&nbsp;Figure&nbsp;15.&nbsp;This&nbsp;helps&nbsp;minimize&nbsp;any&nbsp;differential-mode&nbsp;current&nbsp;from&nbsp;conducting&nbsp;in&nbsp;the&nbsp;LED&nbsp;while&nbsp;the&nbsp;LED&nbsp;is&nbsp;off,&nbsp;due&nbsp;to&nbsp;a&nbsp;common-mode&nbsp;signal&nbsp;occurring&nbsp;on&nbsp;the&nbsp;input&nbsp;VCC&nbsp;(anode)&nbsp;of&nbsp;the&nbsp;LED.&nbsp;The&nbsp;commonmode&nbsp;signal&nbsp;coupling&nbsp;to&nbsp;the&nbsp;anode&nbsp;and&nbsp;cathode&nbsp;could&nbsp;be&nbsp;slightly&nbsp;different.&nbsp;This&nbsp;could&nbsp;potentially&nbsp;create&nbsp;a&nbsp;LED&nbsp;current&nbsp;to&nbsp;flow&nbsp;that&nbsp;would&nbsp;rival&nbsp;the&nbsp;normal,&nbsp;low&nbsp;input&nbsp;current&nbsp;needed&nbsp;to&nbsp;operate&nbsp;the&nbsp;optocoupler.&nbsp;This&nbsp;additional&nbsp;parallel&nbsp;resistor&nbsp;can&nbsp;help&nbsp;shunt&nbsp;any&nbsp;leakage&nbsp;current&nbsp;around&nbsp;the&nbsp;LED&nbsp;should&nbsp;the&nbsp;drive&nbsp;circuit,&nbsp;in&nbsp;the&nbsp;off&nbsp;state,&nbsp;have&nbsp;any&nbsp;significant&nbsp;leakage&nbsp;current&nbsp;on&nbsp;the&nbsp;order&nbsp;of&nbsp;40&nbsp;μA.<br /><br /><br />［5］With&nbsp;the&nbsp;use&nbsp;of&nbsp;this&nbsp;parallel&nbsp;resistor,&nbsp;the&nbsp;total&nbsp;drive&nbsp;current&nbsp;conducted&nbsp;when&nbsp;the&nbsp;LED&nbsp;is&nbsp;on&nbsp;is&nbsp;the&nbsp;sum&nbsp;of&nbsp;the&nbsp;parallel&nbsp;resistor&nbsp;and&nbsp;LED&nbsp;currents.&nbsp;In&nbsp;the&nbsp;series&nbsp;circuit&nbsp;of&nbsp;Figure&nbsp;15&nbsp;with&nbsp;the&nbsp;LED&nbsp;off,&nbsp;if&nbsp;a&nbsp;common-mode&nbsp;voltage&nbsp;were&nbsp;to&nbsp;couple&nbsp;to&nbsp;the&nbsp;LED&nbsp;cathode,&nbsp;there&nbsp;can&nbsp;be&nbsp;enough&nbsp;imbalance&nbsp;of&nbsp;common-mode&nbsp;voltage&nbsp;across&nbsp;the&nbsp;LED&nbsp;to&nbsp;cause&nbsp;a&nbsp;LED&nbsp;current&nbsp;to&nbsp;flow&nbsp;and,&nbsp;inadvertently,&nbsp;turn&nbsp;on&nbsp;the&nbsp;optocoupler.&nbsp;This&nbsp;series,&nbsp;switching&nbsp;circuit&nbsp;has&nbsp;no&nbsp;protection&nbsp;against&nbsp;a&nbsp;negative-transition,&nbsp;input&nbsp;commonmode&nbsp;signal.<br />

ocon 发表于 2008-3-12 16:36

简单的电路原来有这么多道理，

看来电路上到高频后要考虑的问题真不少。

ocon 发表于 2008-3-12 18:11

接上，

［4］一般不推荐串联开关在低端的办法来控制HCPL-47XX&nbsp;LED的输入电流。这在高共模干扰环境下显而易见。然而，如果一定要这么干，需要依图15所示从LED的负极接个上拉电阻到VCC，这有助于降低LED关闭时串过来的差模干扰电流，原因是，共模干扰信号作用于LED的正极的VCC，而此信号在LED的正负两端耦合情况有些微差异，这将可能在LED上产生电流并导致其工作异常。这个额外并联的电阻将有助于消除泄漏电流对LED的影响，在LED关闭状态，对于工作电流只有40μA的LED来说任何泄漏电流都很重要。

iC921 发表于 2008-3-12 20:55

谢谢老乡！

这句可能没有处理好，我也是在这被堵住了----<br />This&nbsp;could&nbsp;potentially&nbsp;create&nbsp;a&nbsp;LED&nbsp;current&nbsp;to&nbsp;flow&nbsp;that&nbsp;would&nbsp;rival&nbsp;the&nbsp;normal,&nbsp;low&nbsp;input&nbsp;current&nbsp;needed&nbsp;to&nbsp;operate&nbsp;the&nbsp;optocoupler.<br />“这将可能在LED上产生电流并导致其工作异常”

ocon 发表于 2008-3-12 21:06

继续

［5］由于这个并联电阻，当LED点亮时，总驱动电流等于LED电流与并联电阻电流之和。在图15的串联电路里，当LED关闭时，如果一个共模电压耦合到LED负极，将在LED上产生足够的不平衡电压，致使电流流过LED导致光耦意外导通。这个串联开关电路，对输入的共模信号的负跃迁没有保护功能。

ocon 发表于 2008-3-12 21:10

用金山词霸译的，可能有问题，我的英语在高中时就没及格

iC921 发表于 2008-3-12 21:20

没事，我也是瞎碰撞

后面的我正在尝试进行中……

iC921 发表于 2008-3-12 22:38

很不错啊！不过此处turn on译作点亮什么的比较好

<br /><br />［5］With&nbsp;the&nbsp;use&nbsp;of&nbsp;this&nbsp;parallel&nbsp;resistor,&nbsp;the&nbsp;total&nbsp;drive&nbsp;current&nbsp;conducted&nbsp;when&nbsp;the&nbsp;LED&nbsp;is&nbsp;on&nbsp;is&nbsp;the&nbsp;sum&nbsp;of&nbsp;the&nbsp;parallel&nbsp;resistor&nbsp;and&nbsp;LED&nbsp;currents.&nbsp;<font color=#0000FF>In&nbsp;the&nbsp;series&nbsp;circuit&nbsp;of&nbsp;Figure&nbsp;15&nbsp;with&nbsp;the&nbsp;LED&nbsp;off,&nbsp;if&nbsp;a&nbsp;common-mode&nbsp;voltage&nbsp;were&nbsp;to&nbsp;couple&nbsp;to&nbsp;the&nbsp;LED&nbsp;cathode,&nbsp;there&nbsp;can&nbsp;be&nbsp;enough&nbsp;imbalance&nbsp;of&nbsp;common-mode&nbsp;voltage&nbsp;across&nbsp;the&nbsp;LED&nbsp;to&nbsp;cause&nbsp;a&nbsp;LED&nbsp;current&nbsp;to&nbsp;flow&nbsp;and,&nbsp;inadvertently,&nbsp;turn&nbsp;on&nbsp;the&nbsp;optocoupler.</font>&nbsp;This&nbsp;series,&nbsp;switching&nbsp;circuit&nbsp;has&nbsp;no&nbsp;protection&nbsp;against&nbsp;a&nbsp;negative-transition,&nbsp;input&nbsp;commonmode&nbsp;signal.<br /><br />用了并联电阻后，发光二极管LED导通时，总驱动电流为该电阻和二极管的电流之和。如图15所示的一类电路，发光二极管LED关闭时，如果有共模电压耦合到它的阴极，就有足够的不平衡电压致使流过而意外地点亮光耦的LED。也就是说，这种开关串联于LED的电路，对负向共模输入信号不提供保护。<br />

iC921 发表于 2008-3-12 22:40

问：什么情况下会在阴极出现“负”干扰呢？

decoder8 发表于 2008-3-13 12:55

做个记号慢慢看!

iC921 发表于 2008-3-13 18:51

内容不多，有E文才显得太长

不知道谁给我加酷，打乱了我要记录55000分的计划。当然，这也得谢谢它，省得老hot谰我“腐败”……

mxh0506 发表于 2008-3-14 22:27

版主选的**真不错

又学习了

asunmad 发表于 2008-3-17 22:44

RE：电路精选：曾经有人说不明白的驱动电路

［4］&nbsp;In&nbsp;general,&nbsp;series&nbsp;switching&nbsp;the&nbsp;low&nbsp;input&nbsp;current&nbsp;of&nbsp;the&nbsp;HCPL-47XX&nbsp;LED&nbsp;is&nbsp;not&nbsp;recommended.&nbsp;This&nbsp;is&nbsp;particularly&nbsp;valid&nbsp;when&nbsp;in&nbsp;a&nbsp;high&nbsp;common-mode&nbsp;interference&nbsp;environment.&nbsp;However,&nbsp;if&nbsp;series&nbsp;switching&nbsp;of&nbsp;the&nbsp;LED&nbsp;current&nbsp;must&nbsp;be&nbsp;done,&nbsp;use&nbsp;an&nbsp;additional&nbsp;pull-up&nbsp;resistor&nbsp;from&nbsp;the&nbsp;cathode&nbsp;of&nbsp;the&nbsp;LED&nbsp;to&nbsp;the&nbsp;input&nbsp;VCC&nbsp;as&nbsp;shown&nbsp;in&nbsp;Figure&nbsp;15.&nbsp;This&nbsp;helps&nbsp;minimize&nbsp;any&nbsp;differential-mode&nbsp;current&nbsp;from&nbsp;conducting&nbsp;in&nbsp;the&nbsp;LED&nbsp;while&nbsp;the&nbsp;LED&nbsp;is&nbsp;off,&nbsp;due&nbsp;to&nbsp;a&nbsp;common-mode&nbsp;signal&nbsp;occurring&nbsp;on&nbsp;the&nbsp;input&nbsp;VCC&nbsp;(anode)&nbsp;of&nbsp;the&nbsp;LED.&nbsp;The&nbsp;commonmode&nbsp;signal&nbsp;coupling&nbsp;to&nbsp;the&nbsp;anode&nbsp;and&nbsp;cathode&nbsp;could&nbsp;be&nbsp;slightly&nbsp;different.&nbsp;This&nbsp;could&nbsp;potentially&nbsp;create&nbsp;a&nbsp;LED&nbsp;current&nbsp;to&nbsp;flow&nbsp;that&nbsp;would&nbsp;rival&nbsp;the&nbsp;normal,&nbsp;low&nbsp;input&nbsp;current&nbsp;needed&nbsp;to&nbsp;operate&nbsp;the&nbsp;optocoupler.&nbsp;This&nbsp;additional&nbsp;parallel&nbsp;resistor&nbsp;can&nbsp;help&nbsp;shunt&nbsp;any&nbsp;leakage&nbsp;current&nbsp;around&nbsp;the&nbsp;LED&nbsp;should&nbsp;the&nbsp;drive&nbsp;circuit,&nbsp;in&nbsp;the&nbsp;off&nbsp;state,&nbsp;have&nbsp;any&nbsp;significant&nbsp;leakage&nbsp;current&nbsp;on&nbsp;the&nbsp;order&nbsp;of&nbsp;40&nbsp;μA.<br /><br />&nbsp;LED附件的泄漏电流得以分流。一般不建议对小电流的HCPL-47XX&nbsp;LED实行串联开关驱动，在共模干扰强的环境中尤其如此。如果必须使用串联开关驱动，则应该在LED的阴极到VCC输入之间增加一个上拉电阻，如图15所示。这样，在LED关闭期间LED的VCC输入端（阳极）出现的共模信号时，尽可能地减小了差模电流使LED导通的可能性。耦合到阳极和阴极的共模信号可能略有差别，这将可能建立起足以强过使光耦工作的正常模式小电流。如果驱动电路在关态有40μA数量级的明显的泄漏电流，增加这个并联电阻使LED附近的泄漏电流得以分流。<br />

iC921 发表于 2008-3-18 01:03

OK，终于完全明白了！无论翻译还是理解，这句是关键----

The&nbsp;commonmode&nbsp;signal&nbsp;coupling&nbsp;to&nbsp;the&nbsp;anode&nbsp;and&nbsp;cathode&nbsp;could&nbsp;be&nbsp;slightly&nbsp;different.<br />共模电压（电源VCC）加到发光二极管LED两极后会小有不同，原因就是那个串联的开关会漏电吧。<br /><br />［这样，在LED关闭期间LED的VCC输入端（阳极）出现的共模信号时，尽可能地减小了差模电流使LED导通的可能性。耦合到阳极和阴极的共模信号可能略有差别］<br />［原因是，共模干扰信号作用于LED的正极的VCC，而此信号在LED的正负两端耦合情况有些微差异］。<br /><br /><br /><br />［4］&nbsp;In&nbsp;general,&nbsp;series&nbsp;switching&nbsp;the&nbsp;low&nbsp;input&nbsp;current&nbsp;of&nbsp;the&nbsp;HCPL-47XX&nbsp;LED&nbsp;is&nbsp;not&nbsp;recommended.&nbsp;This&nbsp;is&nbsp;particularly&nbsp;valid&nbsp;when&nbsp;in&nbsp;a&nbsp;high&nbsp;common-mode&nbsp;interference&nbsp;environment.&nbsp;However,&nbsp;if&nbsp;series&nbsp;switching&nbsp;of&nbsp;the&nbsp;LED&nbsp;current&nbsp;must&nbsp;be&nbsp;done,&nbsp;use&nbsp;an&nbsp;additional&nbsp;pull-up&nbsp;resistor&nbsp;from&nbsp;the&nbsp;cathode&nbsp;of&nbsp;the&nbsp;LED&nbsp;to&nbsp;the&nbsp;input&nbsp;VCC&nbsp;as&nbsp;shown&nbsp;in&nbsp;Figure&nbsp;15.&nbsp;This&nbsp;helps&nbsp;minimize&nbsp;any&nbsp;differential-mode&nbsp;current&nbsp;from&nbsp;conducting&nbsp;in&nbsp;the&nbsp;LED&nbsp;while&nbsp;the&nbsp;LED&nbsp;is&nbsp;off,&nbsp;due&nbsp;to&nbsp;a&nbsp;common-mode&nbsp;signal&nbsp;occurring&nbsp;on&nbsp;the&nbsp;input&nbsp;VCC&nbsp;(anode)&nbsp;of&nbsp;the&nbsp;LED.&nbsp;<font color=#0000FF>The&nbsp;commonmode&nbsp;signal&nbsp;coupling&nbsp;to&nbsp;the&nbsp;anode&nbsp;and&nbsp;cathode&nbsp;could&nbsp;be&nbsp;slightly&nbsp;different.&nbsp;</font>This&nbsp;could&nbsp;potentially&nbsp;create&nbsp;a&nbsp;LED&nbsp;current&nbsp;to&nbsp;flow&nbsp;that&nbsp;would&nbsp;rival&nbsp;the&nbsp;normal,&nbsp;low&nbsp;input&nbsp;current&nbsp;needed&nbsp;to&nbsp;operate&nbsp;the&nbsp;optocoupler.&nbsp;This&nbsp;additional&nbsp;parallel&nbsp;resistor&nbsp;can&nbsp;help&nbsp;shunt&nbsp;any&nbsp;leakage&nbsp;current&nbsp;around&nbsp;the&nbsp;LED&nbsp;should&nbsp;the&nbsp;drive&nbsp;circuit,&nbsp;in&nbsp;the&nbsp;off&nbsp;state,&nbsp;have&nbsp;any&nbsp;significant&nbsp;leakage&nbsp;current&nbsp;on&nbsp;the&nbsp;order&nbsp;of&nbsp;40&nbsp;μA.<br /><br />&nbsp;LED附件的泄漏电流得以分流。一般不建议对小电流的HCPL-47XX&nbsp;LED实行串联开关驱动，在共模干扰强的环境中尤其如此。如果必须使用串联开关驱动，则应该在LED的阴极到VCC输入之间增加一个上拉电阻，如图15所示。这样，在LED关闭期间LED的VCC输入端（阳极）出现的共模信号时，尽可能地减小了差模电流使LED导通的可能性。耦合到阳极和阴极的共模信号可能略有差别，这将可能建立起足以强过使光耦工作的正常模式小电流。如果驱动电路在关态有40μA数量级的明显的泄漏电流，增加这个并联电阻使LED附近的泄漏电流得以分流。<br /><br /><br />［4］一般不推荐串联开关在低端的办法来控制HCPL-47XX&nbsp;LED的输入电流。这在高共模干扰环境下显而易见。然而，如果一定要这么干，需要依图15所示从LED的负极接个上拉电阻到VCC，这有助于降低LED关闭时串过来的差模干扰电流，原因是，共模干扰信号作用于LED的正极的VCC，而此信号在LED的正负两端耦合情况有些微差异，这将可能在LED上产生电流并导致其工作异常。这个额外并联的电阻将有助于消除泄漏电流对LED的影响，在LED关闭状态，对于工作电流只有40μA的LED来说任何泄漏电流都很重要。<br /><br />

asunmad 发表于 2008-3-18 21:53

不小心把尾巴贴到开头了。

昨晚匆匆一贴，突然发现有名字，改一下，却把错误的尾巴弄到开头来了:(<br /><br />最后一句自己也感觉译得拗口，但觉得意义是我理解的那个。<br /><br />This&nbsp;additional&nbsp;parallel&nbsp;resistor&nbsp;can&nbsp;help&nbsp;shunt&nbsp;any&nbsp;leakage&nbsp;current&nbsp;around&nbsp;the&nbsp;LED&nbsp;should&nbsp;the&nbsp;drive&nbsp;circuit,&nbsp;in&nbsp;the&nbsp;off&nbsp;state,&nbsp;have&nbsp;any&nbsp;significant&nbsp;leakage&nbsp;current&nbsp;on&nbsp;the&nbsp;order&nbsp;of&nbsp;40&nbsp;μA.<br />&nbsp;=<br />This&nbsp;additional&nbsp;parallel&nbsp;resistor&nbsp;can&nbsp;help&nbsp;shunt&nbsp;any&nbsp;leakage&nbsp;current&nbsp;around&nbsp;the&nbsp;LED&nbsp;if&nbsp;the&nbsp;drive&nbsp;circuit&nbsp;should&nbsp;have&nbsp;any&nbsp;significant&nbsp;leakage&nbsp;current&nbsp;on&nbsp;the&nbsp;order&nbsp;of&nbsp;40&nbsp;μA&nbsp;when&nbsp;it&nbsp;is&nbsp;in&nbsp;the&nbsp;off&nbsp;state.<br />所以on&nbsp;the&nbsp;order&nbsp;of&nbsp;40&nbsp;μA指的是leakage&nbsp;current。<br />其中should&nbsp;....&nbsp;=&nbsp;if&nbsp;....&nbsp;should，是虚拟语句的一种表达方式。<br />shunt是旁路，如果没有这个电阻，驱动电路的关态漏电流只能从LED流过，有了这个电阻，就可以从这个电阻流过了，这就把LED给旁路了。

iC921 发表于 2008-3-18 22:20

哈哈，我以为你是突出那一句的呢……

从应用上说，确实是这一句有意义。但从理解上说，我倒是觉得共模电压在两点上存在差异是关键，比如开关泄漏电流，共模电压波动而通过开关两端的电容耦合成回路等。

anewlife 发表于 2008-3-19 12:55

学习学习~~~

apcfy 发表于 2008-3-20 11:13

牛人,谢谢!

5_0_5 发表于 2008-3-20 12:54

学习的

要多动脑筋 发表于 2008-3-27 18:45

学习

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