原创 电路精选：曾经有人说不明白的驱动电路【转】

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？

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？

Common-Mode Rejection & LED Driver Circuits

［1］With the combination of a high efficiency AlGaAs LED and a high-gain amplifier in the HCPL- 47XX optocoupler, a few circuit techniques can enhance the common-mode rejection (CMR) of this optocoupler. First, use good high-frequency circuit layout practices to minimize coupling of common-mode signals between input and output circuits. Keep input traces away from output traces to minimize capacitive coupling of interference between input and output sections. If possible, parallel, or shunt switch the LED current as shown in Figure 13, rather than series switch the LED current as illustrated in Figure 15. Not only will CMR be enhanced with these circuits (Figures 13 and 14), but the switching speed of the optocoupler will be improved as well. This is because in the parallel switched case the LED current is current-steered into or away from the LED, rather than being fully turned off as in the series switched case.

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- 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 13 and 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。

［2］ Figure 13 illustrates this type of circuit. The Schottky diode helps quickly to discharge and pre-bias the LED in the off state. If a common-mode voltage across the optocoupler suddenly attempts to inject a current into the off LED anode, the Schottky diode would divert the interfering current to ground. The combination of the Schottky diode forward voltage and the Vol saturation voltage of the driver output stage (on-condition) will keep the LED voltage at or below 0.8 V. This will prevent the LED (off-condition) from conducting any significant forward current that might cause the HCPL-47XX to turn on. Also, if the driver stage is an active totem-pole output, the Schottky diode allows the active output pull-up section to disconnect from the LED and pull high.

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 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 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。



［3］As shown in Figure 14, most active output driver integrated circuits can source directly the forward current needed to operate the LED of the HCPL-47XX optocoupler. The advantage of using the silicon diode in this circuit is to conduct charge out of the LED quickly when the LED is turned off. Upon turn-on of the LED, the silicon diode capacitance will provide a rapid charging path (peaking current) for the LED. In addition, this silicon diode prevents commonmode current from entering the LED anode when the driver IC is on and no operating LED current exists.

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。



以下部分怎么也译不好，看看哪位来？！

［4］ In general, series switching the low input current of the HCPL-47XX LED is not recommended. This is particularly valid when in a high common-mode interference environment. However, if series switching of the LED current must be done, use an additional pull-up resistor from the cathode of the LED to the input VCC as shown in Figure 15. This helps minimize any differential-mode current from conducting in the LED while the LED is off, due to a common-mode signal occurring on the input VCC (anode) of the LED. The commonmode signal coupling to the anode and cathode could be slightly different. This could potentially create a LED current to flow that would rival the normal, low input current needed to operate the optocoupler. This additional parallel resistor can help shunt any leakage current around the LED should the drive circuit, in the off state, have any significant leakage current on the order of 40 μA.


［5］With the use of this parallel resistor, the total drive current conducted when the LED is on is the sum of the parallel resistor and LED currents. In the series circuit of Figure 15 with the LED off, if a common-mode voltage were to couple to the LED cathode, there can be enough imbalance of common-mode voltage across the LED to cause a LED current to flow and, inadvertently, turn on the optocoupler. This series, switching circuit has no protection against a negative-transition, input commonmode signal.