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ACPL-064L Datasheet(PDF) 12 Page - AVAGO TECHNOLOGIES LIMITED |
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ACPL-064L Datasheet(HTML) 12 Page - AVAGO TECHNOLOGIES LIMITED |
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12 / 16 page  12 Optocoupler CMR Performance The principal protection against common mode noise, comes from the fundamental isolation properties of the optocoupler, and this in turn is directly related to the Input-Output leakage capacitance of the optocoupler. To provide maximum protection to circuitry connected to the input or output of the optocoupler the leakage capac- itance is minimized by having large separation distances at all points in the optocoupler construction, including the LED/photodiode interface. In addition to the optocouplers basic physical construc- tion, additional circuit design steps mitigate the effects of common mode noise. The most important of these is the Faraday shield on the photodetector stage. A Faraday shield is effective in optocouplers because the internal modulation frequency (light) is many orders of magnitude higher than the common mode noise frequency. Improving CMR Performance at the Application Level In an end application it desirable that the optocouplers common mode isolation be as close as possible to that indicated in the data sheet specifications. The first step in meeting this goal is to ensure maximum separation between PCB interconnects on either side of the opto- coupler is maintained and that PCB tracks beneath the optocoupler are avoided. It is inevitable that a certain amount of CMR noise will be coupled into the inputs and this can potentially result in false-triggering of the input. This problem is frequently observed in devices with input high input impedance. In some cases this can cause momentary missing pulses and may even cause input circuitry to latch-up in some alternate technologies. The ACPL-x6xL optocoupler family does not have an input latch-up issue. Even at very high CMR levels such as those experienced in end equipment level tests (for example IEC61000-4-4) the ACPL-x6xL series is immune to latch-up because of the simple diode structure of the LED. In some cases achieving the rated data sheet CMR per- formance level is not possible in an application. This is often because of the practical need to actually connect the isolator input to the output of a dynamically changing signal rather than tying the input statically to VDD or GND. A data sheet CMR “specmanship” issue is often seen with alternative technology isolators that are based on AC encoding techniques. To address the need to define achievable end application performance on data sheets, the ACPL-x6xL optocouplers include an additional typical performance specification for dynamic CMR in the electrical parameter table. The dynamic CMR specification indicates the typical achiev- able CMR performance as the input is being toggled on or off during a CMR transient. The logic output the ACPL-x6xL optocouplers is mainly controlled by LED current level, and since the LED current features very fast rise and fall times, dynamic noise immunity is essentially the same as static noise immunity. Despite their immunity to input latch-up and the excellent dynamic CMR immunity, ACPL-x6xL opto- coupler devices are still potentially vulnerable to miss- operation caused by the LED being turned either on or off during a CMR disturbance. If the LED status could be ensured by design, the overall application level CMR performance would be that of the photodetector. To benefit from the inherently high CMR capabilities of the ACPL-x6xL family, some simple steps about operating the LED at the application level should be taken. In particular, ensure that the LED stays either on or off during a CMR transient. Some common design techniques to accomplish this are: Keep the LED On: i) Overdrive the LED with a higher than required forward current. Keep the LED Off: i) Reverse bias the LED during the off state. ii) Minimize the off-state impedance across the anode and cathode of the LED during the off state. All these methods allow the full CMR capability of the ACPL-x6xL family to be achieved, but they do have practical implementation issues or require a compromise on power consumption. There is, however, an effective method to meet the goal of maintaining the LED status during a CMR event with no other design compromises other than adding a single resistor. This CMR optimization takes advantage of the differential connection to the LED. By ensuring the common mode impedances at both the cathode and anode of the LED are equal, the CMR transient on the LED is effectively canceled. As shown in Figure 11, this is easily achieved by using two, instead of one, input bias resistors. |
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Similar Description - ACPL-064L |
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