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AN729 Datasheet(PDF) 1 Page - Silicon Laboratories |
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AN729 Datasheet(HTML) 1 Page - Silicon Laboratories |
1 / 8 page Rev. 0.1 8/12 Copyright © 2012 by Silicon Laboratories AN729 AN729 R EPLACING TRADITIONAL O PTOCOUPLERS WITH Si87 XX D IGITAL I SOLATORS 1. Introduction Opto-couplers are a decades-old technology widely used for signal isolation, typically providing safety isolation, signal level shifting, and ground loop mitigation. They are commonly used in a wide range of end applications, including data communication circuits, switch mode power systems, measurement and test systems, and isolated data acquisition systems. Optocouplers have several weaknesses, including parametric instability with temperature and device aging, significant internal parasitic couplings, long propagation delay times, narrow operating temperature ranges, and relatively low reliability. Optocouplers have been the “go-to” isolation device of choice for the past 30+ years because they were the only integrated solutions to the problem of signal isolation. Today's advanced CMOS signal isolation products offer better timing performance, higher reliability, and lower power consumption compared to optocouplers and are capturing sockets traditionally held by optocouplers. However, converting to CMOS isolation devices has, most often, required circuit changes and PCB modifications that cost money and create design risks, until now. The Si87xx family can be used as a pin-compatible optocoupler upgrade in existing systems or as a design-in component for new product designs. Package and pin compatibility allow the Si87xx to replace optocouplers without PCB modifications, with substantial gains in device performance and reliability. Device operation is simple: the Si87xx output is held low when anode current is above the turn-on current threshold and pulled high by an internal or external pull-up resistor when anode current is below the turn-off current threshold. (For more information about current thresholds and hysteresis values, see the Si87xx data sheet.) 2. Si87xx Operation Figure 1 shows an Si87xx block diagram in which the input-side circuit contains a diode emulator, high-frequency transmitter, and galvanic isolator, all of which are powered by the voltage present on the anode pin. The diode emulator mimics the behavior of an optocoupler LED to ensure compatibility with existing optocoupler input circuits. Device operation is straightforward: the diode emulator enables the transmitter when anode current is above its turn-on threshold. This action causes the transmitter to propagate a high-frequency carrier across the isolation barrier to the receiver, which, in turn, forces the output driver low. Conversely, an anode current below the turn-off threshold disables the transmitter, causing the receiver to release the output pin to be pulled high by the pull-up resistor. Figure 1. Si8710 Digital Isolator Block Diagram Si8710 Digital Isolator OUTPUT DIE INPUT DIE ANODE VDD GND NC NC XMIT VDDI VO CATHODE RF VBIAS ENABLE VF IF GNDI VDD2 e GND2 RECV BUFFER NC |
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