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HCPL-2602 Datasheet(PDF) 14 Page - Agilent(Hewlett-Packard) |
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HCPL-2602 Datasheet(HTML) 14 Page - Agilent(Hewlett-Packard) |
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14 / 15 page  1-327 different circuit configuration could make t PHL <tPLH, in which case NOR gates would be pre- ferred. If it is not known whether t PHL > tPLH or tPHL < tPLH, or if the drive conditions may vary over the boundary for these conditions, the exclusive-OR flip-flop of Figure (d) should be used. RS-422 and RS-423 Line drivers designed for RS-422 and RS-423 generally provide adequate voltage and current for operating the HCPL-2602/12. Most drivers also have characteristics allowing the HCPL-2602/12 to be connected directly to the driver terminals. Worst case drive conditions, however, would require current shunting to prevent overstress of the HCPL-2602/12. Propagation Delay, Pulse- Width Distortion and Propagation Delay Skew Propagation delay is a figure of merit which describes how quickly a logic signal propagates through a system. The propaga- tion delay from low to high (t PLH) is the amount of time required for an input signal to propagate to the output, causing the output to change from low to high. Similarly, the propagation delay from high to low (t PHL) is the amount of time required for the input signal to propagate to the output, causing the output to change from high to low (see Figure 6). Pulse-width distortion (PWD) results when t PLH and tPHL differ in value. PWD is defined as the difference between t PLH and tPHL and often determines the maximum data rate capability of a transmission system. PWD can be expressed in percent by dividing the PWD (in ns) by the minimum pulse width (in ns) being transmitted. Typically, PWD on the order of 20-30% of the minimum pulse width is tolerable; the exact figure depends on the particular application (RS232, RS422, T-1, etc.). Propagation delay skew, t PSK, is an important parameter to consider in parallel data applications where synchronization of signals on parallel data lines is a concern. If the parallel data is being sent through a group of optocouplers, differences in propagation delays will cause the data to arrive at the outputs of the optocouplers at different times. If this difference in propagation delays is large enough, it will determine the maximum rate at which parallel data can be sent through the optocouplers. Propagation delay skew is defined as the difference between the minimum and maximum propagation delays, either t PLH or t PHL, for any given group of optocouplers which are operating under the same conditions (i.e., the same drive current, supply voltage, output load, and operating temperature). As illustrated in Figure 16, if the inputs of a group of optocouplers are switched either ON or OFF at the same time, t PSK is the difference between the shortest propagation delay, either t PHL or t PHL, and the longest propagation delay, either t PLH or tPHL. As mentioned earlier, t PSK can determine the maximum parallel data transmission rate. Figure 17 is the timing diagram of a typical parallel data application with both the clock and the data lines being sent through optocouplers. The figure shows data and clock signals at the inputs and outputs of the optocouplers. To obtain the maximum data transmission rate, both edges of the clock signal are being used to clock the data; if only one edge were used, the clock signal would need to be twice as fast. Propagation delay skew represents the uncertainty of where an edge might be after being sent through an optocoupler. Figure 17 shows that there will be uncertainty in both the data and the clock lines. It is important that these two areas of uncertainty not overlap, otherwise the clock signal might arrive before all of the data outputs have settled, or some of the data outputs may start to change before the clock signal has arrived. From these considerations, the absolute minimum pulse width that can be sent through optocouplers in a parallel application is twice t PSK. A cautious design should use a slightly longer pulse width to ensure that any additional uncertainty in the rest of the circuit does not cause a problem. The t PSK specified optocouplers offer the advantages of guaranteed specifications for propagation delays, pulse-width distortion and propagation delay skew over the recommended temperature, input current, and power supply ranges. |
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Similar Description - HCPL-2602 |
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