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ISL81487 Datasheet(PDF) 9 Page - Intersil Corporation |
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ISL81487 Datasheet(HTML) 9 Page - Intersil Corporation |
9 / 13 page 9 Twisted pair is the cable of choice for RS-485/RS-422 networks. Twisted pair cables tend to pick up noise and other electromagnetically induced voltages as common mode signals, which are effectively rejected by the differential receivers in these ICs. To minimize reflections, proper termination is imperative when using the 5Mbps device. Short networks using the 250kbps versions need not be terminated, but, terminations are recommended unless power dissipation is an overriding concern. In point-to-point, or point-to-multipoint (single driver on bus) networks, the main cable should be terminated in its characteristic impedance (typically 120 Ω) at the end farthest from the driver. In multi-receiver applications, stubs connecting receivers to the main cable should be kept as short as possible. Multipoint (multi-driver) systems require that the main cable be terminated in its characteristic impedance at both ends. Stubs connecting a transceiver to the main cable should be kept as short as possible. Built-In Driver Overload Protection As stated previously, the RS-485 spec requires that drivers survive worst case bus contentions undamaged. These devices meet this requirement via driver output short circuit current limits, and on-chip thermal shutdown circuitry. The driver output stages incorporate short circuit current limiting circuitry which ensures that the output current never exceeds the RS-485 spec, even at the common mode voltage range extremes. Additionally, these devices utilize a foldback circuit which reduces the short circuit current, and thus the power dissipation, whenever the contending voltage exceeds either supply. In the event of a major short circuit condition, these devices also include a thermal shutdown feature that disables the drivers whenever the die temperature becomes excessive. This eliminates the power dissipation, allowing the die to cool. The drivers automatically re-enable after the die temperature drops about 15 degrees. If the contention persists, the thermal shutdown/re-enable cycle repeats until the fault is cleared. Receivers stay operational during thermal shutdown. Low Power Shutdown Mode (Excluding ISL81487) These CMOS transceivers all use a fraction of the power required by their bipolar counterparts, but the ISL8487 and ISL81483 include a shutdown feature that reduces the already low quiescent ICC to a 1nA trickle. They enter shutdown whenever the receiver and driver are simultaneously disabled (RE =VCC and DE = GND) for a period of at least 600ns. Disabling both the driver and the receiver for less than 50ns guarantees that shutdown is not entered. Note that receiver and driver enable times increase when enabling from shutdown. Refer to Notes 5-9, at the end of the Electrical Specification table, for more information. Typical Performance Curves VCC = 5V, TA = 25oC, ISL8487, ISL81483 and ISL81487; Unless Otherwise Specified FIGURE 6. DRIVER OUTPUT CURRENT vs DIFFERENTIAL OUTPUT VOLTAGE FIGURE 7. DRIVER DIFFERENTIAL OUTPUT VOLTAGE vs TEMPERATURE DIFFERENTIAL OUTPUT VOLTAGE (V) 0123 45 0 10 20 30 40 50 60 70 80 90 -40 0 50 85 2 2.2 2.4 2.6 2.8 3 3.2 3.4 3.6 TEMPERATURE (oC) -25 25 75 RDIFF = 54Ω RDIFF = 100Ω ISL8487, ISL81483, ISL81487 |
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