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ISL3179EIRZ Datasheet(PDF) 10 Page - Intersil Corporation |
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ISL3179EIRZ Datasheet(HTML) 10 Page - Intersil Corporation |
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10 / 17 page  10 FN6365.2 April 22, 2008 higher than ±9kV. The RS-485 pins of the ISL3179E survive ±9kV contact discharges, while the ISL3180E’s RS-485 pins withstand ±5kV. Hot Plug Function When a piece of equipment powers up, there is a period of time where the processor or ASIC driving the RS-485 control lines (DE, RE) is unable to ensure that the RS-485 Tx and Rx outputs are kept disabled. If the equipment is connected to the bus, a driver activating prematurely during power up may crash the bus. To avoid this scenario, the ISL3179E and ISL3180E incorporate a “Hot Plug” function. Circuitry monitoring VCC ensures that, during power up and power down, the Tx and Rx outputs remain disabled, regardless of the state of DE and RE, if VCC is less than ~2.4V. This gives the processor/ASIC a chance to stabilize and drive the RS-485 control lines to the proper states. Data Rate, Cables, and Terminations RS-485/RS-422 are intended for network lengths up to 4000’, but the maximum system data rate decreases as the transmission length increases. Devices operating at 40Mbps are limited to lengths less than 100’. 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 receiver in this IC. Proper termination is imperative to minimize reflections. 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. The ISL3179E, and ISL3180E may also be used at slower data rates over longer cables, but there are some limitations. The Rx is optimized for high speed operation, so its output may glitch if the Rx input differential transition times are too slow. Keeping the transition times below 500ns, which equates to the Tx driving a 1000’ (305m) CAT 5 cable, yields excellent performance over the full operating temperature range. Built-In Driver Overload Protection As stated previously, the RS-485 specification requires that drivers survive worst case bus contentions undamaged. These transmitters 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 specification, even at the common mode voltage range extremes. In the event of a major short circuit condition, the device also includes 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 reenable after the die temperature drops about +15°C. If the contention persists, the thermal shutdown/reenable cycle repeats until the fault is cleared. Receivers stay operational during thermal shutdown. Low Power Shutdown Mode This BiCMOS transceiver uses a fraction of the power required by their bipolar counterparts, but it also includes a shutdown feature that reduces the already low quiescent ICC to a 50nA trickle. It enters 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 60ns guarantees that the transceiver will not enter shutdown. Note that receiver and driver enable times increase when the transceiver enables from shutdown. Refer to Notes 6, 7, 8, 9 and 10, at the end of the “Electrical Specification” table on page 6, for more information. FIGURE 7. HOT PLUG PERFORMANCE (ISL3179E) vs ISL83485 WITHOUT HOT PLUG CIRCUITRY TIME (40µs/DIV) VCC 2 4 2 4 RL = 1kΩ RO 0 2 4 0 0 A /Y RL = 1kΩ 2.3V 2.5V DE, DI = VCC ISL3179E RE = GND ISL3179E ISL3179E, ISL3180E |
Similar Part No. - ISL3179EIRZ |
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