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ISL3159EFRZ Datasheet(PDF) 10 Page - Intersil Corporation |
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ISL3159EFRZ Datasheet(HTML) 10 Page - Intersil Corporation |
10 / 16 page 10 FN6364.0 July 26, 2007 design of equipment meeting level 4 criteria without the need for additional board level protection on the RS-485 port. AIR-GAP DISCHARGE TEST METHOD For this test method, a charged probe tip moves toward the IC pin until the voltage arcs to it. The current waveform delivered to the IC pin depends on approach speed, humidity, temperature, etc., so it is more difficult to obtain repeatable results. The ISL3159E RS-485 pins withstand ±15kV air-gap discharges. CONTACT DISCHARGE TEST METHOD During the contact discharge test, the probe contacts the tested pin before the probe tip is energized, thereby eliminating the variables associated with the air-gap discharge. The result is a more repeatable and predictable test, but equipment limits prevent testing devices at voltages higher than ±9kV. The RS-485 pins of the ISL3159E survive ±8kV contact discharges. 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 ISL3159E incorporates 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 ~3.2V. 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 Twisted pair is the cable of choice for RS-485, RS-422, and PROFIBUS 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. According to guidelines in the RS-422 and PROFIBUS specifications, networks operating at data rates in excess of 3Mbps should be limited to cable lengths of 100m (328 ft) or less, and the PROFIBUS specification recommends that the more expensive “Type A” (22AWG) cable be used. The ISL3159E’s large differential output swing, fast transition times, and high drive-current output stages allow operation even at 40Mbps over standard “CAT5” cables in excess of 100m (328 ft). Figure 17 details the ISL3159E performance at this condition, with a 120 Ω termination resistor at both the driver and the receiver ends. Note that the differential signal delivered to the receiver at the end of the cable (A-B) still exceeds 1V, so even longer cables could be driven if lower noise margins are acceptable. Of course, jitter or some other criteria may limit the network to shorter cable lengths than those discussed here. If more noise margin is desired, shorter cables produce a larger receiver input signal as illustrated in Figure 16. Performance should be even better if the “Type A” cable is utilized. The ISL3159E 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. To minimize reflections, proper termination is imperative when using this high data rate transceiver. 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 for “CAT5”, and 220Ω for “Type A”) 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 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 FIGURE 7. HOT PLUG PERFORMANCE (ISL3159E) vs ISL83088E WITHOUT HOT PLUG CIRCUITRY TIME (40 μs/DIV) VCC 2.5 5.0 2.5 5.0 RL = 1kΩ RO 0 2.5 5.0 0 0 A /Y RL = 1kΩ 3.1V 3.3V DE, DI = VCC ISL3159E ISL3159E RE = GND ISL3159E |
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