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ISL3173E Datasheet(PDF) 11 Page - Intersil Corporation |
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ISL3173E Datasheet(HTML) 11 Page - Intersil Corporation |
11 / 19 page 11 FN6307.5 June 8, 2012 Receivers easily meet the data rates supported by the corresponding driver, and all receiver outputs (except on the ISL3171E, ISL3174E and ISL3177E) are tri-statable via the active low RE input. Driver Features The RS-485/422 driver is a differential output device that delivers at least 1.5V across a 54 Ω load (RS-485) and at least 2V across a 100 Ω load (RS-422). The drivers feature low propagation delay skew to maximize bit width and to minimize EMI. All drivers are tri-statable via the active high DE input, except on the ISL3171E, ISL3174E and ISL3177E. The 250kbps and 500kbps driver outputs are slew rate limited to minimize EMI and to reduce reflections in unterminated or improperly terminated networks. Outputs of the ISL3176E through ISL3178E drivers are not limited, so faster output transition times allow data rates of at least 20Mbps. 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 ISL317XE versions with output enable pins incorporate a “Hot Plug” function. During power up, circuitry monitoring VCC ensures that the Tx and Rx outputs remain disabled for a period of time, regardless of the state of DE and RE. This gives the processor/ASIC a chance to stabilize and drive the RS-485 control lines to the proper states. ESD Protection All pins on these devices include class 3 (>7kV) Human Body Model (HBM) ESD protection structures, but the RS-485 pins (driver outputs and receiver inputs) incorporate advanced structures allowing them to survive ESD events in excess of ±15kV HBM and ±15kV IEC61000. The RS-485 pins are particularly vulnerable to ESD damage because they typically connect to an exposed port on the exterior of the finished product. Simply touching the port pins, or connecting a cable, can cause an ESD event that might destroy unprotected ICs. These new ESD structures protect the device whether or not it is powered up, and without degrading the RS-485 common mode range of -7V to +12V. This built-in ESD protection eliminates the need for board level protection structures (e.g., transient suppression diodes), and the associated, undesirable capacitive load they present. IEC61000-4-2 Testing The IEC61000 test method applies to finished equipment, rather than to an individual IC. Therefore, the pins most likely to suffer an ESD event are those that are exposed to the outside world (the RS-485 pins in this case), and the IC is tested in its typical application configuration (power applied) rather than testing each pin-to-pin combination. The lower current limiting resistor coupled with the larger charge storage capacitor yields a test that is much more severe than the HBM test. The extra ESD protection built into this device’s RS-485 pins allows the 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 difficult to obtain repeatable results. The ISL317XE 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 ±8kV. The ISL317XE survive ±8kV contact discharges on the RS-485 pins. Data Rate, Cables, and Terminations RS-485/422 are intended for network lengths up to 4000’, but the maximum system data rate decreases as the transmission length increases. Devices operating at 20Mbps are limited to lengths less than 100’, while the 250kbps versions can operate at full data rates with lengths of several thousand feet. Twisted pair is the cable of choice for RS-485/422 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. Proper termination is imperative, when using the 20Mbps devices to minimize reflections. 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. ISL3170E, ISL3171E, ISL3172E, ISL3173E, ISL3174E, ISL3175E,ISL3176E, ISL3177E, ISL3178E |
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