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Moteur de recherche de fiches techniques de composants électroniques |
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Moteur de recherche de fiches techniques de composants électroniques |
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ISL3179EFRZ Fiches technique(PDF) 9 Page - Intersil Corporation |
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ISL3179EFRZ Fiches technique(HTML) 9 Page - Intersil Corporation |
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9 / 17 page  9 FN6365.2 April 22, 2008 Application Information RS-485 and RS-422 are differential (balanced) data transmission standards for use in long haul or noisy environments. RS-422 is a subset of RS-485, so RS-485 transceivers are also RS-422 compliant. RS-422 is a point-to-multipoint (multidrop) standard, which allows only one driver and up to 10 (assuming one unit load devices) receivers on each bus. RS-485 is a true multipoint standard, which allows up to 32 one unit load devices (any mix of drivers and receivers) on each bus. To allow for multipoint operation, the RS-485 spec requires that drivers must handle bus contention without sustaining any damage. Another important advantage of RS-485 is the extended common mode range (CMR), which specifies that the driver outputs and receiver inputs withstand signals that range from +12V to -7V. RS-422 and RS-485 are intended for runs as long as 4000’ (~1200m), so the wide CMR is necessary to handle ground potential differences, as well as voltages induced in the cable by external fields. Receiver (Rx) Features This transceiver utilizes a differential input receiver for maximum noise immunity and common mode rejection. Input sensitivity is ±200mV, as required by the RS-422 and RS-485 specifications. Receiver inputs function with common mode voltages as great as +9/-7V outside the power supplies (i.e., +12V and -7V), making them ideal for long networks, or industrial environments, where induced voltages are a realistic concern. The receiver input resistance of 50k Ω surpasses the RS-422 specification of 4k Ω, and is 5x the RS-485 “Unit Load” (UL) requirement of 12k Ω minimum. Thus, the ISL3179E is known as a “one-fifth UL” transceiver, and there can be up to 160 devices on the RS-485 bus while still complying with the RS-485 loading specification. The receiver is a “Full Fail-Safe” version that guarantees a high level receiver output if the receiver inputs are unconnected (floating), shorted together, or connected to a terminated bus with all the transmitters disabled (terminated/undriven). Rx outputs deliver large low state currents (typically 28mA at VOL = 1V) to ease the design of optically coupled isolated networks. Receivers easily meet the 40Mbps data rate supported by the driver, and the receiver output is tri-statable via the active low RE input. Driver (Tx) Features The RS-485/RS-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. Outputs of the drivers are not slew rate limited, so faster output transition times allow data rates of at least 40Mbps. Driver outputs are tri-statable via the active high DE input. For parallel applications, bit-to-bit skews between any two transmitter and receiver pairs are guaranteed to be no worse than 8ns (4ns max for any two Tx, 4ns max for any two Rx). ESD Protection All pins on the ISL3179E and ISL3180E include class 3 (>9kV) 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 ±16.5kV HBM (ISL3179E) or ±12kV HBM (ISL3180E), and ±16.5kV (ISL3179E) or ±4kV (ISL3180E) IEC61000-4-2. The RS-485 pins are particularly vulnerable to ESD strikes 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 IEC61000 standard’s 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 the ISL3179E’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 more difficult to obtain repeatable results. The ISL3179E RS-485 pins withstand ±16.5kV air-gap discharges, while the ISL3180E’s RS-485 pins withstand ±4kV. 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 ISL3179E, ISL3180E |
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Description similaire - ISL3179EFRZ |
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