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ISL3298EMRTEP-TK Scheda tecnica(PDF) 5 Page - Intersil Corporation |
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ISL3298EMRTEP-TK Scheda tecnica(HTML) 5 Page - Intersil Corporation |
5 / 9 page ISL3298EMRTEP 5 FN7607.1 July 28, 2011 Applications 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 transmitters and receivers 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 combination of drivers and receivers) on each bus. To allow for multipoint operation, the RS-485 specification 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’, so the wide CMR is necessary to handle ground potential differences, as well as voltages induced in the cable by external fields. Driver Features This 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. The driver is tri-statable via the active high DE input. If the Tx enable function isn’t needed, tie DE to VCC (or VL) through a 1kΩ to 3k Ω resistor. The ISL3298EMRTEP’s output transition times allow data rates of at least 16Mbps. Wide Supply Range The ISL3298EMRTEP is optimized for 3.3V operation, but can be operated with supply voltages as high as 5.5V. This device meets the RS-422 and RS-485 specifications for supply voltages less than 4V, and is RS-422 and RS-485 compatible for supplies greater than 4V. Operation at +125°C requires VCC ≤ 3.6V, while 5V operation requires adding output current limiting resistors (as described in the “Driver Overload Protection” on page 6) if output short circuits (e.g., from bus contention) are a possibility. 5.5V Tolerant Logic Pins Logic input pins (DI, DE) contain no ESD nor parasitic diodes to VCC (nor to VL), so they withstand input voltages exceeding 5.5V regardless of the VCC and VL voltages (see Figure 5). Logic Supply (VL Pin) Note: Power-up VCC before powering up the VL supply. The ISL3298EMRTEP includes a VL pin that powers the logic inputs (DI and DE). These pins interface with “logic” devices such as UARTs, ASICs, and µcontrollers, and today most of these devices use power supplies significantly lower than 3.3V. Thus, the logic device’s low VOH might not exceed the VIH of a 3.3V or 5V powered DI or DE input. Connecting the VL pin to the power supply of the logic device (as shown in Figure 5) reduces the DI and DE input switching points to values compatible with the logic device’s output levels. Tailoring the logic pin input switching points and output levels to the supply voltage of the UART, ASIC, or µcontroller eliminates the need for a level shifter/translator between the two ICs. VL can be anywhere from VCC down to 1.35V, but the input switching points may not provide enough noise margin, and 16Mbps data rates may not be achievable, when VL <1.5V. The E.C. table in the SMD indicates typical VIH and VIL values for various VL settings so the user can ascertain whether or not a particular VL voltage meets his/her needs. The VL supply current (IL) is typically much less than 20µA, as shown in Figure 9, when DE and DI are above/below VIH/VIL. GND TXD DEN VCC = +2V UART/PROCESSOR GND DI DE VCC = +3.3V ISL3293E VOH ≤ 2V VIH ≥ 2V GND TXD DEN VCC = +2V UART/PROCESSOR GND DI DE VCC = +3.3V ISL3296E VOH ≤ 2V VIH = 1.4V VIH = 1.4V VL VOH ≤ 2V VIH ≥ 2V VOH ≤ 2V FIGURE 5. USING VL PIN TO ADJUST LOGIC LEVELS |
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