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ISL3283E Scheda tecnica(PDF) 9 Page - Intersil Corporation |
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ISL3283E Scheda tecnica(HTML) 9 Page - Intersil Corporation |
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9 / 15 page  9 FN6543.2 October 18, 2007 All the receivers include a “full fail-safe” function that guarantees a high level receiver output if the receiver inputs are unconnected (floating), shorted together, or connected to a terminated but undriven bus. Fail-safe with shorted inputs is achieved by setting the Rx upper switching point to -50mV, thereby ensuring that the Rx sees 0V differential as a high input level. All receivers easily support a 20Mbps data rate, and all receiver outputs (except on the ISL3280E and ISL3284E) are tri-statable via the active low RE input or by the active high RE input. Wide Supply Range The ISL3280E, ISL3281E, ISL3282E, ISL3283E, ISL3284E, ISL3285E are designed to operate with a wide range of supply voltages from 3.0V to 5.5V. These devices meet the RS-422 and RS-485 specifications over this full range. Logic Supply (VL Pin, ISL3282E, ISL3284E, ISL3285E Only) Note: Power-up VCC before powering up the VL supply. The ISL3282E, ISL3284E, and ISL3285E include a VL pin that powers the logic input (RE or RE) and / or the Rx output. These pins interface with “logic” devices such as UARTs, ASICs, and microcontrollers and today most of these devices use power supplies significantly lower than 3.3V. Thus, a 3.3V output level from a 3.3V powered RS-485 IC might seriously overdrive and damage the logic device input. Similarly, the logic device’s low VOH might not exceed the VIH of a 3.3V or 5V powered RE input. Connecting the VL pin to the power supply of the logic device (as shown in Figure 3) limits the ISL3282E, ISL3284E, ISL3285E’s Rx output VOH to VL (see Figures 6 through 10), and reduces the RE / RE input switching point to a value compatible with the logic device’s output levels. Tailoring the logic pin input switching point and output levels to the supply voltage of the UART, ASIC, or microcontroller 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 when VL < 1.6V. Table 2 indicates typical VIH, VIL, and data rate values for various VL settings so the user can ascertain whether or not a particular VL voltage meets his/her needs. The quiescent, RO unloaded, VL supply current (IL) is typically less than 60µA for VL ≤ 3.3V, as shown in Figure 5. ESD Protection All pins on these devices include class 3 (>4kV) Human Body Model (HBM) ESD protection structures, but the RS-485 pins (receiver inputs) incorporate advanced structures allowing them to survive ESD events in excess of ±16.5kV HBM and ±16.5kV 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. TABLE 2. VIH, VIL AND DATA RATE vs VL FOR VCC = 3.3V OR 5V VL (V) VIH (V) VIL (V) DATA RATE (Mbps) 1.35 0.55 0.5 11 1.6 0.7 0.6 16 1.8 0.8 0.7 23 2.3 1 0.9 27 2.7 1.1 1 30 3.3 1.3 1.2 30 5.5 (i.e., VCC)2 1.8 24 FIGURE 3. USING VL PIN TO ADJUST LOGIC LEVELS GND RXD RXEN VCC = +2V UART/PROCESSOR GND RO RE VCC = +3.3V ISL3283E VOH ≤ 2V VOH = 3.3V VIH ≥ 2V ESD DIODE GND RXD RXEN VCC = +2V UART/PROCESSOR GND RO RE VCC = +3.3V TO 5V ISL3282E VOH ≤ 2V VOH = 2V VIH = 1V ESD DIODE VL ISL3280E, ISL3281E, ISL3282E, ISL3283E, ISL3284E, ISL3285E |
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