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SM712 数据表(PDF) 4 Page - Semtech Corporation |
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SM712 数据表(HTML) 4 Page - Semtech Corporation |
4 / 6 page 4 2004 Semtech Corp. www.semtech.com PROTECTION PRODUCTS PROTECTION PRODUCTS SM712 RS-485 Common Mode Voltages Device Connection for Protection of Two RS-485 Data Lines EIA RS-485 specifies a ±7V ground difference between devices on the bus. This permits the bus voltage to range from +12V (5V + 7V) to -7V (0 - 7V). The SM712 is designed to protect two RS-485 data lines in extended common mode applications. The SM712 may be used to protect devices from transient voltages resulting from ESD, EFT, and lightning. The device is designed with asymmetrical operating volt- ages for optimum protection. The TVS diodes at pins 1 and 2 have a working voltage of 12 volts. These pins are connected to the differential data line pairs. The TVS diodes at pin 3 have a working voltage of 7 volts. Pin 3 is connected to ground. The internal TVS diodes of the SM712 will protect the transceiver input from positive transient voltage spikes greater than 12V and negative spikes greater than 7V. A series current limiting resistor may be added in applications requiring enhanced surge immunity. Circuit Board Layout Recommendations. Good circuit board layout is critical for the suppression of fast rise-time transients such as ESD. The following guidelines are recommended: Place the SM712 near the input terminals or connectors to restrict electromagnetic coupling. Minimize the path length between the SM712 and the protected line. This minimizes voltage over- shoot due to parasitic inductance of board traces. Use ground planes whenever possible. Long, single trace ground conductors should be avoided. The ground pin (Pin 3) should be con- nected directly to a ground plane on the circuit board for best results. Minimize all conductive loops including power and ground loops. Never run critical signals near board edges. RS-485 Protection Circuit Applications Information |
类似零件编号 - SM712 |
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类似说明 - SM712 |
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