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MAX253 Datasheet(PDF) 10 Page - Maxim Integrated Products |
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MAX253 Datasheet(HTML) 10 Page - Maxim Integrated Products |
10 / 18 page Transformer Driver for Isolated RS-485 Interface 10 ______________________________________________________________________________________ * PC-Series Optocouplers, Sharp Electronics USA Phone: (206) 834-2500 FAX: (206) 834-8903 Sharp Electronics, Europe GmbH Germany Phone: (040) 2376-0 FAX: (040) 230764 __________Applications Information Figures 3–5 are typical isolated RS-485/RS-232 data-inter- face circuits. These circuits withstand 1800VRMS (1sec) and are intended for industrial communications and control applications where very high voltage transients, differential ground potentials, or high noise may be encountered. Table 2 lists transformer characteristics for the applica- tions of Figures 3–10. Some suggested manufacturers of transformers, transformer cores, and optocouplers are listed in Table 3, along with their respective phone and fax numbers. Important layout considerations include: ♦♦ For maximum isolation, the “isolation barrier” should not be breached. Connections and components from one side should not be located near those of the other side. ♦♦ Since the optocoupler outputs are relatively high- impedance nodes, they should be located as close as possible to the Maxim interface IC. This mini- mizes stray capacitance and maximizes data rate. Refer to the µMAX package information for pin spacing and physical dimensions. Isolated RS-485 Data Interface The MAX253 power-supply transformer driver is designed specifically for isolated RS-485 data-interface applications. The application circuits of Figures 3 and 4 combine the MAX253 with a low-dropout linear regulator, a transformer, several high-speed optocouplers, and a Maxim RS-485 interface device. With a few modifica- tions to these circuits, full-duplex communications can be implemented by substituting the MAX481/MAX485 with the MAX490/MAX491 (for data rates up to 2.5Mbps) or substituting the MAX483/MAX487 with the MAX488/MAX489 (for data rates up to 250kbps). The data transfer rates of the application circuits in Figures 3 and 4 are critically limited by the optocou- plers. Table 1 lists suggested optocouplers and the appropriate Maxim interface device for data-transfer rates up to 2.5Mbps. Refer to the MAX1480 data sheet for a complete isolat- ed RS-485 solution in one package. Isolated RS-232 Data Interface The MAX253 is ideal for isolated RS-232 data-interface applications requiring more than four transceivers. The 1W power output capability of the MAX253 enables it to drive more than 10 transceivers simultaneously. Figure 5 shows the typical application circuit for a complete 120kbps isolated RS-232 data interface. The figure also shows how the Sharp PC417 optocouplers can be replaced by the lower-cost 4N25 devices to achieve data-transfer rates up to 9.6kbps. For 3.3V operation, substitute the primary portion of Figure 5 with the circuit of Figure 7. For applications requiring two transceivers or fewer, refer to the MAX250/MAX251 or MAX252 data sheet. Isolated Power Supplies The MAX253 is a versatile isolated power driver, capa- ble of driving a center-tapped transformer primary from a 5V or a 3.3V DC power supply (Figures 6 and 7). The secondary can be wound to provide any isolated volt- age needed at power levels up to 1W with a 5V supply, or 600mW with a 3.3V supply. Figure 6 shows a typical 5V to isolated 5V application circuit that delivers up to 200mA of isolated 5V power. In Figure 7, the MAX253 is configured to operate from a 3.3V supply, deriving a “boost” VCC for the MAX253 by connecting diodes to both ends of the transformer pri- mary. This produces nearly double the input supply, and may be useful for other applications, as shown in Figure 4. The average current in each MAX253 switch must still be limited to less than 200mA, so the total power available is approximately 600mW. Table 1. Optocouplers and RS-485 Interface ICs for Various Data Rates DATA RATE FULL DUPLEX RS-485 IC HALF DUPLEX RS-485 IC OPTOCOUPLER FOR DI / RO OPTOCOUPLER FOR DE 250kbps MAX488/MAX489 MAX483/MAX487 PC417* PC357T* 2.5Mbps MAX490/MAX491 MAX481/MAX485 PC410* PC357T |
Similar Part No. - MAX253_10 |
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