Datasheet MAX253 (Maxim) - 10

MAX253 Transformer Driver for Isolated RS-485 Interface
Applications Information
appropriate Maxim interface device for data-transfer rates up to 2.5Mbps. Figures 3–5 are typical isolated RS-485/RS-232 data-inter- face circuits. These circuits withstand 1800V Refer to the MAX1480 data sheet for a complete isolat- RMS (1sec) and are intended for industrial communications and control ed RS-485 solution in one package. applications where very high voltage transients, differential
Isolated RS-232 Data Interface
ground potentials, or high noise may be encountered. The MAX253 is ideal for isolated RS-232 data-interface Table 2 lists transformer characteristics for the applica- applications requiring more than four transceivers. The tions of Figures 3–10. Some suggested manufacturers 1W power output capability of the MAX253 enables it to of transformers, transformer cores, and optocouplers drive more than 10 transceivers simultaneously. Figure 5 are listed in Table 3, along with their respective phone shows the typical application circuit for a complete and fax numbers. 120kbps isolated RS-232 data interface. The figure Important layout considerations include: also shows how the Sharp PC417 optocouplers can be o replaced by the lower-cost 4N25 devices to achieve For maximum isolation, the “isolation barrier” should not data-transfer rates up to 9.6kbps. be breached. Connections and components from one side should not be located near those of the other side. For 3.3V operation, substitute the primary portion of o Figure 5 with the circuit of Figure 7. Since the optocoupler outputs are relatively high- impedance nodes, they should be located as close For applications requiring two transceivers or fewer, as possible to the Maxim interface IC. This mini- refer to the MAX250/MAX251 or MAX252 data sheet. mizes stray capacitance and maximizes data rate.
Isolated Power Supplies
Refer to the µMAX package information for pin spacing The MAX253 is a versatile isolated power driver, capa- and physical dimensions. ble of driving a center-tapped transformer primary from a 5V or a 3.3V DC power supply (Figures 6 and 7). The
Isolated RS-485 Data Interface
secondary can be wound to provide any isolated volt- The MAX253 power-supply transformer driver is age needed at power levels up to 1W with a 5V supply, designed specifically for isolated RS-485 data-interface or 600mW with a 3.3V supply. Figure 6 shows a typical applications. The application circuits of Figures 3 and 4 5V to isolated 5V application circuit that delivers up to combine the MAX253 with a low-dropout linear regulator, 200mA of isolated 5V power. a transformer, several high-speed optocouplers, and a Maxim RS-485 interface device. With a few modifica- In Figure 7, the MAX253 is configured to operate from a tions to these circuits, full-duplex communications can 3.3V supply, deriving a “boost” VCC for the MAX253 by be implemented by substituting the MAX481/MAX485 connecting diodes to both ends of the transformer pri- with the MAX490/MAX491 (for data rates up to 2.5Mbps) mary. This produces nearly double the input supply, or substituting the MAX483/MAX487 with the and may be useful for other applications, as shown in MAX488/MAX489 (for data rates up to 250kbps). Figure 4. The average current in each MAX253 switch must still be limited to less than 200mA, so the total The data transfer rates of the application circuits in power available is approximately 600mW. Figures 3 and 4 are critically limited by the optocou- plers. Table 1 lists suggested optocouplers and the
Table 1. Optocouplers and RS-485 Interface ICs for Various Data Rates FULL DUPLEX HALF DUPLEX OPTOCOUPLER OPTOCOUPLER DATA RATE RS-485 IC RS-485 IC FOR DI / RO FOR DE
250kbps MAX488/MAX489 MAX483/MAX487 PC417* PC357T* 2.5Mbps MAX490/MAX491 MAX481/MAX485 PC410* PC357T * 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 www.maximintegrated.com Maxim Integrated | 10