Transformer Driver for

Isolated RS-485 Interface

10

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* 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-

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

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