Transformer Driver for
Isolated RS-485 Interface
__________Applications Information
Figures 3–5 are typical isolated RS-485/RS-232 data-inter-
face circuits. These circuits withstand 1800V
RMS
(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.
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.
MAX253
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 (see Figures 6 and 7).
The secondary can be wound to provide any isolated
voltage needed at power levels up to 1W with a 5V sup-
ply, 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” V
CC
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.
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
Table 1. Optocouplers and RS-485 Interface ICs for Various Data Rates
DATA RATE
250kbps
2.5Mbps
FULL DUPLEX
RS-485 IC
MAX488/MAX489
MAX490/MAX491
HALF DUPLEX
RS-485 IC
MAX483/MAX487
MAX481/MAX485
OPTOCOUPLER
FOR DI / RO
PC417*
PC410*
OPTOCOUPLER
FOR DE
PC357T*
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
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