High CMRR RS-485 Transceiver

with ±60V Isolation

10

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Detailed Description

The MAX3158 is a high CMRR RS-485/RS-422 data-

communications interface providing ±60V isolation in a

hybrid microcircuit. A single +5V supply on the logic

side powers both sides of the interface, with external

100V capacitors transferring power from the logic side

to the isolated side (see Block Diagram). The MAX3158

typically draws 25mA of supply current when unloaded

or when fully loaded with the driver disabled. Supply

current drops to 25µA when the device is placed in

shutdown mode (see Low-Power Shutdown Mode sec-

tion). The MAX3158 transceiver for RS-485/RS-422

communication contains one driver and one receiver.

This device features fail-safe circuitry, which guaran-

tees a logic-high receiver output when the receiver

inputs are open or shorted, or when they are connected

to a terminated transmission line with all drivers dis-

abled (see Fail-Safe section). The MAX3158 is selec-

table between half- and full-duplex communication by

connecting a selector pin to ISOVCC or ISOCOM,

respectively. Drivers are output short-circuit current lim-

ited. Thermal shutdown circuitry protects drivers

against excessive power dissipation. When activated,

the thermal shutdown circuitry places the driver outputs

into a high-impedance state. The device also features

independently programmable receiver and transmitter

output phase through separate pins. The MAX3158 is a

low-cost replacement for opto-isolated transceivers.

Fail-Safe

The MAX3158 guarantees a logic-high receiver output

when the receiver inputs are shorted or open, or when

they are connected to a terminated transmission line with

all drivers disabled. The receiver threshold is fixed

between -50mV and -200mV. If the differential receiver

input voltage (A - B) is greater than or equal to -50mV,

RO is logic high. If A - B is less than or equal to -200mV,

RO is logic low. In the case of a terminated bus with all

transmitters disabled, the receiver’s differential input volt-

age is pulled to 0V by the termination. With the receiver

thresholds of the MAX3158, this results in a logic high

with a 50mV minimum noise margin. Unlike competitor’s

fail-safe devices, the -50mV to -200mV threshold com-

plies with the ±200mV EIA/TIA-485 standard.

MAX3158 Programming

The MAX3158 has several programmable operating

modes. Occasionally, twisted-pair lines are reversed.

The MAX3158 has two pins that invert the phase of the

driver and the receiver to correct for this problem. For

normal operation, drive TXP and RXP low, connect

them to ground, or leave them unconnected (internal

pulldown). To invert the driver phase, drive TXP high or

threshold is positive when RXP is high.

The MAX3158 can operate in full- or half-duplex mode.

Connect H/F to ISOCOM for full-duplex mode, and con-

nect it to ISOVCC for half-duplex operation. In half-

duplex mode, the receiver inputs are switched to the

driver outputs, connecting outputs Y and Z to inputs A

and B, respectively. In half-duplex mode, the internal

full-duplex receiver input resistors are still connected to

pins A and B.

Low-Power Shutdown Mode

The low-power shutdown mode is initiated by bringing

both RE high and DE low. In shutdown, this device typi-

cally draws only 25µA of supply current, and no power

is transferred across the isolation capacitors in this

mode. RE and DE may be driven simultaneously; the

parts are guaranteed not to enter shutdown if RE is

high and DE is low for less than 50ns. If the inputs are

in this state for at least 700ns, the parts are guaranteed

Switching Characteristics table assume the part was

not in a low-power shutdown state. Enable times

down. It takes drivers and receivers longer to become

Driver Output Protection

Two mechanisms prevent excessive output current and

power dissipation caused by faults or by bus con-

tention. The first, a foldback current limit on the output

stage, provides immediate protection against short cir-

cuits over the whole common-mode voltage range (see

Typical Operating Characteristics). The second, a ther-

mal shutdown circuit, forces the driver outputs into a

high-impedance state if the die temperature becomes

excessive—typically around +150°C.

Applications Information

Capacitor and Grounding Resistor

Selection

The value for the charge-pump capacitors C1 and C2

should be between 47nF and 100nF. Smaller values will

result in insufficient supply voltage on the isolated side.

Larger values are allowed but will not result in better

charge-pump capacity. The values for C1 and C2, as

well as that of C4, determine the maximum frequency

and amplitude of the voltage difference (under operat-

ing conditions) between the local and isolated ground.

Besides the capacitor values, this maximum frequency