MAX3157
High CMRR RS-485 Transceiver
with ±50V Isolation
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Detailed Description
The MAX3157 is a high CMRR RS-485/RS-422 data-
communications interface providing ±50V isolation in a
hybrid microcircuit. A single +5V supply on the logic
side powers both sides of the interface, with external
±50V capacitors transferring power from the logic side
to the isolated side (see Block Diagram). The MAX3157
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). The
MAX3157 transceiver for RS-485/RS-422 communica-
tion 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). The MAX3157 is selectable
between half- and full-duplex communication by con-
necting a selector pin to ISOVCC or ISOCOM, respec-
tively. Drivers are output short-circuit current limited.
Thermal shutdown circuitry protects drivers against
excessive power dissipation. When activated, the ther-
mal shutdown circuitry places the driver outputs into a
high-impedance state. The device also features inde-
pendently programmable receiver and transmitter out-
put phase via separate pins. The MAX3157 is a
low-cost replacement for opto-isolated transceivers.
Fail-Safe
The MAX3157 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 MAX3157, 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.
MAX3157 Programming
The MAX3157 has several programmable operating
modes. Occasionally, twisted-pair lines are reversed.
The MAX3157 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
connect it to V
CC
. To invert the receiver phase, drive
RXP high or connect it to V
CC
. Note that the receiver
threshold is positive when RXP is high.
The MAX3157 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
to enter shutdown. Enable times t_
ZH
and t_
ZL
in the
Switching Characteristics table assume the part was
not in a low-power shutdown state. Enable times
t_
ZH
(SHDN)
and t_
ZL
(SHDN)
assume the parts were shut
down. It takes drivers and receivers longer to become
enabled from low-power shutdown mode t_
ZH
(SHDN)
,
t_
ZL
(SHDN)
than from driver/receiver-disable mode
(t_
ZH
, t_
ZL
).
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
