MAX14852 Datasheet, PDF (16/22 Pages) Maxim Integrated Products – 2.75kVRMS Isolated 500kbps/25Mbps Full-Duplex

MAX14852/MAX14854
2.75kV_RMS Isolated 500kbps/25Mbps Full-Duplex
RS-485/RS-422 Transceivers
with ±35kV ESD Protection
Detailed Description
The MAX14852/MAX14854 isolated RS-485/RS-422
transceivers provide 5000VRMS (60s) of galvanic
isolation between the RS-485/RS-422 cable side of
the transceiver and the UART side. These devices
allow up to 500kbps (MAX14852)/25Mbps (MAX14854)
communication across an isolation barrier when a large
potential exists between grounds on each side of the barrier.
Isolation
Data isolation is achieved using high-voltage capacitors
that allow data transmission between the UART side and
the RS-485/RS-422 cable side of the transceiver.
Integrated LDO
The devices include an internal low-dropout regulator with
a set 3.3V (typ) output that is used to power the cable side
of the IC. The output of the LDO is V_DDB. The LDO has
a 300mA (typ) current limit. If the LDO is unused, connect
V_LDO to GNDB and apply +3.3V directly to V_DDB.
True Fail-Safe
The devices guarantee a logic-high on the receiver output
when the receiver inputs are shorted or open, or when
connected to a terminated transmission line with all
drivers disabled. The receiver threshold is fixed between
-10mV and -200mV. If the differential receiver input voltage
(V_A – V_B) is greater than or equal to -10mV, RXD is logic-
high. In the case of a terminated bus with all transmitters
disabled, the receiver’s differential input voltage is pulled
to zero by the termination resistors. Due to the receiver
thresholds of the devices, this results in a logic-high at
RXD.
Driver Output Protection
Two mechanisms prevent excessive output current and
power dissipation caused by faults or bus contention.
The first, a current limit on the output stage, provides
immediate protection against short circuits over the entire
common-mode voltage range. The second, a thermal-
shutdown circuit, forces the driver outputs into a
high-impedance state if the die temperature exceeds
+160°C (typ).
Thermal Shutdown
The devices are protected from overtemperature
damage by integrated thermal-shutdown circuitry. When the
junction temperature (T_J) exceeds +160°C (typ), the driver
outputs go high-impedance. The device resumes normal
operation when T_J falls below +145°C (typ).
Applications Information
128 Transceivers on the Bus
The standard RS-485 receiver input impedance is one unit
load. A standard driver can drive up to 32 unit-loads. The
MAX14852/MAX14854 transceivers have a 1/4-unit load
receiver, which allows up to 128 transceivers, connected
in parallel, on one communication line. Connect any
combination of these devices, and/or other RS-485
devices, for a maximum of 32 unit-loads to the line.
Typical Application
The MAX14852/MAX14854 full-duplex transceivers are
designed for bidirectional data communications on multi-
point bus transmission lines. Figure 9 and Figure 10 show
typical network application circuits. To minimize reflections,
the bus should be terminated at both ends in its characteristics
impedance, and stub lengths off the main line should be
kept as short as possible.
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