MAX14945 Datasheet, PDF (15/21 Pages) Maxim Integrated Products – 2.75kVRMS Isolated 500kbps Half-Duplex RS-485

MAX14945
2.75kV_RMS Isolated 500kbps Half-Duplex RS-485/
RS-422 Transceiver with ±30kV ESD Protection
Detailed Description
The MAX14945 isolated RS-485/RS-422 transceiver provides
2750V_RMS (60s) of galvanic isolation between the RS-485/
RS-422 cable side of the transceiver and the UART side. This
device allows up to 500kbps communication across an isolation
barrier when a large potential exists between grounds on each
side of the barrier.
Isolation
Data isolation is achieved using integrated capacitive iso-
lation that allows data transmission between the UART-
side and the cabel-side of the transceiver.
Integrated LDO
The device includes an internal low-dropout regulator with
a set 5V (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 +5V directly to V_DDB.
True Fail-Safe
The device guarantees 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
-50mV and -200mV. If the differential receiver input
voltage (V_A – V_B) is greater than or equal to -50mV,
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 MAX14945, 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 foldback 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 device is protected from overtemperature damage
by integrated thermal shutdown circuitry. When the junc-
tion 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
256 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 MAX14945 transceiver has a 1/8-unit load receiver,
which allows up to 256 transceivers, connected in parallel,
on one communication line. Connect any combination of
these devices, and/or other RS-485 devices, for a maxi-
mum of 32 unit-loads to the line.
Typical Application
The MAX14945 transceiver is designed for bidirectional
data communications on multipoint bus transmission lines.
Figure 9 shows a typical network application circuit. 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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