MAX14948 Datasheet, PDF (15/21 Pages) Maxim Integrated Products – 5kVRMS Isolated 500kbps Half-Duplex RS-485

MAX14948
5kV_RMS Isolated 500kbps Half-Duplex RS-485/
RS-422 Transceiver with ±30kV ESD Protection
Detailed Description
The MAX14948 isolated RS-485/RS-422 transceiver
provides 5000V_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
isolation that allows data transmission between the UART
side and the cable 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 device, this
results in a logic-high at RXD.
Driver Output Protection
Two mechanisms prevent excessive output current and
power dissipation caused by faults or by bus contention.
The first, a foldback mode 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 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
256 Transceivers on the Bus
The standard RS-485 receiver input impedance is one
unit load, and a standard driver can drive up to 32 unit
loads. The device 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/RS-422 devices, for a
maximum of 32 unit loads to the line.
Typical Application
The transceiver is designed for bidirectional data
communications on multipoint bus-transmission lines.
Figure 9 shows typical network application circuits. To mini-
mize 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.
Layout Considerations
It is recommended to design an isolation or keep out channel
underneath the isolator that is free from ground and signal
planes. Any galvanic or metallic connection between the
cable side and UART side defeats the isolation.
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