MAX13448E Datasheet, PDF (11/14 Pages) Maxim Integrated Products – 80V Fault-Protected Full-Duplex RS-485 Transceiver

±80V Fault-Protected Full-Duplex
RS-485 Transceiver
Detailed Description
The MAX13448E ±80V fault-protected RS-485/RS-422
transceiver contains one driver and one receiver. This
device features fail-safe circuitry, guaranteeing 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 disabled. The device
has a hot-swap input structure that prevents distur-
bances on the differential signal lines when a circuit
board is plugged into a hot backplane. All receiver
inputs and driver outputs are protected to ±8kV ESD
using the Human Body Model. The MAX13448E
features a reduced slew-rate driver that minimizes
EMI and reduces reflections caused by improperly
terminated cables, allowing error-free data transmis-
sion up to 500kbps.
Driver
The driver accepts a single-ended, logic-level input
(DI) and converts it to a differential, RS-485/RS-422
level output (A and B). Deasserting the driver enable
places the driver outputs (A and B) into a high-imped-
ance state.
Receiver
The receiver accepts a differential, RS-485/RS-422
level input (A and B), and translates it to a single-
ended, logic-level output (RO). Deasserting the receiv-
er enable places the receiver outputs (RO) into a
high-impedance state (see Table 1).
Low-Power Shutdown
Low-power shutdown is initiated by bringing DE low
and RE high. In shutdown, the device draws a maxi-
mum of 100µA of supply current.
The device is guaranteed to not enter shutdown if DE is
low and RE is high for 1µs. If the inputs are in this state
for at least 1ms, the device is guaranteed to enter shut-
down. In the shutdown state, the driver outputs (A and
B) as well as the receiver output (RO) are in a high-
impedance state.
±80V Fault Protection
In certain applications, such as industrial control, driver
outputs and receiver inputs of an RS-485 device some-
times experience common-mode voltages in excess of
the -7V to +12V range specified in the EIA/TIA-485
standard. In these applications, ordinary RS-485
devices (typical absolute maximum ratings of -8V to
+12.5V) may experience damage without the addition
of external protection devices.
Table 1. Function Table
TRANSMITTING
INPUT
RE
DE
DI
OUTPUT
Z
Y
X
1
0
1
0
X
1
1
0
1
0
0
X
High
High
Impedance Impedance
1
0
X
High Impedance (Shutdown)
RECEIVING
INPUT
RE
DE
A-B
OUTPUT
RO
0
X
-50mV
1
0
X
-200mV
0
1
1
X
Disabled
1
0
X
High Impedance (Shutdown)
X = Don’t care; shutdown mode, driver, and receiver outputs are
high impedance.
To reduce system complexity and the need for external
protection, the driver outputs and receiver inputs of the
MAX13448E withstand voltage faults of up to ±80V with
respect to ground without damage (see the Absolute
Maximum Ratings section, Note 1). Protection is guar-
anteed regardless of whether the device is active, in
shutdown, or without power. Certain parasitic effects
present while driving an unterminated cable may cause
the voltage seen at driver outputs to exceed the
absolute maximum limit, while the DI input is switched
during a ±80V fault on the A or B input. Therefore, a
termination resistor is recommend in order to maximize
the overvoltage fault protection while the DI input is
being switched. If the DI input does not change state
while the fault voltage is present, the MAX13448E will
withstand up the ±80V on the RS-485 inputs, regard-
less of the presence of a termination resistor. While the
MAX13448E is not damaged by up to ±80V common-
mode voltages, the RO, Y, and Z outputs will be in an
indeterminate state if the common-mode voltage
exceeds -7V to +12V.
True Fail-Safe
The MAX13448E guarantees a logic-high receiver out-
put when the receiver inputs are shorted or open, or
when they are connected to a terminated transmission
line with all drivers disabled. This is done by setting the
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