Datasheet

LTC2870/LTC2871
22
28701fb
For more information www.linear.com/LTC2870
Figure16. RS485 Receiver Input Threshold Characteristics
applicaTions inForMaTion
V
L
Logic Supply and Logic Pins
A separate logic supply pin V
L
allows the LTC2870 and
LTC2871 to interface with any logic signal from 1.7V to
5.5V. All logic I/Os use V
L
as their high supply. For proper
operation, V
L
should not be greater than V
CC
. During
power-up, if V
L
is higher than V
CC
, the device will not be
damaged, but behavior of the device is not guaranteed.
If V
L
is not connected to V
CC
, bypass V
L
with a 0.1µF
capacitor to GND.
RS232 and RS485 driver outputs are undriven and the
RS485 termination resistors are disabled when V
L
or V
CC
is grounded or V
CC
is disconnected.
Although all logic input pins reference V
L
as their high
supply, they can be driven up to 7V, independent of V
L
and
V
CC
, with the exception of FEN, which must not exceed V
L
by more than 1V for proper operation. Logic input pins
do not have internal biasing devices to pull them up or
down. They must be driven high or low to establish valid
logic levels; do not float.
RS485 Driver
The RS485 driver provides full RS485/RS422 compat-
ibility. When enabled, if DI is high, Y – Z is positive. With
the driver disabled the Y and Z output resistance is greater
than 96kΩ (typically 125kΩ) to ground over the entire
common mode range of –7V to +12V. This resistance is
equivalent to the input resistance on these lines when the
driver is configured in half-duplex mode and Y and Z act
as the RS485 receiver inputs.
Driver Overvoltage and Overcurrent Protection
The RS232 and RS485 driver outputs are protected from
short circuits to any voltage within the absolute maximum
range ±15V. The maximum current in this condition is
90mA for the RS232 driver and 250mA for the RS485
driver.
If the RS485 driver output is shorted to a voltage greater
than V
CC
, when it is active, positive current of up to 150mA
may flow from the driver output back to V
CC
. If the system
power supply or loading cannot sink this excess current,
clamp V
CC
to GND with a Zener diode (e.g., 5.6V, 1W,
1N4734) to prevent an overvoltage condition on V
CC
.
All devices also feature thermal shutdown protection that
disables the drivers, receivers, and RS485 terminators in
case of excessive power dissipation (see Note 6).
RS485 Balanced Receiver with Full Failsafe Operation
The LTC2870 and LTC2871 receivers use a window com
-
parator with two voltage thresholds centered around zero
for low pulse width distortion. As illustrated in Figure16,
for a differential signal approaching from a negative direc-
tion, the threshold is typically +110mV. When approach-
ing from the positive direction, the threshold is typically
–110mV. Each of these thresholds has about 35mV of
hysteresis (not shown in the figure). The state of RO
reflects the polarity of A–B in full-duplex mode or Y–Z in
half-duplex mode.
28701 F15
RECEIVER
OUTPUT LOW
–200mV –110mV 0V
RO
110mV 200mV
V
AB
RECEIVER
OUTPUT HIGH
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