LTC1535 Datasheet, PDF (8/12 Pages) Linear Technology – Isolated RS485 Transceiver

LTC1535
APPLICATIO S I FOR ATIO
Driver Output and Slow Slew Rate Control
The LTC1535 uses a proprietary driver output stage that
allows a common mode voltage range that extends be-
yond the power supplies. Thus, the high impedance state
is maintained over the full RS485 common mode range.
The output stage provides 100mA of short-circuit current
limiting in both the positive and negative directions. Thus,
even under short-circuit conditions, the supply voltage
from the open-loop DC converter will not be pulled too low
to prevent proper communication across the isolation
barrier. The driver output will be disabled in the event of a
thermal shutdown and a fault condition will be indicated
through the RE weak output.
The CMOS level SLO pin selects slow or fast slew rates on
the RS485 driver output. The SLO input has an internal
100k pull-up resistor. When SLO is low, the driver outputs
are slew rate limited to reduce high frequency edges. Left
open or tied high, SLO defaults to fast edges. The part
draws more current during slow slew rate edges.
Monitoring Faults on RE
The RE pin can be used to monitor the following fault
conditions: low supply voltages, thermal shutdown or a
time-out fault when there is no data communication across
the barrier. Open circuit or short-circuit conditions on the
twisted pair do not cause a fault indication. However, the
RS485 receiver defaults to a high output state when the
receiver input is open or short-circuited.
The RE pin has a weak current drive output mode for
indicating fault conditions. This fault state can be polled
using the circuit in Figure 10 where the control to RE is
three-stated and the fault condition read back from the RE
pin. The weak drive has 100µA pull-up current to indicate
a fault and 50µA pull-down current for no fault. This allows
the RE pin to be polled without disabling RE on nonfault
conditions.
Both sides contain a low voltage detect circuit. A voltage
less than 4.2V on the isolated side disables communica-
tion.
RE
POLL
FAULT
BUFFER
V_CC
RO
V_CC
RE
LTC1535
DI
FAULT
DE
GND
POLL
FAULT
FAULT INDICATED WHEN RE IS THREE-STATED
1535 F10
Figure 10. Detecting Fault Conditions
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