ltc1535csw-trpbf Linear Technology Corporation, ltc1535csw-trpbf Datasheet - Page 10
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ltc1535csw-trpbf
Manufacturer Part Number
ltc1535csw-trpbf
Description
Isolated Rs485 Transceiver
Manufacturer
Linear Technology Corporation
Datasheet
1.LTC1535CSW-TRPBF.pdf
(16 pages)
APPLICATIO S I FOR ATIO
LTC1535
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 remain high enough
for 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 (see Figures 15, 16, 17, 18 for
typical waveforms). 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.
10
U
U
FAULT
FAULT
FAULT INDICATED WHEN RE IS THREE-STATED
W
POLL
POLL
RE
Figure 11. Detecting Fault Conditions
BUFFER
U
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. During a fault, the receiver output, RO, defaults
to a high state (see Table 2). Open circuit or short-circuit
conditions on the twisted pair do not cause a fault indica-
tion. 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 a bidirectional microcontroller I/O line or by using
the circuit in Figure 11, where the control to RE is three-
stated and the fault condition read back from the RE pin.
The weak drive has 100 A pull-up current to indicate a
fault and 50 A 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
communication.
RO
RE
DI
DE
FAULT
GND
V
V
CC
CC
LTC1535
1535 F11
1535fa