8

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 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 100

µA pull-up current to indicate

a fault and 50

µA 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.

Figure 10. Detecting Fault Conditions

RE

RE

POLL

FAULT

RO

DI

DE

GND

LTC1535

FAULT

FAULT INDICATED WHEN RE IS THREE-STATED

1535 F10

POLL

FAULT

BUFFER

APPLICATIO S I FOR ATIO