Datasheet

LTC1535
13
1535fc
For more information www.linear.com/LTC1535
applicaTions inForMaTion
High Voltage Considerations
The LTC1535 eliminates ground loops on data commun-
ication lines. However, such isolation can bring potentially
dangerous voltages onto the circuit board. An example
would be accidental faulting to 117V AC at some point
on the cable which is then conducted to the PC board.
Figure 12 shows how to detect and warn the user or
installer that a voltage fault condition exists on the twisted
pair or its shield. A small (3.2mm) glow lamp is connected
between GND2 (the isolated ground) and the equipment’s
safety “earth” ground. If a potential of more than 75V
AC is present on the twisted pair or shield, B1 will light,
indicating a wiring fault. Resistors R3 and R4 are used
to ballast the current in B1. Two resistors are necessary
because they can only stand off 200V each, as well as for
power dissipation. As shown, the circuit can withstand a
direct fault to a 440V 3-phase system.
Other problems introduced by floating the twisted pair
include the collection of static charge on the twisted
pair, its shield and the attached circuitry. Resistors R1
and R2 provide a path to shunt static charge safely to
ground. Again, two resistors are necessary to withstand
high voltage faults. Electrostatic spikes, electromagneti-
cally induced transients and radio frequency pickup are
shunted by addition capacitor
C1.
Receiver Inputs Fail-Safe
The LTC1535 features an input common mode range
covering the entire RS485 specified range of –7V to 12V.
Differential signals of greater than ± 200mV within the
specified input common mode range will be converted
to TTL compatible signals at the receiver outputs, RO
and RO2. A small amount of input hysteresis is included
to minimize the effects of noise on the line signals. If
the receiver inputs are floating or shorted, a designed-
in receiver offset guarantees a fail-safe logic high at the
receiver outputs. If a fail-safe logic low is desired, connect
as shown in Figure 19.
TWISTED-PAIR
NETWORK
2
EQUIPMENT SAFETY GROUND
EARTH GROUND
2
2
FLOATING RS485 COMMON
* IRC WCR1206
** IRC WCR1210
*** PANASONIC ECQ-U2A103MV
B1
CN2R (JKL)
1535 F12
R1*
470k
R2*
470k
R3**
100k
R4**
100k
C1***
10nF
2
Y
A
B
Z
GND2
LTC1535
Figure 12. Detecting Fault Conditions
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