LTC1535 Linear Technology, LTC1535 Datasheet - Page 9

• Manufacturer Part Number: LTC1535
• Description: Isolated RS485 Transceiver
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
High Voltage Considerations
The LTC1535 eliminates ground loops on data communi-
cation 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 11 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
U
U
W
2
EQUIPMENT SAFETY GROUND
EARTH GROUND
FLOATING RS485 COMMON
LTC1535
Figure 11. Detecting Wiring Faults
GND2
U
2
2
R1*
470k
R2*
470k
A
Y
B
Z
C1***
10nF
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 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 resisitors are necessary to withstand high
voltage faults. Electrostatic spikes and transients can be
limited by the addition of capacitor C1 and discharged
through R1–R4.
*
** IRC WCR1210
*** PANASONIC ECQ-U2A103MV
2
IRC WCR1206
R3**
100k
R4**
100k
B1
CN2R (JKL)
TWISTED-PAIR
NETWORK
1535 F11
LTC1535
9