LTC491
APPLICATIO S I FOR ATIO
Typical Application
A typical connection of the LTC491 is shown in Figure 9.
Two twisted-pair wires connect up to 32 driver/receiver
pairs for full duplex data transmission. There are no
restrictions on where the chips are connected to the wires,
and it isn’t necessary to have the chips connected at the
ends. However, the wires must be terminated only at the
ends with a resistor equal to their characteristic imped-
ance, typically 120Ω. The input impedance of a receiver is
typically 20kΩ to GND, or 0.6 unit RS-485 load, so in
practice 50 to 60 transceivers can be connected to the
same wires. The optional shields around the twisted pair
help reduce unwanted noise, and are connected to GND at
one end.
The LTC491 can also be used as a line repeater as shown
in Figure 10. If the cable length is longer than 4000 feet, the
LTC491 is inserted in the middle of the cable with the
receiver output connected back to the driver input.
Thermal Shutdown
The LTC491 has a thermal shutdown feature which pro-
tects the part from excessive power dissipation. If the
12
RX
2
3
4
10
DX
5
DRIVER
120Ω
9
120Ω
RECEIVER
120Ω
11
120Ω
LTC491
LTC491
U
outputs of the driver are accidently shorted to a power
supply or low impedance source, up to 250mA can flow
through the part. The thermal shutdown circuit disables
the driver outputs when the internal temperature reaches
150°C and turns them back on when the temperature cools
to 130°C. If the outputs of two or more LTC491 drivers are
shorted directly, the driver outputs can not supply enough
current to activate the thermal shutdown. Thus, the ther-
mal shutdown circuit will not prevent contention faults
when two drivers are active on the bus at the same time.
12
RX
2
3
4
10
DX
5
DRIVER
120Ω
9
DATA OUT
RECEIVER
120Ω
11
DATA IN
LTC491
LTC491 • F10
W
U U
Figure 10. Line Repeater
12
11
RECEIVER
2
3
4
10
9
DRIVER
5
DX
RX
9
10
11
12
LTC491
RECEIVER
DRIVER
LTC491 • F09
5
DX
4
3
2
RX
Figure 9. Typical Connection
491fa
7
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