LTC491IS Linear Technology, LTC491IS Datasheet - Page 7

• Manufacturer Part Number: LTC491IS
• Description: IC TXRX RS485 LOWPWR DIFF 14SOIC
• Manufacturer: Linear Technology
• Type: Transceiverr

Specifications of LTC491IS

• Number Of Drivers/receivers: 1/1
• Protocol: RS422, RS485
• Voltage - Supply: 4.75 V ~ 5.25 V
• Mounting Type: Surface Mount
• Package / Case: 14-SOIC (3.9mm Width), 14-SOL
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

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 120 . The input impedance of a receiver is
typically 20k
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
RX
DX
2
3
4
5
to GND, or 0.6 unit RS-485 load, so in
DRIVER
U
RECEIVER
LTC491
U
12
11
10
9
W
120
120
LTC491
U
Figure 9. Typical Connection
DRIVER
9
DX
5
10
4
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.
RECEIVER
11
3 2
RX
DX
RX
12
2
3
4
5
LTC491 • F09
DRIVER
RECEIVER
LTC491
Figure 10. Line Repeater
120
120
12
11
10
9
12
11
10
9
LTC491 • F10
RECEIVER
120
120
LTC491
DRIVER
2
3
4
5
LTC491
DX
RX
DATA OUT
DATA IN
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