7

LTC491

491fa

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

Ω. The input impedance of a receiver is

typically 20k

Ω 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

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

°Candturnsthembackonwhenthetemperaturecools

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.

APPLICATIO S I FOR ATIO

Figure 9. Typical Connection

Figure 10. Line Repeater

LTC491 • F09

120

Ω

120

Ω

LTC491

DRIVER

RECEIVER

LTC491

DRIVER

DX

RX

DX

RX

2

5

9

10

11

12

RECEIVER

9

10

11

12

5

2

120

Ω

3

4

3

4

120

Ω

RECEIVER

LTC491

DRIVER

9

10

11

12

5

4

3

2

DX

RX

LTC491 • F10

120

Ω

LTC491

DRIVER

DX

RX

2

5

9

10

11

12

RECEIVER

DATA IN

DATA OUT

3

4

120

Ω