LTC490

6

490fb

Typical Application

A typical connection of the LTC490 is shown in Figure 5.

Two twisted-pair wires connect two driver/receiver pairs

for full duplex data transmission. Note that the driver and

receiver outputs are always enabled. If the outputs must

be disabled, use the LTC491.

There are no restrictions on where the chips are con-

nected, and it isn’t necessary to have the chips connected

at the ends of the wire. However, the wires must be

terminated only at the ends with a resistor equal to their

characteristic impedance, typically 120

Ω. Because only

one driver can be connected on the bus, the cable can be

terminated only at the receiving end. The optional shields

around the twisted pair help reduce unwanted noise, and

are connected to GND at one end.

The LTC490 can also be used as a line repeater as shown

in Figure 6. If the cable length is longer than 4000 feet, the

LTC490 is inserted in the middle of the cable with the

receiver output connected back to the driver input.

Thermal Shutdown

The LTC490 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

°C and turns them back on when the temperature

cools to 130

°C. If the outputs of two or more LTC490

drivers are shorted directly, the driver outputs can not

supply enough current to activate the thermal shutdown.

Thus, the thermal shutdown circuit will not prevent con-

tention faults when two drivers are active on the bus at the

same time.

Cables and Data Rate

The transmission line of choice for RS485 applications is

a twisted pair. There are coaxial cables (twinaxial) made

for this purpose that contain straight pairs, but these are

Figure 5. Typical Connection

Figure 6. Line Repeater

APPLICATIO S I FOR ATIO

LTC490 • F05

120

Ω

120

Ω

SHIELD

LTC490

DRIVER

RECEIVER

LTC490

DRIVER

DX

RX

DX

RX

2

3

5

6

7

8

RECEIVER

0.01

µF

1

4

5V

SHIELD

8

7

6

5

0.01

µF

5V

4

2

3

1

+

+

LTC490 • F06

120

Ω

LTC490

DRIVER

DX

RX

2

3

5

6

7

8

RECEIVER

DATA IN

DATA OUT