LTC486

8

486fc

Typical Application

A typical connection of the LTC486 is shown in Figure 6.

A twisted pair of wires connect up to 32 drivers and

receivers for half 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

endswitharesistorequaltotheircharacteristicimpedance,

typically 120Ω. The optional shields around the twisted

pair help reduce unwanted noise, and are connected to

GND at one end.

Thermal Shutdown

TheLTC486hasathermalshutdownfeaturewhichprotects

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 LTC486 drivers are shorted

directly, the driver outputs cannot supply enough current

to activate the thermal shutdown. Thus, the thermal shut-

down circuit will not prevent contention faults when two

drivers are active on the bus at the same time.

Cable 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

less flexible, more bulky, and more costly than twisted

pairs. Many cable manufacturers offer a broad range of

120Ω cables designed for RS485 applications.

Lossesinatransmissionlineareacomplexcombinationof

DCconductorloss,AClosses(skineffect),leakage,andAC

losses in the dielectric. In good polyethylene cables such

as the Belden 9841, the conductor losses and dielectric

losses are of the same order of magnitude, with relatively

low overall loss (Figure 7).

APPLICATIONS INFORMATION

Figure 7. Attenuation vs Frequency for Belden 9841

Figure 6. Typical Connection

EN

12

EN

4

486 F06

120

DX

1

2

3

SHIELD

120

RX

RX

SHIELD

3

DX

EN

12

EN

4

2

EN

EN

4

1

2

3

RX

RX

3

DX

EN

12

EN

4

1

DX

1/4 LTC488

1/4 LTC486

1/4 LTC488

1/4 LTC486

2

1

12

FREQUENCY (MHz)

0.1

0.1

1

10

1

10

100

486 F07