LTC487



487fc

TesT circuiT

applicaTions inForMaTion

LTC487 TA02

A

B

R

R

OD

V

OC

V

DRIVER 1

LTC487 TA03

DI

A

B

EN12

LTC487 TA04

OUTPUT

UNDER TEST

S1

500

CC

V

S2

Figure 5. Driver Timing Test Load #2

Figure 4. Driver Timing Test Circuit

Figure 3. Driver DC Test Load

Typical Application

A typical connection of the LTC487 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 ends with a resistor equal to their characteristic im-

pedance, typically 120Ω. The optional shields around the

twistedpairhelpreduceunwantednoise,andareconnected

to GND at one end.

Thermal Shutdown

TheLTC487hasathermalshutdownfeaturewhichprotects

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 LTC487 drivers are shorted

directly, the driver outputs can not supply enough cur-

rent to activate the thermal shutdown. Thus, the thermal

shutdown 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.

EN12

4

LTC487 • TA07

120

DX

1

2

3

SHIELD

120

RX

RX

SHIELD

3

DX

EN12

4

2

EN12

4

1

2

3

RX

RX

3

DX

EN12

4

1

DX

1/4 LTC489

1/4 LTC487

1/4 LTC489

1/4 LTC487

2

1

Figure 6. Typical Connection