5

LTC486

Figure 1. Driver DC Test Load

Figure 3. Driver Timing Test Load #2

Figure 2. Driver Timing Test Circuit

DRIVER

LTC486 • TA03

EN

DI

A

B

EN

CI1

CI2

TEST CIRCUITS

GND (Pin 8): Ground Connection.

DI3 (Pin 9): Driver 3 Input. Refer to DI1.

DO3A (Pin 10): Driver 3 Output.

DO3B (Pin 11): Driver 3 Output.

EN (Pin 12): Driver Outputs Disabled. See Function Table

for details.

DO4B (Pin 13): Driver 4 Output.

DO4A (Pin 14): Driver 4 Output.

DI4 (Pin 15): Driver 4 Input. Refer to DI1.

DI1 (Pin 1): Driver 1 Input. If Driver 1 is enabled, then a low

on DI1 forces the driver outputs DO1A low and DO1B high.

A high on DI1 with the driver outputs enabled will force

DO1A high and DO1B low.

DO1A (Pin 2): Driver 1 Output.

DO1B (Pin 3): Driver 1 Output.

EN (Pin 4): Driver Outputs Enabled. See Function Table for

details.

DO2B (Pin 5): Driver 2 Output.

DO2A (Pin 6): Driver 2 Output.

DI2 (Pin 7): Driver 2 Input. Refer to DI1.

PI

FU C

S

TI

S

APPLICATI

I FOR ATIO

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 ends with a resistor equal to their characteristic

impedance, typically 120

Ω. The optional shields around

the twisted pair help reduce unwanted noise, and are

connected to GND at one end.

Thermal Shutdown

The LTC486 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 LTC486 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.

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 120

Ω

cables designed for RS485 applications.

LTC486 • TA02

A

B

R

R

OD

V

OC

V

LTC486 • TA04

OUTPUT

UNDER TEST

S1

500

CC

V

Ω

S2