9

LTC1690

Fault Protection

When shorted to –7V or 10V at room temperature, the

short-circuit current in the driver outputs is limited by

internal resistance or protection circuitry to 250mA maxi-

mum. Over the industrial temperature range, the absolute

maximum positive voltage at any driver output should be

limited to 10V to avoid damage to the driver outputs. At

higher ambient temperatures, the rise in die temperature

due to the short-circuit current may trip the thermal

shutdown circuit.

The receiver inputs can withstand the entire –7V to 12V

RS485 common mode range without damage.

The LTC1690 includes a thermal shutdown circuit that

protects the part against prolonged shorts at the driver

outputs. If a driver output is shorted to another output or

If the die temperature rises above 150

°C, the thermal

shutdown circuit three-states the driver outputs to open

the current path. When the die cools down to about 130

°C,

the driver outputs are taken out of three-state. If the short

persists, the part will heat again and the cycle will repeat.

This thermal oscillation occurs at about 10Hz and protects

the part from excessive power dissipation. The average

fault current drops as the driver cycles between active and

three-state. When the short is removed, the part will return

to normal operation.

If the outputs of two or more LTC1690 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.

3

5

6

D

DRIVER

LTC1690

120

Ω

2

8

7

R

RECEIVER

DATA

OUT

DATA

IN

1690 F07

Figure 7. Line Repeater