Datasheet

9
LTC1690
APPLICATIONS INFORMATION
WUU
U
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
to V
CC
, the current will be limited to a maximum of 250mA.
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
120Ω
2
8
7
R
RECEIVER
DATA
OUT
DATA
IN
1690 F07
Figure 7. Line Repeater