Datasheet

9
LTC1685
Output Short-Circuit Protection
The LTC1685 employs voltage sensing short-circuit pro-
tection at the output terminals of both the driver and
receiver. For a given input polarity, this circuitry deter-
mines what the correct output level should be. If the output
level is different from the expected, it shuts off the big
output devices. For example, if the driver input is >2V, it
expects the “A” output to be >3.25V and the “B” output to
be <1.75V. If the “A” output is subsequently shorted to a
voltage below V
DD
/2, this circuitry shuts off the big output
devices and turns on a smaller device in its place (the
converse applies for the “B” output). The outputs then
appear as ±10mA current sources. Note that under normal
operation, the output drivers can sink/source >50mA. A
time-out period of about 50ns is used in order to maintain
normal high frequency operation, even under heavy ca-
pacitive loads.
If the cable is shorted at a large distance from the device
outputs, it is possible for the short to go unnoticed at the
driver outputs due to parasitic cable resistance. Addition-
ally, when the cable is shorted, it no longer appears as an
ideal transmission line, and the parasitic Ls and Cs might
give rise to ringing and even oscillation. All these conditions
disappear once the device comes out of short-circuit mode.
For cables with the typical RS485 termination (no DC bias
on the cable, such as Figure 10), the LTC1685 will auto-
matically come out of short-circuit mode once the physical
short has been removed. With cable terminations with a
DC bias (such as Fast-20 and Fast-40 differential SCSI
APPLICATIONS INFORMATION
WUU
U
terminators, see Figure 15), the LTC1685 will not come
out of short-circuit mode automatically upon release of the
physical short. In order to resume normal operation, the
DE pin has to be pulsed low for at least 200ns.
High Speed Twisted Pair Transmission
Data rates up to 52Mbps can be transmitted over 100ft of
category 5 twisted pair. Figure 10 shows the LTC1685
receiving differential data from another LTC1685 trans-
ceiver. Figure 11a shows a 26MHz (52Mbps) square wave
propagated over 100ft of category 5 UTP. Figure 11b
shows a more stringent case of propagating a single 20ns
pulse over 100ft of category 5 UTP. Figure 12 shows a
4Mbps square wave over 1000ft of category 5 unshielded
twisted pair.
1685 F11
DRIVER
INPUT
RECEIVER
OUTPUT
10ns/DIV
2V/DIV
2V/DIV
Figure 11a. 100ft of Category 5 UTP: 50Mbps
1685 F11b
RECEIVER
INPUT
DRIVER
INPUT
RECEIVER
OUTPUT
20ns/DIV
2V/DIV
5V/DIV
2V/DIV
CABLE DELAY
Figure 11b. 100ft of Category 5 UTP: 20ns Pulse
Figure 10
100Ω
A 1
4
EN
RO
1685 F10b
1/4 LTC1518 LTC1685LTC1685
12
3
2 B
7
6
3
2
DE
DI
RO
RE
4
1
100Ω
7
6
3
2
DE
DI
RO
RE
4
1
EN