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LTC1687CS#PBF

P1-P3P4-P6P7-P9P10-P12
7
LTC1686/LTC1687
SWITCHI G TI E WAVEFOR S
UW W
Figure 5. Driver Propagation Delays
D
3V
1.5V
t
PLH
t
r
t
SKEW
1/2 V
O
90%
10%
0V
Z
Y
V
O
–V
O
0V
90%
1.5V
t
PHL
t
SKEW
1/2 V
O
f = 1MHz, t
r
≤ 3ns, t
f
≤ 3ns
10%
t
f
V
DIFF
= V(Y) – V(Z)
1686/87 F05
V
O
1.5V
t
ZL
2.5V
2.5V
t
ZH
1.5V
t
LZ
0.5V
0.5V
t
HZ
f = 1MHz, t
r
≤ 3ns, t
f
≤ 3ns
OUTPUT NORMALLY LOW
OUTPUT NORMALLY HIGH
3V
0V
DE
5V
V
OL
V
OH
0V
Y, Z
Y, Z
1686/87 F06
Figure 6. Driver Enable and Disable Times
2.5V
t
PHL
f = 1MHz, t
r
≤ 3ns, t
f
≤ 3ns
R
–V
OD2
A – B
0V
2.5V
t
PLH
OUTPUT
INPUT
V
OD2
V
OL
V
OH
1686/87 F07
Figure 7. Receiver Propagation Delays
1.5V
t
ZL
2.5V
2.5V
t
ZH
1.5V
t
LZ
0.5V
0.5V
t
HZ
f = 1MHz, t
r
≤ 3ns, t
f
≤ 3ns
OUTPUT NORMALLY LOW
OUTPUT NORMALLY HIGH
3V
0V
RE
5V
0V
R
R
1686/87 F08
Figure 8. Receiver Enable and Disable Times
8
LTC1686/LTC1687
EQUIVALE T I PUT NETWORKS
U
U
Figure 9. Input Thevenin Equivalent
A
B
1686/87 F09
≥22k
3.3V
≥22k
3.3V
RE = 0 OR 1, V
DD
= 5V V
DD
= 0V
A
B
≥22k
≥22k
APPLICATIONS INFORMATION
WUU
U
THEORY OF OPERATION
Unlike typical CMOS transceivers whose propagation
delay can vary by as much as 500% from package to
package and show significant temperature drift, the
LTC1686/LTC1687 employ a novel architecture that pro-
duces a tightly controlled and temperature compensated
propagation delay. The differential timing skew is also
minimized between rising and falling output edges of the
receiver output and the complementary driver outputs.
The precision timing features of the LTC1686/LTC1687
reduce overall system timing constraints by providing a
narrow ±3.5ns window during which valid data appears at
the receiver/driver output. The driver and receiver will
have propagation delays that typically match to within 1ns.
In clocked data systems, the low skew minimizes duty
cycle distortion of the clock signal. The LTC1686/LTC1687
can be used at data rates of 52Mbps with less than 5% duty
cycle distortion (depending on cable length). When a clock
signal is used to retime parallel data, the maximum recom-
mended data transmission rate is 26Mbps to avoid timing
errors due to clock distortion.
FAIL-SAFE FEATURES
The LTC1686/LTC1687 have a fail-safe feature that guar-
antees the receiver output to be in a logic HIGH state when
the inputs are either shorted or left open (note that when
inputs are left open, large external leakage currents might
override the fail-safe circuitry). In order to maintain good
high frequency performance, it is necessary to slow down
the transient response of the fail-safe feature. When a line
fault is detected, the output will go HIGH typically in 2µs.
Note that the LTC1686/LTC1687 guarantee receiver fail-
safe performance over the
entire
(–7V to 12V) common
mode range!
When the inputs are accidentally shorted (by cutting
through a cable, for example), the short circuit fail-safe
feature will guarantee a high output logic level. Note also
that if the line driver is removed and the ground terminated
resistors are left in place, the receiver will see this as a
“short” and output a logic HIGH. Both of these fail-safe
features will keep the receiver from outputting false data
pulses under line fault conditions.
Thermal shutdown and short-circuit protection prevent
latchup damage to the LTC1686/LTC1687 during fault
conditions.
OUTPUT SHORT-CIRCUIT PROTECTION
The LTC1686/LTC1687 employ voltage sensing short-
circuit protection at the output terminals of both the driver
and receiver. For a given input polarity, this circuitry
determines 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
9
LTC1686/LTC1687
(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 a
simple transmission line impedance, and the parasitic L’s
and C’s 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 LTC1686/LTC1687
will automatically come out of short-circuit mode once the
physical short has been removed.
Cable Termination
The recommended cable termination for the LTC1686/
LTC1687 is a single resistor across the two wires at each
end of the twisted-pair line (see Figure 10). The LTC1687
can also be used with cable terminations with a DC bias
(such as Fast-20 and Fast-40 differential SCSI termina-
tors). When using a biased termination with the LTC1687,
however, the DE pin must be held low for at least 200ns
after the part has been powered up. This ensures proper
start-up into the DC load of the biased termination. Fur-
thermore, when the LTC1687 output is shorted, the DE pin
APPLICATIONS INFORMATION
WUU
U
should be pulsed low for at least 200ns after the short has
been removed. Since the LTC1686 driver is always
enabled, the LTC1686 should only be used with single
resistor termination, as shown in Figure 10.
HIGH SPEED TWISTED-PAIR TRANSMISSION
Data rates up to 52Mbps can be transmitted over 100 feet
of category 5 twisted pair. Figure 10 shows the LTC1687
receiving differential data from another LTC1687 trans-
ceiver. Figure 11a shows a 26MHz (52Mbps) square wave
propagated over 100 feet of category 5 UTP. Figure 11b
shows a more stringent case of propagating a 20ns pulse
over 100 feet of category 5 UTP. Figure 12 shows a 2MHz
(4Mbps) square wave propagated over 1000 feet of
category 5 unshielded twisted pair. Note that the LTC1686/
LTC1687 can still perform reliably at this distance and
speed. Very inexpensive unshielded telephone grade
twisted pair is shown in Figure 13. Despite the noticeable
loss at the receiver input, the LTC1686/LTC1687 can still
transfer at 30Mbps over 100 feet of telephone grade UTP.
Note that under all these conditions, the LTC1686/LTC1687
can pass through a single data pulse equal to the inverse
of the data rate (e.g., 20ns for 50Mbps data rate).
TRANSMISSION OVER LONG DISTANCES
1Mbps Over 4000 Feet Category 5 UTP
The LTC1685/LTC1686/LTC1687 family of high speed
transceivers is capable of 1Mbps transmission over 4000
feet of category 5 UTP. High quality cable provides lower
Figure 10
LTC1686/87 • F10
100Ω 100Ω
100Ω 100Ω
LTC1687
R
D
R
D
5
2
11
12
10
9
4
DE
3
RE
DE
RE
CATEGORY 5 UTP
RECEIVER
LTC1687
DRIVER
RECEIVER
DRIVER
P1-P3P4-P6P7-P9P10-P12

LTC1687CS#PBF

Mfr. #:
Buy LTC1687CS#PBF
Manufacturer:
Analog Devices Inc.
Description:
RS-485 Interface IC 52Mbps Prec Delay RS485 Fail-Safe Trans
Lifecycle:
New from this manufacturer.
Delivery:
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