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LTC1685CS8#PBF

P1-P3P4-P6P7-P9P10-P12
7
LTC1685
SWITCHI G TI E WAVEFOR S
UW W
Figure 5. Driver Propagation Delays
DI
3V
1.5V
t
PLH
t
r
t
SKEW
1/2 V
O
90%
10%
0V
B
A
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(A) – V(B)
1586 F05
V
O
Figure 8. Receiver Enable and Disable Times
Figure 7. Receiver Propagation Delays
2.5V
t
PHL
f = 1MHz, t
r
≤ 3ns, t
f
≤ 3ns
RO
–V
OD2
A – B
0V
2.5V
t
PLH
OUTPUT
INPUT
V
OD2
V
OL
V
OH
1686 F07
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
A, B
A, B
1686 F06
Figure 6. Driver Enable and Disable Times
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
RO
RO
1685 F08
8
LTC1685
A
B
1685 F09
≥22k
3.3V
≥22k
3.3V
DE = 0, 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
LTC1685 employs a novel architecture that produces a
tightly controlled and temperature compensated propaga-
tion 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 LTC1685 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 pair 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 LTC1685 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 LTC1685 has a fail-safe feature that guarantees 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 was 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 LTC1685 guarantees 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 termination
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 LTC1685 during fault conditions.
Figure 9. Input Thevenin Equivalent
EQUIVALENT INPUT NETWORKS
UU U
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
P1-P3P4-P6P7-P9P10-P12

LTC1685CS8#PBF

Mfr. #:
Buy LTC1685CS8#PBF
Manufacturer:
Analog Devices Inc.
Description:
RS-485 Interface IC 52Mbps, Prec Delay, RS485 Fail-Safe Tran
Lifecycle:
New from this manufacturer.
Delivery:
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