1
LTC1520
50Mbps Precision Quad
Line Receiver
May 1996
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen-
tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
Final Electrical Specifications
TYPICAL APPLICATIO
N
U
DESCRIPTIO
N
U
s
Precision Propagation Delay: 18ns
3ns Over
Temperature
s
Data Rate: 50Mbps
s
Low t
PLH
/t
PHL
Skew: 500ps Typ
s
Low Channel-to-Channel Skew: 400ps Typ
s
Rail-to-Rail Input Common Mode Range
s
High Input Resistance:
18k, Even When Unpowered
s
Hot Swap Capable
s
Can Withstand Input DC Levels of
10V
s
Short-Circuit Protected
s
Single 5V Supply
s
LVDS Compatible
s
Will Not Oscillate with Slow Input Signals
FEATURES
The LTC
1520 is a high speed, precision differential line
receiver that can operate at data rates as high as 50Mbps.
A unique architecture provides very stable propagation
delays and low skew over a wide input common mode,
input overdrive and ambient temperature range. Propaga-
tion delay is 18ns
3ns, while typically t
PLH
/t
PHL
skew is
500ps and channel-to-channel skew is 400ps.
Each receiver translates differential input levels (V
ID
100mV) into valid CMOS and TTL output levels. Its high
input resistance (
18k) allows many receivers to be con-
nected to the same driver. The receiver outputs go into a
high impedance state when disabled.
Protection features include thermal shutdown and a con-
trolled maximum short-circuit current (50mA max) that
does not oscillate in and out of short-circuit mode. Input
resistance remains
18k when the device is unpowered or
disabled, thus allowing the LTC1520 to be hot swapped into
a backplane without loading the data lines.
The LTC1520 operates from a single 5V supply and draws
12mA of supply current. The part is available in a 16-lead
narrow SO package.
APPLICATIO
N
S
U
s
High Speed Backplane Interface
s
Line Collision Detector
s
PECL and LVDS Line Receivers
s
Level Translator
s
Ring Oscillator
s
Tapped Delay Line
, LTC and LT are registered trademarks of Linear Technology Corporation.
High Speed Backplane Receiver
Propagation Delay Guaranteed to Fall
Within Shaded Area (
3ns)
+
+
+
+
LTC1520
LTC1520 TA01
3.3k
5V
3.3k
0.01
F
LTC1520 TA02
V
IN
=
1V/DIV
V
OUT
=
5V/DIV
5
5
15
TIME (ns)
25
35
0
10
20
30
40 45
RECEIVER
INPUT
V
ID
= 500mV
RECEIVER
OUTPUT
V
DD
= 5V
2
LTC1520
A
U
G
W
A
W
U
W
A
R
BSOLUTE
XI
TI
S
W
U
U
PACKAGE/ORDER I FOR ATIO
ORDER PART
NUMBER
(Note 1)
Supply Voltage ....................................................... 10V
Digital Input Currents ..................... 100mA to 100mA
Digital Input Voltages ............................... 0.5V to 10V
Receiver Input Voltages ........................................
10V
Receiver Output Voltages ............. 0.5V to V
DD
+ 0.5V
Short-Circuit Duration .................................... Indefinite
Operating Temperature Range .................... 0
C to 70
C
Storage Temperature Range ................ 65
C to 150
C
Lead Temperature (Soldering, 10 sec) ................. 300
C
LTC1520CS
T
JMAX
= 150
C,
JA
= 90
C/ W
Consult factory for Industrial and Military grade parts.
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
V
CM
Input Common Mode Voltage
A, B Inputs
q
0.2
V
DD
+ 0.2
V
V
IH
Input High Voltage
Enable Input
q
2
V
V
IL
Input Low Voltage
Enable Input
q
0.8
V
I
IN1
Input Current
Enable Input
q
1
1
A
I
IN2
Input Current (A, B)
V
A
, V
B
= 5V
q
250
A
V
A
, V
B
= 0
q
250
A
R
IN
Input Resistance (Figure 5)
0.2V
V
CM
V
DD
+ 0.2V
q
18
k
C
IN
A, B Input Capacitance
3
pF
V
OC
Open-Circuit Input Voltage (Figure 5)
V
DD
= 5V (Note 4)
q
3.2
3.3
3.4
V
V
ID(MIN)
Differential Input Threshold Voltage
0.2V < V
CM
< V
DD
+ 0.2V
q
0.1
0.1
V
dV
ID
Input Hysteresis
V
CM
= 2.5V
q
20
mV
V
OH
Output High Voltage
I
OUT
= 4mA, V
ID
= 0.1V, V
DD
= 5V
q
4.6
V
V
OL
Output Low Voltage
I
OUT
= 4mA, V
ID
= 0.1V, V
DD
= 5V
q
0.4
V
I
OZR
Three-State Output Current
0V
V
OUT
5V
q
10
10
A
I
DD
Total Supply Current All 4 Receivers
V
ID
0.1V, No Load, Enable = 5V
q
12
20
mA
I
OSR
Short-Circuit Current
V
OUT
= 0V, V
OUT
= 5V
q
50
50
mA
CMRR
Common Mode Rejection Ratio
V
CM
= 2.5V, f = 25MHz
45
dB
V
DD
= 5V (Notes 2, 3) per receiver, unless otherwise noted.
DC ELECTRICAL CHARACTERISTICS
TOP VIEW
S PACKAGE
16-LEAD PLASTIC SO
1
2
3
4
5
6
7
8
16
15
14
13
12
11
10
9
B1
A1
OUT 1
ENABLE
OUT 2
A2
B2
GND
V
DD
B4
A4
OUT 4
NC
OUT 3
A3
B3
3
LTC1520
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
t
PLH
, t
PHL
Input-to-Output Propagation Delay
C
L
= 15pF (Figure 1)
q
15
18
21
ns
t
r
, t
f
Rise/Fall Times
C
L
= 15pF
2.5
ns
t
SKD
t
PLH
t
PHL
Skew
C
L
= 15pF, Same Receiver (Note 5)
q
500
ps
t
ZL
Enable to Output Low
C
L
= 15pF (Figure 2)
q
10
25
ns
t
ZH
Enable to Output High
C
L
= 15pF (Figure 2)
q
10
25
ns
t
LZ
Disable from Output Low
C
L
= 15pF (Figure 2)
q
20
35
ns
t
HZ
Disable from Output High
C
L
= 15pF (Figure 2)
q
20
35
ns
t
CH-CH
Channel-to-Channel Skew
C
L
= 15pF (Figure 3) (Note 6)
q
400
ps
t
PKG-PKG
Package-to-Package Skew
C
L
= 15pF, Same Temperature
1.5
ns
(Figure 4, Note 4)
Minimum Input Pulse Width
(Note 4)
12
ns
f
IN
Maximum Input Frequency
(Note 4)
40
MHz
V
DD
= 5V (Notes 2, 3) V
ID
= 500mV, V
CM
= 2.5V, unless otherwise noted.
SWITCHI G TI E CHARACTERISTICS
U
W
The
q
denotes specifications which apply over the full operating
temperature range.
Note 1: Absolute Maximum Ratings are those values beyond which the
safety of the device cannot be guaranteed. Recommended: V
DD
= 5V
5%.
Note 2: All currents into the device pins are positive; all currents out of the
device pins are negative.
Note 3: All typicals are given for V
DD
= 5V, T
A
= 25
C.
Note 4: Guaranteed by design, but not tested.
Note 5: Worst-case
t
PLH
t
PHL
skew for a single receiver in a package
over the full operating temperature range.
Note 6: Maximum difference between any two t
PLH
or t
PHL
transitions in a
single package over the full operating temperature range.
TYPICAL PERFOR
M
A
N
CE CHARACTERISTICS
U
W
TEMPERATURE (
C)
50
25
0
PROPAGATION DELAY (ns)
10
25
0
50
75
LTC1520 G01
5
20
15
25
100
125
V
CM
= 2.5V
V
ID
= 500mV
Propagation Delay (t
PLH
/ t
PHL
)
vs Temperature
Propagation Delay (t
PLH
/t
PHL
)
vs Input Overdrive
INPUT OVERDRIVE (V)
0.05
0
PROPAGATION DELAY (ns)
15
20
25
0.1
1
5
10
1520 G02
10
5
T
A
= 25
C
V
CM
= 2.5V
4
LTC1520
TYPICAL PERFOR
M
A
N
CE CHARACTERISTICS
U
W
FREQUENCY (Hz)
10
42.0
COMMON MODE REJECTION RATIO (dB)
42.5
43.5
44.0
44.5
46.5
LTC1520 G04
43.0
1k
100k
10M
45.0
45.5
46.0
T
A
= 25
C
CMRR vs Frequency
INPUT COMMON MODE (V)
0
0
PROPAGATION DELAY (ns)
15
2
4
5
LTC1520 G03
10
5
1
3
20
25
T
A
= 25
C
V
ID
= 500mV
Figure 1. Propagation Delay Test Circuit and Waveforms
B1 (Pin 1): Receiver 1 Inverting Input.
A1 (Pin 2): Receiver 1 Noninverting Input.
RO1 (Pin 3): Receiver 1 Output.
Enable (Pin 4): Receiver Output Enable Pin. A logic high
input enables the receiver outputs. A logic low input
forces the receiver outputs into a high impedance state.
Do not float.
RO2 (Pin 5): Receiver 2 Output.
A2 (Pin 6): Receiver 2 Noninverting Input.
B2 (Pin 7): Receiver 2 Inverting Input.
GND (Pin 8): Ground Pin. A ground plane is recommended
for all LTC1520 applications.
B3 (Pin 9): Receiver 3 Inverting Input.
A3 (Pin 10): Receiver 3 Noninverting Input.
RO3 (Pin 11): Receiver 3 Output.
NC (Pin 12): No Connection.
RO4 (Pin 13): Receiver 4 Output.
A4 (Pin 14): Receiver 4 Noninverting Input.
B4 (Pin 15): Receiver 4 Inverting Input.
V
DD
(Pin 16): 5V Supply Pin. This pin should be decoupled
with a 0.1
F ceramic capacitor as close as possible to the
pin. Recommended: V
DD
= 5V
5%.
PI
N
FU
N
CTIO
N
S
U
U
U
Propagation Delay (t
PLH
/t
PHL
)
vs Input Common Mode
SWITCHI G TI E WAVEFOR S
U
W
W
+
INPUT
2.5V
OUTPUT
15pF
1520 F01b
1/4 LTC1520
2.5V
2.5V
V
DD
/2
V
DD
/2
3V
2V
INPUT
OUTPUT
t
PLH
t
PHL
1520 F01
5
LTC1520
SWITCHI G TI E WAVEFOR S
U
W
W
APPLICATIO
N
S I
N
FOR
M
ATIO
N
W
U
U
U
3.3V
18k
18k
A
B
RECEIVER ENABLED, V
DD
= 5V
RECEIVER DISABLED OR V
DD
= 0V
3.3V
18k
A
B
1520 F05
18k
Figure 5. Input Thevenin Equivalent
EQUIVALE T I PUT NETWORKS
U
U
Theory of Operation
Unlike typical line receivers whose propagation delay can
vary by as much as 500% from package to package and
show significant temperature drift, the LTC1520 employs
a novel architecture that produces a tightly controlled and
temperature compensated propagation delay. The differ-
ential timing skew is also minimized between rising and
V
DD
/2
V
DD
/2
V
DD
/2
V
DD
/2
3V
B1, B2 = 2.5V
2V
CH1 OUT
INPUT
A1, A2
CH2 OUT
t
CH-CH
t
CH-CH
1520 F03
Figure 3. Any Channel to Any Channel Skew, Same Package
Figure 4. Package-to-Package Propagation Delay Skew
falling output edges, and the propagation delays of any
two receivers within a package are very tightly matched.
The precision timing features of the LTC1520 reduce
overall system timing constraints by providing a narrow
6ns window during which valid data appears at the re-
ceiver output. This output timing window applies to all
receivers in all packages over all operating temperatures
Figure 2. Receiver Enable and Disable Timing
Test Circuit and Waveforms
1.5V
1.5V
1.5V
C
L
1k
1k
S1
S2
t
ZH
t
ZL
1.5V
t
LZ
0.2V
0.2V
t
HZ
V
DD
OUTPUT
NORMALLY LOW
OUTPUT
NORMALLY HIGH
0V
3V
ENABLE
5V
V
OL
V
OH
0V
RECEIVER
OUTPUT
OUT 1
OUT 1
1520 F02
SAME INPUT FOR BOTH PACKAGES
INPUT
A1, B1
V
ID
= 500mV
PACKAGE 1
OUT 1
1520 F04
t
PKG-PKG
t
PKG-PKG
PACKAGE 2
OUT 1
6
LTC1520
APPLICATIO
N
S I
N
FOR
M
ATIO
N
W
U
U
U
thereby making the LTC1520 well suited for high speed
parallel data transmission applications such as backplanes.
In clocked data systems, the low skew minimizes duty
cycle distortion of the clock signal. The LTC1520 can
propagate signals at frequencies up to 25MHz (50Mbps)
with less than 5% duty cycle distortion. When a clock
signal is used to retime parallel data, the maximum recom-
mended data transmission rate is 25Mbps to avoid timing
errors due to clock distortion.
Rail-to-rail input common mode range enables the LTC1520
to be used in both single-ended and differential applica-
tions with transmission distances up to 100 feet. Thermal
shutdown and short-circuit protection prevent latchup
damage to the LTC1520 during fault conditions.
Single-Ended Applications
Over short distances, the LTC1520 can be configured to
receive single-ended data by tying one input to a fixed bias
voltage and connecting the other input to the driver output.
In such applications, standard high speed CMOS logic
may be used as a driver for the LTC1520. The receiver trip
points may be easily adjusted to accommodate different
driver output swings by changing the resistor divider at the
fixed input. Figure 6a shows a single-ended receiver
configuration with the driver and receiver connected via
PC traces. Note that at very high speeds, transmission line
and driver ringing effects have to be considered. Motorola's
MECL System Design Handbook serves as an excellent
reference for transmission line and termination effects. To
mitigate transmission errors and duty cycle distortion due
to driver ringing, a small output filter or a dampening
resistor on V
DD
may be needed as shown in Figure 6b. To
transmit single-ended data over distances up to 10 feet,
twisted pair is recommended with the unused wire
grounded at both ends (Figure 7).
Figure 6a. Single-Ended Receiver
Figure 6b. Techniques to Minimize Driver Ringing
Figure 8. Differential Transmission Over Long Distances
10
PC TRACE OR
PC TRACE
0.01
F
MC74AC04
1520 F06b
10pF
10
+
2.2k
5V
PC TRACE
1/4 LTC1520
MC74ACT04
(TTL INPUT)
MC74AC04
(CMOS INPUT)
2.2k
1520 F06a
0.01
F
100-FT TWISTED PAIR
*MC10116
R
T
120
1520 F08
100
100
100
5V
5V
5V
*
100
+
1/4 LTC1520
Figure 7. Medium Distance Single-Ended Transmission
Using a CMOS Driver
Differential Transmission
The LTC1520 is well suited for medium distance differen-
tial transmission due to its rail-to-rail input common mode
range. Clock rates up to 25MHz can be transmitted over
100 feet of high quality twisted pair. Figure 8 shows the
LTC1520 receiving differential data from a PECL driver. As
in the single-ended configurations, care must be taken to
properly terminate the differential data lines to avoid
unwanted reflections, etc.
+
1/4 LTC1520
10-FT TWISTED PAIR
MC74ACT04
MC74AC04
0.01
F
120
3.3k
5V
2.2k
1520 F07
7
LTC1520
APPLICATIO
N
S I
N
FOR
M
ATIO
N
W
U
U
U
Alternate Uses
The tightly controlled propagation delay of the LTC1520
allows the part to serve as a fixed delay element. Figure 9
shows the LTC1520 used as a tapped delay line with 18ns
3ns steps. Several LTC1520s may be connected in series
to form longer delay lines. Each tap in the delay line is
accurate to within
17% over temperature.
As shown in Figure 10, the LTC1520 can be used to create
a temperature stable ring oscillator with period increments
of 36ns. Low skew and good channel-to-channel match-
ing enable this oscillator to achieve better than a 45/55
duty cycle (the duty cycle approaches 50/50 as more
LTC1520s are used for lower frequencies). Note that the
fixed voltage bias may either be created externally with a
resistor divider or generated internally using a bypass
capacitor and the internal open circuit bias point (approxi-
mately 3.3V). The use of the internal bias point will result
in a 1% to 2% distortion of the duty cycle.
+
0ns DELAY
INPUT
1/4 LTC1520
+
18ns DELAY
1/4 LTC1520
+
36ns DELAY
1/4 LTC1520
+
54ns DELAY
72ns DELAY
1/4 LTC1520
3.3k
5V
0.01
F
3.3k
1520 F09
Figure 9. Tapped Delay Line with 18ns Steps
Figure 10. Temperature Stable Ring Oscillators
+
+
1/4 LTC1520
+
1/4 LTC1520
9.3MHz OSCILLATOR
WITH BETTER THAN
45/55 DUTY CYCLE
TYPICAL STABILITY
5% OVER TEMPERATURE
1/4 LTC1520
3.3k
5V
0.01
F
3.3k
+
+
1/4 LTC1520
+
1/4 LTC1520
+
1/4 LTC1520
6.9MHz
OSCILLATOR OUTPUT
1/4 LTC1520
0.01
F
1520 F10
8
LTC1520
PART NUMBER
DESCRIPTION
COMMENTS
LTC486/487
Low Power Quad RS485 Driver
10Mbps, 7V to 12V Common Mode Range
LTC488/489
Low Power Quad RS485 Receiver
10Mbps, 7V to 12V Common Mode Range
LT
1016
Ultrafast Precision Comparator
Single 5V Supply, 10ns Propagation Delay
LTC1518
High Speed Quad RS485 Receiver
50Mbps, 7V to 12V Common Mode Range
LTC1519
High Speed Quad RS485 Receiver
50Mbps, 7V to 12V Common Mode Range
LINEAR TECHNOLOGY CORPORATION 1996
LT/GP 0596 6K PRINTED IN THE USA
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
q
FAX
: (408) 434-0507
q
TELEX
: 499-3977
PACKAGE DESCRIPTIO
N
U
S Package
16-Lead Plastic Small Outline (Narrow 0.150)
(LTC DWG # 05-08-1610)
RELATED PARTS
0.016 0.050
0.406 1.270
0.010 0.020
(0.254 0.508)
45
0
8
TYP
0.008 0.010
(0.203 0.254)
1
2
3
4
5
6
7
8
0.150 0.157**
(3.810 3.988)
16
15
14
13
0.386 0.394*
(9.804 10.008)
0.228 0.244
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12
11
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S16 0695
0.053 0.069
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(0.355 0.483)
0.004 0.010
(0.101 0.254)
0.050
(1.270)
TYP
*DIMENSION DOES NOT INCLUDE MOLD FLASH. MOLD FLASH
SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE
**DIMENSION DOES NOT INCLUDE INTERLEAD FLASH. INTERLEAD
FLASH SHALL NOT EXCEED 0.010" (0.254mm) PER SIDE
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