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LTC1686/LTC1687 52Mbps Precision Delay RS485 Fail-Safe Transceivers
FEATURES
s s s
DESCRIPTION
The LTC(R)1686/LTC1687 are high speed, precision delay, full-duplex RS485 transceivers that can operate at data rates as high as 52Mbps. The devices also meet the requirements of RS422. A unique architecture provides very stable propagation delays and low skew over a wide common mode and ambient temperature range. The driver and receiver feature three-state outputs, with disabled driver outputs maintaining high impedance over the entire common mode range. A short-circuit feature detects shorted outputs and substantially reduces driver output current. A similar feature also protects the receiver output from short circuits. Thermal shutdown circuitry protects from excessive power dissipation. The receiver has a fail-safe feature that guarantees a high output state when the inputs are shorted or are left floating. The LTC1686/LTC1687 RS485 transceivers guarantee receiver fail-safe operation over the entire common mode range (- 7V to 12V). Receiver input resistance remains 22k when the device is unpowered or disabled. The LTC1686/LTC1687 operate from a single 5V supply and draw only 7mA of supply current.
, LTC and LT are registered trademarks of Linear Technology Corporation.
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s s s s
s s s
Precision Propagation Delay Over Temperature: Receiver/Driver: 18.5ns 3.5ns High Data Rate: 52Mbps Low tPLH/tPHL Skew: Receiver/Driver: 500ps Typ -7V to 12V RS485 Input Common Mode Range Guaranteed Fail-Safe Operation Over the Entire Common Mode Range High Input Resistance: 22k, Even When Unpowered Short-Circuit Protected Thermal Shutdown Protected Driver Maintains High Impedance in Three-State or with Power Off Single 5V Supply Pin Compatible with LTC490/LTC491 45dB CMRR at 26MHz
APPLICATIONS
s s s s s
High Speed RS485/RS422 Full Duplex Transceivers Level Translator Backplane Transceiver STS-1/OC-1 Data Transceiver Signal Repeaters
TYPICAL APPLICATION
LTC1686 5 D 3 DRIVER 100 6 100 RECEIVER R LTC1686
2V/DIV CABLE DELAY
1V/DIV
8 R 2 RECEIVER 100 7 100 DRIVER D
5V/DIV
400 FT OF CATEGORY 5 UTP
LTC1686/87 * TA01
U
U
U
10Mbps Data Pulse 400 Feet Category 5 UTP
DRIVER INPUT
RECEIVER INPUT
RECEIVER OUTPUT
100ns/DIV
1686/87 TA02
1
LTC1686/LTC1687 ABSOLUTE
(Note 1)
AXI U
RATI GS
Driver Short-Circuit Duration (VOUT: - 7V to 10V)...................................... Indefinite Receiver Short-Circuit Duration (VOUT: 0V to VDD) ........................................ Indefinite Operating Temperature Range .................... 0C to 70C Storage Temperature Range ................ - 65C to 150C Lead Temperature (Soldering, 10 sec)................. 300C
Supply Voltage (VDD) .............................................. 10V Control Input Currents .................... - 100mA to 100mA Control Input Voltages .................. - 0.5V to VDD + 0.5V Driver Input Voltages .................... - 0.5V to VDD + 0.5V Driver Output Voltages ................................. + 12V/- 7V Receiver Input Voltages ................................ + 12V/- 7V Receiver Output Voltages ............. - 0.5V to VDD + 0.5V Receiver Input Differential ...................................... 10V
PACKAGE/ORDER I FOR ATIO
TOP VIEW VDD 1 R2 D3
D
ORDER PART NUMBER
8 A B Z Y 7 6 5
R
LTC1686CS8 S8 PART MARKING 1686
GND 4
S8 PACKAGE 8-LEAD PLASTIC SO
TJMAX = 125C, JA = 150C/ W
Consult factory for Industrial and Military grade parts.
DC ELECTRICAL CHARACTERISTICS
SYMBOL VOD1 VOD2 VOD PARAMETER Differential Driver Output (Unloaded) Differential Driver Output (With Load) Change in Magnitude of Driver Differential Output Voltage for Complementary Output States Driver Common Mode Output Voltage Change in Magnitude of Driver Common Mode Output Voltage for Complementary Output States Input High Voltage Input Low Voltage Input Current Input Current (A, B) Differential Input Threshold Voltage for Receiver Receiver Input Hysteresis Receiver Output High Voltage CONDITIONS IOUT = 0
R = 50 (RS422) R = 27 (RS485), Figure 1 R = 27 or 50, Figure 1
VOC VOC
R = 27 or 50, VDD = 5V, Figure 1 R = 27 or 50, Figure 1
VIH VIL IIN1 IIN2 VTH VTH VOH
D, DE, RE D, DE, RE D, DE, RE VA, VB = 12V, VDD = 0V or 5.25V VA, VB = - 7V, VDD = 0V or 5.25V - 7V VCM 12V VCM = 0V IOUT = - 4mA, VID = 300mV
2
U
U
W
WW
U
W
TOP VIEW NC R RE DE D GND GND 1 2 3 4 5 D 6 7 S PACKAGE 14-LEAD PLASTIC SO 9 8 Y NC R 14 VDD 13 NC 12 A 11 B 10 Z
ORDER PART NUMBER LTC1687CS
TJMAX = 125C, JA = 90C/ W
VDD = 5V 5% unless otherwise noted (Notes 2, 3).
MIN
q q q
TYP
MAX VDD
UNITS V V V V
2.0 1.5
VDD 0.2
q q
2
3 0.2
V V
q q q q q q
2 0.8 -1 - 500 - 0.3 25 0.3 1 500
V V A A A V mV V
q
3.5
4.8
LTC1686/LTC1687
DC ELECTRICAL CHARACTERISTICS
SYMBOL VOL IOZR IOZD CLOAD IDD IOSD1 IOSD2 IOSR RIN CIN Fail-Safe Time CMRR PARAMETER Receiver Output Low Voltage Three-State (High Impedance) Output Current at Receiver Three-State (High Impedance) Output Current at Driver Supply Current Driver Short-Circuit Current, VOUT = HIGH Driver Short-Circuit Current, VOUT = LOW Receiver Short-Circuit Current Input Resistance Input Capacitance Open-Circuit Input Voltage Time to Detect Fail-Safe Condition Receiver Input Common Mode Rejection Ratio CONDITIONS
VDD = 5V 5% unless otherwise noted (Notes 2, 3).
MIN
q q q q q q q q q
TYP
MAX 0.4 1 200 500
UNITS V A A pF mA mA mA mA k pF
IOUT = 4mA, VID = - 300mV 0.4V VOUT 2.4V VOUT = - 7V to 12V
-1 - 200
Receiver and Driver Output Load Capacitance (Note 4) No Load, Pins D, DE, RE = 0V or VDD VOUT = - 7V or 10V (Note 5) VOUT = - 7V or 10V (Note 5) VOUT = 0V or VDD (Note 5) - 7V VCM 12V A, B, D, DE, RE Inputs (Note 4) VDD = 5V (Note 4), Figure 5
7
12 20 20 20
22 3 3.2 3.3 2 3.4
q
V s dB
VCM = 2.5V, f = 26MHz
45
SWITCHING CHARACTERISTICS
SYMBOL tPLH, tPHL tSKEW t r , tf tZH tZL tLZ t HZ tPLH, tPHL tSQD tZL tZH tLZ t HZ PARAMETER Driver Input-to-Output Propagation Delay Driver Output A-to-Output B Skew Driver Rise/Fall Time Driver Enable to Output High Driver Enable to Output Low Driver Disable from Low Driver Disable from High
Receiver Input-to-Output Propagation Delay CL = 15pF, Figures 3, 7 Receiver Skew tPLH - t PHL CL = 15pF, Figures 3, 7 Receiver Enable to Output Low Receiver Enable to Output High Receiver Disable from Low Receiver Disable from High Maximum Receiver Input Rise/Fall Times CL = 15pF, S1 Closed, Figures 2, 8 CL = 15pF, S2 Closed, Figures 2, 8 CL = 15pF, S1 Closed, Figures 2, 8 CL = 15pF, S2 Closed, Figures 2, 8 (Note 4) CL = 15pF, Same Temperature (Note 4) VDD = 5V 5% (Note 4) VDD = 5V 5% (Note 4) VDD = 5V 5% (Note 4)
tPKG-PKG
Package-to-Package Skew Minimum Input Pulse Width Maximum Data Rate Maximum Input Frequency
U
VDD = 5V, unless otherwise noted (Notes 2, 3).
MIN
q
CONDITIONS RDIFF = 54, CL1 = CL2 = 100pF, Figures 3, 5 RDIFF = 54, CL1 = CL2 = 100pF, Figures 3, 5 RDIFF = 54, CL1 = CL2 = 100pF, Figures 3, 5 CL = 100pF, S2 Closed, Figures 4, 6 CL = 100pF, S1 Closed, Figures 4, 6 CL = 15pF, S1 Closed, Figures 4, 6 CL = 15pF, S2 Closed, Figures 4, 6
q q q q q
TYP 18.5 500 3.5 25 25 25 25
MAX 22
UNITS ns ps ns
15
50 50 50 50 22 50 50 50 50 2000
ns ns ns ns ns ps ns ns ns ns ns ns
15
18.5 500 25 25 25 25
q q q q q
1.5
q q q
17 52 26 60 30
19.2
ns Mbps MHz
3
LTC1686/LTC1687
ELECTRICAL CHARACTERISTICS
The q denotes specifications which apply over the full operating temperature range. Note 1: Absolute Maximum Ratings are those values beyond which the life of a device may be impaired. 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 VDD = 5V, TA = 25C. Note 4: Guaranteed by design, but not tested. Note 5: Short-circuit current does not represent output drive capability. When the output detects a short-circuit condition, output drive current is significantly reduced (from hundreds of mA to 20mA max) until the short is removed.
TYPICAL PERFORMANCE CHARACTERISTICS
Receiver Input CMRR
46.5
COMMON MODE REJECTION RATIO (dB)
46.0
SUPPLY CURRENT (mA)
45.0 44.5 44.0 43.5 43.0 42.5 TA = 25C 42.0 10 1k 100k FREQUENCY (Hz) 1M
1686/87 G01
50 40 30 20 10 0 1 10 30 DATA RATE (Mbps) 20 40 50
SUPPLY CURRENT (mA)
45.5
Receiver Propagation Delay vs Load Capacitance
30 TA = 25C 25 25
RECEIVER PROPAGATION DELAY (ns)
PROPAGATION DELAY (ns)
20 15 10 5 0
PROPAGATION DELAY (ns)
5
15
25 35 55 105 LOAD CAPACITANCE (pF)
4
UW
1686/87 G04
Supply Current vs Data Rate
70 BOTH DRIVER AND RECEIVER ENABLED AND LOADED 60 T = 25C A
Supply Current vs Temperature
58 57 56 55 54 53 52 51 BOTH DRIVER AND RECEIVER ENABLED AND LOADED 25Mbps DATA RATE 0 25 50 TEMPERATURE (C) 75 100
1686/87 G03
50 - 25
1686/87 G02
Receiver Propagation Delay vs Common Mode
25
Receiver Propagation Delay vs Input Overdrive
TA = 25C 20
TA = 25C
20
15
15
10
10
5
5
205
0 8 10 4 -7 -4 -2 0 6 2 RECEIVER COMMON MODE (V) 12
0 0.3 0.5 0.7 1.0 1.25 1.5 2.0 RECEIVER INPUT OVERDRIVE (V) 2.5
1686/87 G05
1686/87 G06
LTC1686/LTC1687 TYPICAL PERFORMANCE CHARACTERISTICS
Receiver Propagation Delay vs Temperature
25
PROPAGATION DELAY (ns)
15
DATA RATE (Mbps)
50 40 30 20 10
PROPAGATION DELAY (ns)
1.5 0.4 0.5 0.6 0.7 1.0 RECEIVER INPUT DIFFERENTIAL (V) 2.5
20
10
5
0 -50 -25
50 0 75 25 TEMPERATURE (C)
Driver Propagation Delay vs Driver Input Voltage
25 VDD = 5V INPUT THRESHOLD = 1.5V TA = 25C tHL tLH 15
19.0
PROPAGATION DELAY (ns)
PROPAGATION DELAY (ns)
20
10
5
0 2.5
3.0 4.0 4.5 3.5 DRIVER INPUT VOLTAGE (V)
PIN FUNCTIONS
LTC1686 VDD (Pin 1): Positive Supply, 5V to 5%. Bypass with 0.1F ceramic capacitor. R (Pin 2): Receiver Output. If A B by 300mV, then R will be high. If A B by 300mV, then R will be low. D (Pin 3): Driver Input. Controls the states of the Y and Z outputs. Do not float. GND (Pin 4): Ground. Y (Pin 5): Noninverting Driver Output. Z (Pin 6): Inverting Driver Output. B (Pin 7): Inverting Receiver Input. A (Pin 8): Noninverting Receiver Input. LTC1687 NC (Pins 1, 8, 13): No Connection. R (Pin 2): Receiver Output. If A B by 300mV, then R will be high. If A B by 300mV, then R will be low. RE (Pin 3): Receiver Enable. RE = low enables the receiver. RE = high forces receiver output into high impedance state. Do not float.
UW
100
1686/87 G09
Receiver Maximum Data Rate vs Input Overdrive
70 TA = 25C 60 20 25
Driver Propagation Delay vs Temperature
15
10
5
125
0 0.3
0 - 20
0
20 40 60 TEMPERATURE (C)
80
100
1686/87 G10
1686/87 G07
Driver Propagation Delay vs Capacitive Load
TA = 25C 18.5 18.0 17.5 17.0 16.5 16.0
5.0
1686/87 G08
5
15
25 50 75 100 LOAD CAPACITANCE (pF)
150
1686/87 G11
U
U
U
5
LTC1686/LTC1687
PIN FUNCTIONS
DE (Pin 4): Driver Enable. DE = high enables the driver. DE = low will force the driver output into a high impedance state. Do not float. D (Pin 5): Driver Input. Controls the states of the Y and Z outputs when DE = high. Do not float. GND (Pins 6, 7): Ground. Y (Pin 9): Noninverting Driver Output. Z (Pin 10): Inverting Driver Output. B (Pin 11): Inverting Receiver Input. A (Pin 12): Noninverting Receiver Input. VDD (Pin 14): Positive Supply, 5V to 5%. Bypass with 0.1F ceramic capacitor.
FU CTIO TABLES
Transmitting
RE X X X X INPUTS DE 1 1 0 1 D 1 0 X X
TEST CIRCUITS
Y R VOD R Z
1686/87 * F01
Figure 1. Driver DC Test Load
3V DE Y D Z RDIFF CL2 CL1 A R B RE 15pF
OUTPUT UNDER TEST
Figure 3. Driver/Receiver Timing Test Circuit
6
U
U
U
U
U
(LTC1687) Receiving
OUTPUTS Z 0 1 Hi- Z 10mA Current Source Y 1 0 Hi- Z RE 0 0 0 0 1 INPUTS DE X X X X X A-B 300mV - 300mV Inputs Open Inputs Shorted Together A = B = - 7V to 12V X OUTPUT R 1 0 1 1 Hi- Z
LINE CONDITION No Fault No Fault X Fault
TEST POINT RECEIVER OUTPUT CL 15pF 1k
S1
1k VDD
S2
1686/87 F02
VOC
Figure 2. Driver DC Test Load
S1 500 S2 CL
1686/87 F04
VDD
1686/87 F03
Figure 4. Driver Timing Test Load #2
LTC1686/LTC1687
SWITCHI G TI E WAVEFOR S
3V D 0V t PLH Z VO Y 1/2 VO VO 0V -VO 10% tr tSKEW 90% VDIFF = V(Y) - V(Z) tf 90% 10%
1686/87 F05
1.5V
Figure 5. Driver Propagation Delays
3V DE 0V 5V Y, Z VOL VOH Y, Z 0V t ZH 2.5V t ZL 2.5V 1.5V
Figure 6. Driver Enable and Disable Times
VOH R VOL t PHL VOD2 A-B -VOD2 0V 2.5V OUTPUT f = 1MHz, t r 3ns, t f 3ns INPUT t PLH 2.5V
Figure 7. Receiver Propagation Delays
3V RE 0V 5V R t ZL 2.5V OUTPUT NORMALLY LOW t LZ 0.5V 1.5V f = 1MHz, t r 3ns, t f 3ns 1.5V
R 0V t ZH
Figure 8. Receiver Enable and Disable Times
W
W
U
f = 1MHz, t r 3ns, t f 3ns t PHL
1.5V 1/2 VO
t SKEW
f = 1MHz, t r 3ns, t f 3ns t LZ OUTPUT NORMALLY LOW
1.5V
0.5V
OUTPUT NORMALLY HIGH t HZ
0.5V
1686/87 F06
1686/87 F07
2.5V
OUTPUT NORMALLY HIGH t HZ
0.5V
1686/87 F08
7
LTC1686/LTC1687
EQUIVALE T I PUT NETWORKS
22k A 22k B 3.3V B 3.3V A 22k 22k
APPLICATIONS INFORMATION
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 produces 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 recommended 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 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 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 2s. Note that the LTC1686/LTC1687 guarantee receiver failsafe 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 shortcircuit 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 VDD/2, this circuitry shuts off the big output devices and turns on a smaller device in its place
8
U
W
U
UU
U
RE = 0 OR 1, VDD = 5V
VDD = 0V
1686/87 F09
Figure 9. Input Thevenin Equivalent
LTC1686/LTC1687
APPLICATIONS INFORMATION
(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 capacitive 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. Additionally, 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 terminators). 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. Furthermore, when the LTC1687 output is shorted, the DE pin
DE 4
9 D 5 DRIVER 100 10 100 RECEIVER R
LTC1687 12 R 2 RECEIVER 100 11 100
3 RE
U
W
U
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 transceiver. 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
DE
LTC1687
DRIVER
D
CATEGORY 5 UTP RE
LTC1686/87 * F10
Figure 10
9
LTC1686/LTC1687
APPLICATIONS INFORMATION
2V/DIV DRIVER INPUT
2V/DIV
DRIVER INPUT
2V/DIV
2V/DIV
RECEIVER OUTPUT 10ns/DIV
1686/87 F11a
Figure 11a. 100 Feet of Category 5 UTP: 50Mbps
2V/DIV
CABLE DELAY
DRIVER INPUT
2V/DIV
RECEIVER INPUT
5V/DIV
RECEIVER OUTPUT
20ns/DIV
1685 F11b
Figure 11b. 100 Feet of Category 5 UTP: 20ns Pulse
2V/DIV
DRIVER INPUT
2V/DIV
RECEIVER OUTPUT
100ns/DIV
1686/87 F12
Figure 12. 1000 Feet of Category 5 UTP: 4Mbps
DC and AC attenuation over long distances. Figure 14a shows a 1s pulse propagated down 4000 feet of category 5 UTP. Notice the significant attenuation at the receiver input and the clean pulse at the receiver output. The DC attenuation is due to the parasitic resistance of the cable. Figure 14b shows a 1Mbps square wave over the same 4000 feet of cable.
10
U
W
U
U
DIFFERENTIAL RECEIVER INPUT
2V/DIV
RECEIVER OUTPUT
20ns/DIV
1686/87 F13
Figure 13. 100 Feet of Telephone Grade UTP: 30Mbps
2V/DIV
CABLE DELAY
DRIVER INPUT
1V/DIV
RECEIVER INPUT
5V/DIV
RECEIVER OUTPUT
1s/DIV
1685 F14a
Figure 14a. 4000 Feet of Category 5 UTP 1s Pulse
2V/DIV
DRIVER INPUT
5V/DIV
RECEIVER OUTPUT
1s/DIV
1685 F14b
Figure 14b. 4000 Feet of Category 5 UTP 1Mbps Square Wave
1.6Mbps Over 8000 Feet (1.5 Miles) Category 5 UTP Using Repeaters The LTC1686/LTC1687 can be used as repeaters to extend the effective length of a high speed twisted-pair line. Figure 15a shows a three repeater configuration using 2000 feet segments of category 5 UTP. Figure 15b shows the
LTC1686/LTC1687
APPLICATIONS INFORMATION
LTC1687 2000 FT D1 LTC1687 R2 D 2000 FT LTC1687 R3 D 2000 FT LTC1687 R4 D 2000 FT R5 LTC1687
REPEATER R
Figure 15a. 1.6Mbps, 8000 Feet (1.5 Miles) Using Three Repeaters
2V/DIV
DELAY OF 8000 FT OF CABLE
DRIVER 1 INPUT
5V/DIV
RECEIVER 5 OUTPUT DRIVER 1 INPUT
2V/DIV
5V/DIV 2s/DIV
1686/87 F15b
RECEIVER 5 OUTPUT
Figure 15b. 1.6Mbps Pulse and Square Wave Signals Over 8000 Feet Category 5 UTP Using Three Repeaters
propagation of a 600ns pulse through the network of Figure 15A. The bottom two traces show a 1.6Mbps square wave. Notice that the duty cycle does not noticeably degrade. For the case of the single pulse, however, there is a slight degradation of the pulse width. By slowing down the data rate slightly to 1Mbps, one can obtain minimal pulse width degradation as the signal traverses through the repeater network. Figure 16 shows that the output pulse (bottom trace) is nearly the same width to the input pulse (top trace). The middle three traces of Figure 16 show the signal at the end of each of the first three 2000 feet sections of category 5 UTP. Notice how the LTC1687 repeaters are able to regenerate the signal with little loss. This implies that we can cascade more repeater networks and potentially achieve 1Mbps operation at total distances of over 10,000 feet! A higher data rate can be achieved if the repeaters are spaced closer together. HIGH SPEED BACKPLANE TRANSMISSION The LTC1686/LTC1687 can also be used in backplane point-to-point transceiver applications, where the user wants to assure operation even when the common mode
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 representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
U
W
U
U
REPEATER
REPEATER
1686/87 F15a
2V/DIV
DRIVER 1 INPUT RECEIVER 2 INPUT RECEIVER 3 INPUT RECEIVER 4 INPUT RECEIVER 5 OUTPUT
1V/DIV 1V/DIV 1V/DIV
5V/DIV
2s/DIV
1686/87 F16
Figure 16. Intermediate Signals of a 1s Pulse
goes above or below the rails. It is advisable to terminate the PC traces when approaching maximum speeds. Since the LTC1686/LTC1687 are not intended to drive parallel terminated cables with characteristic impedances much less than that of twisted pair, both ends of the PC trace must be series terminated with the characteristic impedance of the trace. For best results, the signal should be routed differentially. The true and complement outputs of the LTC1686/LTC1687 should be routed on adjacent layers of the PC board. The two traces should be routed very symmetrically, minimizing and equalizing parasitics to nearby signal and power/ground layers. For single-ended transmission, route the series terminated single-ended trace over an adjacent ground plane. Then set the (bypassed) negative input of the receiver to roughly 2.5V. Note that single-ended operation might not reach maximum speeds. LAYOUT CONSIDERATIONS A ground plane is recommended when using high frequency devices like the LTC1686/LTC1687. A 0.1F ceramic bypass capacitor less than 0.25 inch away from the VDD pin is also recommended.
11
LTC1686/LTC1687
APPLICATIONS INFORMATION
Long traces bounded by a VDD and/or GND planes can add substantial parasitic capacitance. Parasitic capacitances on the receiver/driver outputs can also unduly slow down both the propagation delay and the rise/fall times. The receiver inputs are high bandwidth and high impedance. If they are left floating, any capacitive coupling from any other signal can cause a glitch at the receiver output. Thus, if the receiver is not being used, it is advisable to always ground at least one of the two receiver input pins.
PACKAGE DESCRIPTION
Dimensions in inches (millimeters) unless otherwise noted.
S8 Package 8-Lead Plastic Small Outline (Narrow 0.150)
(LTC DWG # 05-08-1610)
0.189 - 0.197* (4.801 - 5.004) 0.010 - 0.020 x 45 (0.254 - 0.508) 0.008 - 0.010 (0.203 - 0.254) 0- 8 TYP 0.228 - 0.244 (5.791 - 6.197) 0.150 - 0.157** (3.810 - 3.988) 0.053 - 0.069 (1.346 - 1.752) 0.004 - 0.010 (0.101 - 0.254) 8 7 6 5
0.016 - 0.050 0.406 - 1.270
0.014 - 0.019 (0.355 - 0.483)
*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
S Package 14-Lead Plastic Small Outline (Narrow 0.150)
(LTC DWG # 05-08-1610)
0.337 - 0.344* (8.560 - 8.738) 0.010 - 0.020 x 45 (0.254 - 0.508) 0.008 - 0.010 (0.203 - 0.254) 0.053 - 0.069 (1.346 - 1.752) 0 - 8 TYP 0.228 - 0.244 (5.791 - 6.197) 0.150 - 0.157** (3.810 - 3.988) 0.004 - 0.010 (0.101 - 0.254) 14 13 12 11 10 9 8
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 0.016 - 0.050 0.406 - 1.270 0.014 - 0.019 (0.355 - 0.483)
RELATED PARTS
PART NUMBER LTC490 LTC491 LTC1518 LTC1519 LTC1520 LTC1685 DESCRIPTION Low Power RS485 Full-Duplex Transceiver Low Power RS485 Full-Duplex Transceiver High Speed Quad RS485 Receiver High Speed Quad RS485 Receiver High Speed Quad Differential Receiver High Speed RS485 Transceiver COMMENTS ICC = 300A (Typ), SO-8 Package ICC = 300A (Typ), 14-Lead SO Package 52Mbps, Pin Compatible with LTC488 52Mbps, Pin Compatible with LTC489 52Mbps, 100mV Threshold, Rail-to-Rail Common Mode 52Mbps, Pin Compatible with LTC485
16867f LT/TP 1197 4K * PRINTED IN THE USA
12
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417 q (408) 432-1900 FAX: (408) 434-0507q TELEX: 499-3977 q www.linear-tech.com
U
U
W
U
U
0.050 (1.270) TYP
SO8 0996
1
2
3
4
S14 0695
1
2
3
4
5
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(c) LINEAR TECHNOLOGY CORPORATION 1997

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