LTC1686/LTC1687 52Mbps Precision Delay RS485 Fail-Safe Transceivers U DESCRIPTION FEATURES 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 U APPLICATIONS High Speed RS485/RS422 Full Duplex Transceivers Level Translator Backplane Transceiver STS-1/OC-1 Data Transceiver Signal Repeaters 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. U TYPICAL APPLICATION 10Mbps Data Pulse 400 Feet Category 5 UTP LTC1686 LTC1686 D 3 DRIVER DRIVER INPUT 2V/DIV 5 CABLE DELAY 100 100 6 RECEIVER R 1V/DIV RECEIVER INPUT 5V/DIV RECEIVER OUTPUT 8 R 2 RECEIVER 100 100 7 DRIVER D 400 FT OF CATEGORY 5 UTP 100ns/DIV 1686/87 TA02 LTC1686/87 * TA01 1 LTC1686/LTC1687 W W W AXI U U ABSOLUTE RATI GS (Note 1) Driver Short-Circuit Duration (VOUT: - 7V to 10V) ..................................... Indefinite Receiver Short-Circuit Duration (VOUT: 0V to VDD) ........................................ Indefinite Operating Temperature Range LTC1686C/LTC1687C ............................. 0C to 70C LTC1686I/LTC1687I .......................... - 40C to 85C 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 U W U PACKAGE/ORDER I FOR ATIO TOP VIEW VDD 1 R 2 8 R D 3 7 A LTC1686CS8 LTC1686IS8 B 6 Z 5 Y D GND 4 TOP VIEW ORDER PART NUMBER S8 PACKAGE 8-LEAD PLASTIC SO S8 PART MARKING TJMAX = 125C, JA = 150C/ W NC 1 R 13 NC R 2 RE 3 12 A DE 4 11 B LTC1687CS LTC1687IS 10 Z D 5 GND 6 9 Y GND 7 8 NC 1686 1686I ORDER PART NUMBER 14 VDD D S PACKAGE 14-LEAD PLASTIC SO TJMAX = 125C, JA = 90C/ W Consult factory for Industrial and Military grade parts. DC ELECTRICAL CHARACTERISTICS VDD = 5V 5% unless otherwise noted (Notes 2, 3). SYMBOL PARAMETER CONDITIONS VOD1 Differential Driver Output (Unloaded) IOUT = 0 MIN VOD2 Differential Driver Output (With Load) R = 50 (RS422) R = 27 (RS485), Figure 1 2.0 1.5 VOD Change in Magnitude of Driver Differential Output Voltage for Complementary Output States R = 27 or 50, Figure 1 VOC Driver Common Mode Output Voltage R = 27 or 50, VDD = 5V, Figure 1 VOC Change in Magnitude of Driver Common Mode Output Voltage for Complementary Output States R = 27 or 50, Figure 1 VIH Input High Voltage D, DE, RE VIL Input Low Voltage D, DE, RE IIN1 Input Current D, DE, RE IIN2 Input Current (A, B) VA, VB = 12V, VDD = 0V or 5.25V VA, VB = - 7V, VDD = 0V or 5.25V - 500 - 0.3 VTH Differential Input Threshold Voltage for Receiver - 7V VCM 12V VTH Receiver Input Hysteresis VCM = 0V VOH Receiver Output High Voltage IOUT = - 4mA, VID = 300mV 2 TYP 2 MAX UNITS VDD V VDD V V 0.2 V 3 V 0.2 V 2 V 0.8 -1 3.5 V 1 A 500 A A 0.3 V 25 mV 4.8 V LTC1686/LTC1687 DC ELECTRICAL CHARACTERISTICS VDD = 5V 5% unless otherwise noted (Notes 2, 3). SYMBOL PARAMETER CONDITIONS VOL Receiver Output Low Voltage IOUT = 4mA, VID = - 300mV IOZR Three-State (High Impedance) Output Current at Receiver 0.4V VOUT 2.4V IOZD Three-State (High Impedance) Output Current at Driver VOUT = - 7V to 12V CLOAD Receiver and Driver Output Load Capacitance (Note 4) IDD Supply Current No Load, Pins D, DE, RE = 0V or VDD IOSD1 Driver Short-Circuit Current, VOUT = HIGH VOUT = - 7V or 10V (Note 5) IOSD2 Driver Short-Circuit Current, VOUT = LOW IOSR Receiver Short-Circuit Current RIN Input Resistance CIN Input Capacitance A, B, D, DE, RE Inputs (Note 4) Open-Circuit Input Voltage VDD = 5V (Note 4), Figure 5 Fail-Safe Time Time to Detect Fail-Safe Condition CMRR Receiver Input Common Mode Rejection Ratio MIN TYP MAX UNITS 0.4 V -1 1 A - 200 200 A 500 pF 12 mA 20 mA VOUT = - 7V or 10V (Note 5) 20 mA VOUT = 0V or VDD (Note 5) 20 mA - 7V VCM 12V 22 3.2 7 k 3 VCM = 2.5V, f = 26MHz U SWITCHING CHARACTERISTICS pF 3.3 3.4 V 2 s 45 dB VDD = 5V, unless otherwise noted (Notes 2, 3). SYMBOL PARAMETER CONDITIONS tPLH, tPHL Driver Input-to-Output Propagation Delay RDIFF = 54, Figures 3, 5 CL1 = CL2 = 100pF tSKEW Driver Output A-to-Output B Skew RDIFF = 54, CL1 = CL2 = 100pF, Figures 3, 5 500 ps tr, tf Driver Rise/Fall Time RDIFF = 54, CL1 = CL2 = 100pF, Figures 3, 5 3.5 ns tZH Driver Enable to Output High CL = 100pF, S2 Closed, Figures 4, 6 25 50 ns tZL Driver Enable to Output Low CL = 100pF, S1 Closed, Figures 4, 6 25 50 ns tLZ Driver Disable from Low CL = 15pF, S1 Closed, Figures 4, 6 25 50 ns t HZ Driver Disable from High CL = 15pF, S2 Closed, Figures 4, 6 tPLH, tPHL Receiver Input-to-Output Propagation Delay CL = 15pF, Figures 3, 7 tSQD Receiver Skew tPLH - t PHL CL = 15pF, Figures 3, 7 tZL Receiver Enable to Output Low CL = 15pF, S1 Closed, Figures 2, 8 25 50 ns tZH Receiver Enable to Output High CL = 15pF, S2 Closed, Figures 2, 8 25 50 ns tLZ Receiver Disable from Low CL = 15pF, S1 Closed, Figures 2, 8 25 50 ns t HZ Receiver Disable from High CL = 15pF, S2 Closed, Figures 2, 8 25 50 ns Maximum Receiver Input Rise/Fall Times (Note 4) 2000 ns Package-to-Package Skew CL = 15pF, Same Temperature (Note 4) tPKG-PKG LTC1686C/LTC1687C LTC1686I/LTC1687I LTC1686C/LTC1687C LTC1686I/LTC1687I MIN TYP MAX 15 13 18.5 18.5 22 25 15 13 UNITS ns ns 25 50 ns 18.5 18.5 22 25 ns ns 500 1.5 ps ns 3 LTC1686/LTC1687 U SWITCHING CHARACTERISTICS SYMBOL VDD = 5V, unless otherwise noted (Notes 2, 3). PARAMETER CONDITIONS MIN TYP MAX UNITS Minimum Input Pulse Width VDD = 5V 5% (Note 4) LTC1686C/LTC1687C LTC1686I/LTC1687I 17 20 19.2 25 ns ns Maximum Data Rate VDD = 5V 5% (Note 4) LTC1686C/LTC1687C LTC1686I/LTC1687I 52 40 60 50 Mbps Mbps Maximum Input Frequency VDD = 5V 5% (Note 4) LTC1686C/LTC1687C LTC1686I/LTC1687I 26 20 30 25 MHz MHz 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. The 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. U W TYPICAL PERFORMANCE CHARACTERISTICS Receiver Input CMRR Supply Current vs Data Rate 45.0 44.5 44.0 43.5 43.0 57 SUPPLY CURRENT (mA) 45.5 SUPPLY CURRENT (mA) 50 40 30 20 10 42.5 TA = 25C 0 42.0 1k 10 100k FREQUENCY (Hz) 1 1M 10 20 30 DATA RATE (Mbps) 40 Receiver Propagation Delay vs Load Capacitance 25 TA = 25C PROPAGATION DELAY (ns) PROPAGATION DELAY (ns) 25 20 15 10 5 105 25 35 55 LOAD CAPACITANCE (pF) 205 1686/87 G04 4 54 53 52 BOTH DRIVER AND RECEIVER ENABLED AND LOADED 25Mbps DATA RATE 50 - 25 50 0 25 50 TEMPERATURE (C) 75 100 1686/87 G03 Receiver Propagation Delay vs Input Overdrive Receiver Propagation Delay vs Common Mode 30 15 55 1686/87 G02 1686/87 G01 5 56 51 25 TA = 25C RECEIVER PROPAGATION DELAY (ns) COMMON MODE REJECTION RATIO (dB) 58 BOTH DRIVER AND RECEIVER ENABLED AND LOADED 60 T = 25C A 46.0 0 Supply Current vs Temperature 70 46.5 20 15 10 5 TA = 25C 20 15 10 5 0 0 8 10 -7 -4 -2 0 4 6 2 RECEIVER COMMON MODE (V) 12 1686/87 G05 0.3 0.5 0.7 1.0 1.25 1.5 2.0 RECEIVER INPUT OVERDRIVE (V) 2.5 1686/87 G06 LTC1686/LTC1687 U W TYPICAL PERFORMANCE CHARACTERISTICS Receiver Propagation Delay vs Temperature Receiver Maximum Data Rate vs Input Overdrive 25 Driver Propagation Delay vs Temperature 70 25 TA = 25C 15 10 PROPAGATION DELAY (ns) DATA RATE (Mbps) PROPAGATION DELAY (ns) 60 20 50 40 30 20 5 50 25 0 75 TEMPERATURE (C) 100 125 0 0.3 1.5 1.0 0.4 0.5 0.6 0.7 RECEIVER INPUT DIFFERENTIAL (V) 1686/87 G09 0 - 20 2.5 0 20 40 60 TEMPERATURE (C) 80 100 1686/87 G07 Driver Propagation Delay vs Capacitive Load 19.0 25 VDD = 5V INPUT THRESHOLD = 1.5V TA = 25C tHL TA = 25C 18.5 PROPAGATION DELAY (ns) PROPAGATION DELAY (ns) 10 1686/87 G10 Driver Propagation Delay vs Driver Input Voltage 20 15 5 10 0 -50 -25 20 tLH 15 10 5 18.0 17.5 17.0 16.5 16.0 0 2.5 3.0 4.0 4.5 3.5 DRIVER INPUT VOLTAGE (V) 5.0 5 15 100 25 50 75 LOAD CAPACITANCE (pF) 150 1686/87 G11 1686/87 G08 U U U PIN FUNCTIONS LTC1686 VDD (Pin 1): Positive Supply, 5V to 5%. Bypass with 0.1F ceramic capacitor. B (Pin 7): Inverting Receiver Input. 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. LTC1687 NC (Pins 1, 8, 13): No Connection. D (Pin 3): Driver Input. Controls the states of the Y and Z outputs. Do not float. 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. GND (Pin 4): Ground. RE (Pin 3): Receiver Enable. RE = low enables the receiver. RE = high forces receiver output into high impedance state. Do not float. Y (Pin 5): Noninverting Driver Output. A (Pin 8): Noninverting Receiver Input. Z (Pin 6): Inverting Driver Output. 5 LTC1686/LTC1687 U U U 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. 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. Y (Pin 9): Noninverting Driver Output. U U FU CTIO TABLES (LTC1687) Receiving Transmitting RE INPUTS DE D LINE CONDITION Z Y RE INPUTS DE A-B OUTPUT R X 1 1 No Fault 0 1 0 X 300mV 1 X 1 0 No Fault 1 0 0 X - 300mV 0 X 0 X X Hi- Z Hi- Z 0 X Inputs Open 1 X 1 X Fault 0 X Inputs Shorted Together A = B = - 7V to 12V 1 1 X X Hi- Z OUTPUTS 10mA Current Source TEST CIRCUITS Y RECEIVER OUTPUT 1k VDD VOD R S1 TEST POINT R CL 15pF VOC 1k S2 1686/87 F02 Z 1686/87 * F01 Figure 2. Driver DC Test Load Figure 1. Driver DC Test Load 3V DE CL1 Y D RDIFF Z A S1 R B CL2 OUTPUT UNDER TEST RE 15pF VDD 500 S2 CL 1686/87 F04 1686/87 F03 Figure 3. Driver/Receiver Timing Test Circuit 6 Figure 4. Driver Timing Test Load #2 LTC1686/LTC1687 U W W SWITCHI G TI E WAVEFOR S 3V f = 1MHz, t r 3ns, t f 3ns 1.5V D 1.5V 0V t PLH 1/2 VO t PHL Z VO Y tSKEW 1/2 VO VO 0V -VO t SKEW 90% 90% VDIFF = V(Y) - V(Z) 10% 10% tr 1686/87 F05 tf Figure 5. Driver Propagation Delays 3V f = 1MHz, t r 3ns, t f 3ns 1.5V DE 1.5V 0V 5V t ZL Y, Z t LZ 2.5V OUTPUT NORMALLY LOW 0.5V 2.5V OUTPUT NORMALLY HIGH 0.5V VOL VOH Y, Z 0V t HZ t ZH 1686/87 F06 Figure 6. Driver Enable and Disable Times VOH 2.5V R VOL f = 1MHz, t r 3ns, t f 3ns t PHL VOD2 A-B -VOD2 0V 2.5V OUTPUT t PLH INPUT 1686/87 F07 Figure 7. Receiver Propagation Delays 3V 1.5V RE f = 1MHz, t r 3ns, t f 3ns 1.5V 0V 5V t ZL R R t LZ 2.5V OUTPUT NORMALLY LOW 0.5V 2.5V OUTPUT NORMALLY HIGH 0.5V 0V t ZH t HZ 1686/87 F08 Figure 8. Receiver Enable and Disable Times 7 LTC1686/LTC1687 U U EQUIVALE T I PUT NETWORKS 22k A 22k 3.3V A 22k 22k B B 3.3V RE = 0 OR 1, VDD = 5V VDD = 0V 1686/87 F09 Figure 9. Input Thevenin Equivalent U W U U 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 8 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 LTC1686/LTC1687 U U W U 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. 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. 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. 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). 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 HIGH SPEED TWISTED-PAIR TRANSMISSION 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 DE 4 9 D 5 DRIVER 100 100 10 LTC1687 RECEIVER R LTC1687 12 R 2 RECEIVER 3 100 100 11 DRIVER D CATEGORY 5 UTP RE RE LTC1686/87 * F10 Figure 10 9 LTC1686/LTC1687 U W U U APPLICATIONS INFORMATION 2V/DIV DRIVER INPUT 2V/DIV DIFFERENTIAL RECEIVER INPUT 2V/DIV RECEIVER OUTPUT DRIVER INPUT 2V/DIV 2V/DIV RECEIVER OUTPUT 10ns/DIV 20ns/DIV 1686/87 F13 1686/87 F11a Figure 11a. 100 Feet of Category 5 UTP: 50Mbps Figure 13. 100 Feet of Telephone Grade UTP: 30Mbps DRIVER INPUT 2V/DIV 2V/DIV RECEIVER INPUT 1V/DIV RECEIVER INPUT 5V/DIV RECEIVER OUTPUT 5V/DIV RECEIVER OUTPUT 2V/DIV CABLE DELAY 20ns/DIV 1s/DIV 1685 F11b 1685 F14a Figure 11b. 100 Feet of Category 5 UTP: 20ns Pulse 2V/DIV DRIVER INPUT 2V/DIV RECEIVER OUTPUT Figure 14a. 4000 Feet of Category 5 UTP 1s Pulse 2V/DIV DRIVER INPUT 5V/DIV RECEIVER OUTPUT 1s/DIV 100ns/DIV 1685 F14b 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 DRIVER INPUT CABLE DELAY 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 U U W U APPLICATIONS INFORMATION LTC1687 LTC1687 2000 FT D1 2000 FT LTC1687 R2 D LTC1687 R3 2000 FT D R4 D R5 REPEATER REPEATER REPEATER 2000 FT LTC1687 R 1686/87 F15a Figure 15a. 1.6Mbps, 8000 Feet (1.5 Miles) Using Three Repeaters 2V/DIV DRIVER 1 INPUT DELAY OF 8000 FT OF CABLE DRIVER 1 INPUT 2V/DIV RECEIVER 2 INPUT RECEIVER 3 INPUT RECEIVER 4 INPUT 1V/DIV 5V/DIV RECEIVER 5 OUTPUT 2V/DIV DRIVER 1 INPUT 1V/DIV 1V/DIV RECEIVER 5 OUTPUT 5V/DIV RECEIVER 5 OUTPUT 5V/DIV 2s/DIV 2s/DIV 1686/87 F15b 1686/87 F16 Figure 15b. 1.6Mbps Pulse and Square Wave Signals Over 8000 Feet Category 5 UTP Using Three Repeaters Figure 16. Intermediate Signals of a 1s Pulse 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. 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. 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. LAYOUT CONSIDERATIONS 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 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. 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. 11 LTC1686/LTC1687 U U W U 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. U PACKAGE DESCRIPTION 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. 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) 0.053 - 0.069 (1.346 - 1.752) 0.004 - 0.010 (0.101 - 0.254) 0.014 - 0.019 (0.355 - 0.483) 0.050 (1.270) TYP 7 8 5 6 0- 8 TYP 0.016 - 0.050 0.406 - 1.270 *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 0.150 - 0.157** (3.810 - 3.988) 0.228 - 0.244 (5.791 - 6.197) SO8 0996 1 3 2 4 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) 0.004 - 0.010 (0.101 - 0.254) 14 13 12 11 10 9 8 0 - 8 TYP 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) 0.228 - 0.244 (5.791 - 6.197) 0.150 - 0.157** (3.810 - 3.988) S14 0695 1 2 3 4 5 6 7 RELATED PARTS PART NUMBER DESCRIPTION COMMENTS LTC490 Low Power RS485 Full-Duplex Transceiver ICC = 300A (Typ), SO-8 Package LTC491 Low Power RS485 Full-Duplex Transceiver ICC = 300A (Typ), 14-Lead SO Package LTC1518 High Speed Quad RS485 Receiver 52Mbps, Pin Compatible with LTC488 LTC1519 High Speed Quad RS485 Receiver 52Mbps, Pin Compatible with LTC489 LTC1520 High Speed Quad Differential Receiver 52Mbps, 100mV Threshold, Rail-to-Rail Common Mode LTC1685 High Speed RS485 Transceiver 52Mbps, Pin Compatible with LTC485 12 Linear Technology Corporation 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 FAX: (408) 434-0507 TELEX: 499-3977 www.linear-tech.com 16867fs, sn16867 LT/TP 1197 4K * PRINTED IN THE USA LINEAR TECHNOLOGY CORPORATION 1997