LTC1685 52Mbps, Precision Delay, RS485 Fail-Safe Transceiver U DESCRIPTIO 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 Receiver Operation Over the Entire Common Mode Range High Receiver 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 LTC485 45dB CMRR at 26MHz U APPLICATIO S ■ ■ ■ ■ High Speed RS485/RS422 Transceivers Level Translator Backplane Transceiver STS-1/OC-1 Data Transceiver Fast-20, Fast-40 SCSI Transceivers 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 LTC1685 RS485 transceiver guarantees receiver failsafe operation over the entire common mode range (– 7V to 12V). Input resistance will remain ≥ 22k when the device is unpowered or disabled. The LTC1685 operates from a single 5V supply and draws only 7mA of supply current. , LTC and LT are registered trademarks of Linear Technology Corporation. U ■ The LTC®1685 is a high speed, precision delay RS485 transceiver that can operate at data rates as high as 52Mbps. The device also meets the requirements of RS422. TYPICAL APPLICATION RO1 R 10Mbps Data Pulse 400ft Category 5 UTP VCC1 RE1 DI1 2V/DIV Rt DE1 D DRIVER INPUT CABLE DELAY GND1 1V/DIV RECEIVER INPUT 5V/DIV RECEIVER OUTPUT Rt RO2 R VCC2 RE2 DE2 DI2 D GND2 1685 TA01 100ns/DIV 1685 TA02 1 LTC1685 U U RATI GS W W W W AXI U U ABSOLUTE PACKAGE/ORDER I FOR ATIO (Note 1) 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 Short-Circuit Duration (Driver VOUT: – 7V to 10V, Receiver VOUT: 0V to VDD) ............................... Indefinite Operating Temperature Range LTC1685C ............................................... 0°C to 70°C LTC1685I ............................................. –40°C to 85°C Storage Temperature Range ................ – 65°C to 150°C Lead Temperature (Soldering, 10 sec)................. 300°C ORDER PART NUMBER TOP VIEW RO 1 RE 2 DE 3 DI 4 8 VDD 7 B 6 A 5 GND R D LTC1685CS8 LTC1685IS8 S8 PART MARKING S8 PACKAGE 8-LEAD PLASTIC SO 1685 1685I TJMAX = 125°C, θJA = 150°C/ W Consult factory for Military grade parts. DC ELECTRICAL CHARACTERISTICS The ● denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VDD = 5V ± 5%, unless otherwise noted. (Notes 2, 3) SYMBOL PARAMETER CONDITIONS VOD1 Differential Driver Output (Unloaded) IOUT = 0 ● VOD2 Differential Driver Output (With Load) R = 50Ω (RS422) R = 27Ω (RS485), Figure 1 ● Change in Magnitude of Driver Differential Output Voltage for Complementary Output States R = 27Ω or 50Ω, Figure 1 ● VOC Driver Common Mode Output Voltage R = 27Ω or 50Ω, VDD = 5V, Figure 1 ● ∆VOC Change in Magnitude of Driver Common Mode Output Voltage for Complementary Output States R = 27Ω or 50Ω, Figure 1 ● VIH Input High Voltage DE, DI, RE ● VIL Input Low Voltage DE, DI, RE ● IIN1 Input Current DE, DI, RE ● IIN2 Input Current (A, B) VA, VB = 12V, DE = 0, VDD = 0V or 5.25V VA, VB = – 7V, DE = 0, VDD = 0V or 5.25V ● ● – 500 ● – 0.3 ∆VOD MIN 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 ● VOL Receiver Output Low Voltage IOUT = 4mA, VID = – 300mV ● IOZR Three-State (High Impedance) Output Current at Receiver 0.4V ≤ VOUT ≤ 2.4V ● IDD Supply Current No Load, Pins 2, 3, 4 = 0V or VDD ● IOSD1 Driver Short-Circuit Current, VOUT = HIGH VOUT = – 7V or 10V (Note 5) ● 2 TYP 2 1.5 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 µA 500 µA µA 0.3 V 25 mV 4.8 V –1 7 0.4 V 1 µA 12 mA 20 mA LTC1685 DC ELECTRICAL CHARACTERISTICS The ● denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VDD = 5V ±5%, unless otherwise noted. (Notes 2, 3) SYMBOL PARAMETER CONDITIONS MIN TYP MAX IOSD2 Driver Short-Circuit Current, VOUT = LOW VOUT = – 7V or 10V (Note 5) ● 20 mA IOSR Receiver Short-Circuit Current VOUT = 0V or VDD (Note 5) ● 20 mA RIN Input Resistance – 7V ≤ VCM ≤ 12V ● 22 CIN Input Capacitance A, B Inputs, D, DE, RE Open-Circuit Input Voltage, Figure 5 VDD = 5V (Note 4) ● 3.2 kΩ 3 Fail-Safe Time Time to Detect Fail-Safe Condition 3.3 pF 3.4 Receiver Input Common Mode Rejection Ratio VCM = 2.6V, f = 26MHz CLOAD Receiver and Driver Output Load Capacitance (Note 4) 45 dB 500 ● V µs 2 CMRR UNITS pF U SWITCHING CHARACTERISTICS The ● denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VDD = 5V, unless otherwise noted. (Notes 2, 3) SYMBOL PARAMETER tPLH, tPHL Driver Input-to-Output Propagation Delay RDIFF = 54Ω, CL1 = CL2 = 100pF, Figures 3, 5, LTC1685C LTC1685I tSKEW Driver Output A-to-Output B Skew RDIFF = 54Ω, CL1 = CL2 = 100pF, Figures 3, 5 500 ps tr, tf Driver Rise/Fall Time RDIFF = 54Ω, 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 ● 25 50 ns tPLH, tPHL Receiver Input-to-Output Propagation Delay CL = 15pF, Figures 3, 7 LTC1685C LTC1685I ● ● 18.5 18.5 22 25 ns ns tSQD Receiver Skew tPLH – tPHL 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 Maximum Receiver Input Rise/Fall Times (Note 4) ● Package-to-Package Skew Same Temperature (Note 4) Minimum Input Pulse Width VDD = 5V ± 5% (Note 4) LTC1685C LTC1685I ● ● VDD = 5V ± 5% (Note 4) LTC1685C LTC1685I ● ● 52 40 60 50 Mbps Mbps VDD = 5V ± 5% (Note 4) LTC1685C LTC1685I ● ● 26 20 30 25 MHz MHz tPKG-PKG Maximum Data Rate Maximum Input Frequency CONDITIONS ● ● MIN TYP MAX 15 13 18.5 18.5 22 25 15 13 500 ns ns ps 50 ns 2000 ns 1.5 17 20 UNITS ns 19.2 25 ns ns 3 LTC1685 U SWITCHING CHARACTERISTICS 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. 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 = 25°C. U W TYPICAL PERFORMANCE CHARACTERISTICS Receiver Input CMRR Supply Current vs Data Rate 59 BOTH DRIVER AND RECEIVER ENABLED AND LOADED 60 T = 25°C A 45.5 45.0 44.5 44.0 43.5 43.0 58 SUPPLY CURRENT (mA) 46.0 50 40 30 20 10 42.5 TA = 25°C 100k FREQUENCY (Hz) 1 1M 10 20 30 DATA RATE (Mbps) Receiver Propagation Delay vs Load Capacitance 25 TA = 25°C PROPAGATION DELAY (ns) PROPAGATION DELAY (ns) 25 20 15 10 5 105 25 35 55 LOAD CAPACITANCE (pF) 205 1685 G04 4 53 52 BOTH DRIVER AND RECEIVER ENABLED AND LOADED 25Mbps DATA RATE – 25 0 25 50 TEMPERATURE (°C) 75 100 1685 G03 Receiver Propagation Delay vs Input Overdrive Receiver Propagation Delay vs Common Mode 30 15 55 54 1685 G02 1685 G01 5 56 50 –50 50 40 25 TA = 25°C RECEIVER PROPAGATION DELAY (ns) 1k 10 57 51 0 42.0 0 Supply Current vs Temperature 70 SUPPLY CURRENT (mA) COMMON MODE REJECTION RATIO (dB) 46.5 20 15 10 5 TA = 25°C 20 15 10 5 0 0 8 10 –7 –4 –2 0 4 6 2 RECEIVER COMMON MODE (V) 12 1685 G05 0.3 0.5 0.7 1.0 1.25 1.5 2.0 RECEIVER INPUT OVERDRIVE (V) 2.5 1685 G06 LTC1685 U W TYPICAL PERFORMANCE CHARACTERISTICS Receiver Propagation Delay vs Temperature Receiver Maximum Data Rate vs Input Overdrive 25 Driver Propagation Delay vs Temperature 25 70 TA = 25°C 15 10 PROPAGATION DELAY (ns) DATA RATE (Mbps) PROPAGATION DELAY (ns) 60 20 50 40 30 20 5 –25 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) 1680 G09 0 – 40 – 20 2.5 20 40 60 0 TEMPERATURE (°C) 80 100 1685 G07 Driver Propagation Delay vs Capacitive Load 19.0 25 TA = 25°C VDD = 5V INPUT THRESHOLD = 1.5V TA = 25°C tHL 18.5 PROPAGATION DELAY (ns) PROPAGATION DELAY (ns) 10 1685 G10 Driver Propagation Delay vs Driver Input Voltage 20 15 5 10 0 –50 20 tLH 15 10 5 18.0 17.5 17.0 16.5 0 2.5 16.0 3.0 4.0 4.5 3.5 DRIVER INPUT VOLTAGE (V) 5.0 5 15 100 25 50 75 LOAD CAPACITANCE (pF) 150 1685 G11 1685 G08 U U U PIN FUNCTIONS RO (Pin 1): Receiver Output. If A ≥ B by 300mV, then RO will be high. If A ≤ B by 300mV, then RO will be low. RE (Pin 2): Receiver Enable. RE = Low enables the receiver. RE = High forces receiver output into high impedance state. Do not float. DE (Pin 3): Driver Enable. DE = High enables the driver. DE = Low will force the driver output into a high impedance state and the device will function as a line receiver if RE is also low. Do not float. DI (Pin 4): Driver Input. Controls the states of the A and B outputs only if DE = High. If DE = Low, DI will have no effect on A and B pins. Do not float. GND (Pin 5): Ground. A (Pin 6): Noninverting Receiver Input/Driver Output. B (Pin 7): Inverting Receiver Input/Driver Output. VDD (Pin 8): Positive Supply, 5V to ±5%. Bypass with 0.1µF ceramic capacitor. 5 LTC1685 U U FU CTIO TABLES Receiving Transmitting RE INPUTS DE DI LINE CONDITION B A RE INPUTS DE A–B OUTPUT RO X 1 1 No Fault 0 1 0 0 ≥ 300mV 1 X 1 0 No Fault 1 0 0 0 ≤ – 300mV 0 X 0 X X Hi-Z Hi-Z 0 0 Inputs Open 1 X 1 X Fault 0 0 Inputs Shorted Together A = B = – 7V to 12V 1 1 X X Hi-Z OUTPUTS ±10mA Current Source TEST CIRCUITS A RECEIVER OUTPUT 1k VDD VOD R S1 TEST POINT R CL 15pF VOC 1k S2 1685 F02 B 1685 F01 Figure 2. Driver DC Test Load Figure 1. Driver DC Test Load 3V DE CL1 A DI RDIFF B A S1 RO B CL2 OUTPUT UNDER TEST RE 15pF VDD 500Ω S2 CL 1685 F04 1685 F03 Figure 3. Driver/Receiver Timing Test Circuit 6 Figure 4. Driver Timing Test Load #2 LTC1685 U W W SWITCHI G TI E WAVEFOR S 3V f = 1MHz, t r ≤ 3ns, t f ≤ 3ns 1.5V DI 1.5V 0V t PLH 1/2 VO t PHL B VO A tSKEW 1/2 VO VO 0V –VO t SKEW 90% 90% VDIFF = V(A) – V(B) 10% tr 10% 1586 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 A, B t LZ 2.5V OUTPUT NORMALLY LOW 0.5V 2.5V OUTPUT NORMALLY HIGH 0.5V VOL VOH A, B 0V t HZ t ZH 1686 F06 Figure 6. Driver Enable and Disable Times VOH 2.5V RO VOL f = 1MHz, t r ≤ 3ns, t f ≤ 3ns t PHL VOD2 A–B –VOD2 0V 2.5V OUTPUT t PLH INPUT 1686 F07 Figure 7. Receiver Propagation Delays 3V 1.5V RE f = 1MHz, t r ≤ 3ns, t f ≤ 3ns 1.5V 0V 5V t ZL RO RO t LZ 2.5V OUTPUT NORMALLY LOW 0.5V 2.5V OUTPUT NORMALLY HIGH 0.5V 0V t ZH t HZ 1685 F08 Figure 8. Receiver Enable and Disable Times 7 LTC1685 U U U EQUIVALENT INPUT NETWORKS ≥22k ≥22k A 3.3V A ≥22k ≥22k B B 3.3V DE = 0, RE = 0 OR 1 VDD = 5V VDD = 0V 1685 F09 Figure 9. Input Thevenin Equivalent U W U U APPLICATIONS INFORMATION Theory of Operation Fail-Safe Features 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 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 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! 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 recommended data transmission rate is 26Mbps to avoid timing errors due to clock distortion. 8 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. LTC1685 U U W U APPLICATIONS INFORMATION Output Short-Circuit Protection The LTC1685 employs 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 VDD/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 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 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 automatically 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 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 transceiver. 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. 2V/DIV RO 1685 F11 Figure 11a. 100ft of Category 5 UTP: 50Mbps DRIVER INPUT CABLE DELAY RE 2 1 1 7 DI RECEIVER OUTPUT 10ns/DIV 2V/DIV RE 2 DRIVER INPUT 2V/DIV 4 3 DE LTC1685 6 RO 7 100Ω 100Ω EN EN A 1 4 2 B 1/4 LTC1518 12 4 6 3 DE 2V/DIV RECEIVER INPUT 5V/DIV RECEIVER OUTPUT DI LTC1685 3 RO 1685 F10b 20ns/DIV 1685 F11b Figure 10 Figure 11b. 100ft of Category 5 UTP: 20ns Pulse 9 LTC1685 U W U U APPLICATIONS INFORMATION 2V/DIV DRIVER INPUT 2V/DIV 1V/DIV RECEIVER INPUT 5V/DIV RECEIVER OUTPUT RECEIVER OUTPUT 2V/DIV 1µs/DIV 100ns/DIV 1685 F14a 1685 F12 Figure 12. 1000ft of Category 5 UTP: 4Mbps Figure 14a. 4000ft of Category 5 UTP: 1µs Pulse DRIVER INPUT 2V/DIV 2V/DIV DIFFERENTIAL RECEIVER INPUT 2V/DIV RECEIVER OUTPUT 20ns/DIV 1685 F13 Figure 13. 100ft of Telephone Grade UTP: 30Mbps Very inexpensive unshielded telephone grade twisted pair is shown in Figure 13. In spite of the noticeable loss at the receiver input, the LTC1685 can still transfer 30Mbps at 100ft of telephone grade UTP. Note that under all these conditions, the LTC1685 can pass through a single data pulse equal to the inverse of the data rate (e.g., 20ns for 50Mbps data rate). Even at distances of 4000ft, 1Mbps data rates are possible using the LTC1685 and category 5 UTP. Figure 14a shows a 1µs pulse propagated down 4000ft of category 5 UTP. Notice both the DC and the AC losses at the receiver input. The DC attenuation is due to the parasitic resistance of the cable. Figure 14b shows a 1Mbps square wave over 4000ft. To transmit at this speed but using longer cable lengths, see the LTC1686/LTC1687 high speed RS485 full-duplex transceivers. 10 DRIVER INPUT CABLE DELAY 2V/DIV DRIVER INPUT 5V/DIV RECEIVER OUTPUT 1µs/DIV 1685 F14b Figure 14b. 4000ft of Category 5 UTP: 1Mbps Square Wave High Speed Backplane Transmission The LTC1685 can also be used in backplane point-to-point transceiver applications, where the user wants to assure operation even when the common mode goes above or below the rails. It is advisable to terminate the PC traces when approaching maximum speeds. Since the LTC1685 is 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 LTC1685 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 LTC1685 U W U U APPLICATIONS INFORMATION 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. High Speed Differential SCSI (Fast-20, Fast-40 HVD) The LTC1685’s high speed, tight propagation delay window and matched driver/receiver propagation delays make it a natural choice as the external transceiver in high speed differential SCSI applications. Note that the ±3.5ns propagation delay window covers the entire commercial temperature range. If, for example, a group of 16 transceivers is placed on the same board, their temperature difference will be much smaller. Hence, the difference in their propagation delays should be even better than the ±3.5ns specification (typically better than ±2ns). The LTC1685 is the most efficient and reliable implementation that meets the Fast-20 and Fast-40 HVD driver and receiver skew specifications. Power-Up Requirements The LTC1685 has unique short-circuit protection that shuts off the big output devices (and keeps them off) when a short is detected. When the LTC1685 is powered up with the driver outputs enabled (Figure 15 shows a typical connection), the part will power up in short-circuit mode. After power-up, the user must hold the DE pin of the LTC1685 low for at least 200ns in order to start normal operation. Note also that turning the termination power on/off might induce the LTC1685 to see a “short.” Consequently, the DE pin should be held low for 200ns after cable termination power is turned on. This requirement is solely due to the cable termination (the 165Ω parallel resistance to both power and ground). For applications whose connections to the cable are made exclusively with RS485 devices, the cable can be terminated only across the two signal wires (as in Figure 10). With cable distances covering under 25 meters, the common mode range of the LTC1685 should be more than sufficient to account for any ground differences between any two communicating devices. The fact that transmission is differential should greatly improve noise TERM POWER RE TERM POWER 2 RO 1 1 330Ω 122Ω CABLE 4 3 DE 6 LTC1685 RO 330Ω 7 DI RE 2 7 150Ω 150Ω 330Ω EN EN 4 A 1 2 B 330Ω 1/4 LTC1518 12 4 6 DI 3 DE LTC1685 3 RO 1685 F15 Figure 15. Fast-20, Fast-40 Differential SCSI Application margin. Furthermore, the good high frequency CMRR of the receiver will serve to reject any common mode interference. DE, DI Inputs It is not necessary that the driver input (DI) have 0V to 3V signal levels. The DI input can be driven by CMOS levels (0V to 5V) and still achieve 40Mbps operation. However, duty cycle will be slightly compromised when driven by a CMOS device. Care should be taken to minimize the ringing on the DI input in order to achieve a driver propagation delay within the ±3.5ns window. This also improves the package-to-package matching of propagation delays. The DE pin should be held low for 200ns after the powerup sequence has been completed. After fault conditions such as an output short or thermal shutdown, the DE pin should be held low for at least 200ns after the fault has been removed. This is usually necessary only if the driver outputs are connected to DC-biased cable terminations (as in Figure 15). Layout Considerations A ground plane is recommended when using a high frequency device like the LTC1685. A 0.1µF ceramic bypass capacitor less than 1/4 inch away from the VDD pin is recommended. Good bypassing is especially needed when operating at maximum frequency or when package-topackage matching is very important. The PC board traces connected to the “A” and “B” outputs must be kept as symmetrical and short as possible to obtain the same 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 LTC1685 U W U U APPLICATIONS INFORMATION parasitic board capacitance. This maintains the good matching characteristics of the low-to-high and high-tolow transitions of the LTC1685. Note that output “A” to output “B” capacitance should also be minimized. If routed adjacent to each other on the same layer, they should be separated by an amount at least as wide as the trace widths. If output “A” and output “B” are routed on different signal planes, they should not be routed directly on top of U PACKAGE DESCRIPTION each other. A trace width’s lateral separation is also recommended. As mentioned before, care should also be taken when routing the “DI” input. To achieve consistent board-toboard propagation delay, the ringing on this signal should be kept below a few hundred millivolts. 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) 8 7 6 5 0.150 – 0.157** (3.810 – 3.988) 0.228 – 0.244 (5.791 – 6.197) 1 0.010 – 0.020 × 45° (0.254 – 0.508) 0.008 – 0.010 (0.203 – 0.254) 2 3 4 0.053 – 0.069 (1.346 – 1.752) 0.004 – 0.010 (0.101 – 0.254) 0°– 8° TYP 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 0.050 (1.270) BSC SO8 0695 RELATED PARTS PART NUMBER DESCRIPTION COMMENTS LTC1485 High Speed RS485 Transceiver 10Mbps, Pin Compatible with LTC485 LTC1518/LTC1519 High Speed Quad RS485 Receivers 52Mbps, Pin Compatible with LTC488/LTC489 LTC1520 High Speed Quad Differential Receiver 52Mbps, ±100mV Threshold, Rail-to-Rail Common Mode LTC1686/LTC1687 High Speed RS485 Driver/Receiver 52Mbps, Pin Compatible with LTC490/LTC491 LTC1688/LTC1689 High Speed Quad RS485 Drivers 100Mbps, Pin Compatible with LTC486/LTC487 12 Linear Technology Corporation 1685fa LT/LCG 0700 2K REV A • PRINTED IN THE USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408)432-1900 ● FAX: (408) 434-0507 ● www.linear-tech.com LINEAR TECHNOLOGY CORPORATION 1997