LTC491 Differential Driver and Receiver Pair U FEATURES DESCRIPTIO Low Power: ICC = 300A Typical Designed for RS485 or RS422 Applications Single 5V Supply -7V to 12V Bus Common Mode Range Permits 7V Ground Difference Between Devices on the Bus Thermal Shutdown Protection Power-Up/-Down Glitch-Free Driver Outputs Permit Live Insertion or Removal of Package Driver Maintains High Impedance in Three-State or with the Power Off Combined Impedance of a Driver Output and Receiver Allows up to 32 Transceivers on the Bus 70mV Typical Input Hysteresis 28ns Typical Driver Propagation Delays with 5ns Skew for 2.5MB Operation Pin Compatible with the SN75180 Available in 14-Lead PDIP and SO Packages U APPLICATIO S The CMOS design offers significant power savings over its bipolar counterpart without sacrificing ruggedness against overload or ESD damage. The driver and receiver feature three-state outputs, with the driver outputs maintaining high impedance over the entire common mode range. Excessive power dissipation caused by bus contention or faults is prevented by a thermal shutdown circuit which forces the driver outputs into a high impedance state. The receiver has a fail safe feature which guarantees a high output state when the inputs are left open. Both AC and DC specifications are guaranteed from 0C to 70C and 4.75V to 5.25V supply voltage range. , LTC and LT are registered trademarks of Linear Technology Corporation. Low Power RS485/RS422 Transceiver Level Translator U The LTC(R)491 is a low power differential bus/line transceiver designed for multipoint data transmission standard RS485 applications with extended common mode range (12V to -7V). It also meets the requirements of RS422. TYPICAL APPLICATIO DE DE 4 9 D 5 DRIVER 120 120 10 RECEIVER R 4000 FT 24 GAUGE TWISTED PAIR LTC491 LTC491 12 R 2 RECEIVER 120 120 11 DRIVER D 4000 FT 24 GAUGE TWISTED PAIR 3 REB REB LTC491 * TA01 491fa 1 LTC491 U RATI GS W W W Supply Voltage (VCC) ............................................... 12V Control Input Voltages .................... -0.5V to VCC + 0.5V Control Input Currents .......................... -50mA to 50mA Driver Input Voltages ...................... -0.5V to VCC + 0.5V Driver Input Currents ............................ -25mA to 25mA Driver Output Voltages .......................................... 14V Receiver Input Voltages ......................................... 14V Receiver Output Voltages ............... -0.5V to VCC + 0.5V Operating Temperature Range LTC491C ................................................. 0C to 70C LTC491I .............................................. - 40C to 85C Storage Temperature Range ................. - 65C to 150C Lead Temperature (Soldering, 10 sec).................. 300C U AXI U W (Note 1) U ABSOLUTE PACKAGE/ORDER I FOR ATIO ORDER PART NUMBER TOP VIEW NC 1 14 VCC R 13 NC R 2 REB 3 12 A DE 4 11 B D 5 GND 6 9 Y GND 7 8 NC LTC491CN LTC491CS LTC491IN LTC491IS 10 Z D N PACKAGE 14-LEAD PDIP S PACKAGE 14-LEAD PLASTIC SO TJMAX = 100C, JA = 90C/W (N) TJMAX = 100C, JA = 110C/W (S) Consult LTC Marketing for parts specified with wider operating temperature ranges. DC ELECTRICAL CHARACTERISTICS The denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. VCC = 5V 5% SYMBOL PARAMETER CONDITIONS MIN VOD1 Differential Driver Output Voltage (Unloaded) IO = 0 VOD2 Differential Driver Output Voltage (With load) R = 50; (RS422) 2 R = 27; (RS485) (Figure 1) 1.5 R = 27 or R = 50 (Figure 1) TYP MAX 5 UNITS V V 5 V 0.2 V VOD Change in Magnitude of Driver Differential Output Voltage for Complementary Output States VOC Driver Common Mode Output Voltage 3 V VOC Change in Magnitude of Driver Common Mode Output Voltage for Complementary Output States 0.2 V VIH Input High Voltage VIL Input Low Voltage lIN1 Input Current lIN2 Input Current (A, B) D, DE, REB 2.0 V 0.8 VCC = 0V or 5.25V V 2 A VIN = 12V 1.0 mA VIN = -7V - 0.8 mA VTH Differential Input Threshold Voltage for Receiver - 7V VCM 12V VTH Receiver Input Hysteresis VCM = 0V VOH Receiver Output High Voltage IO = -4mA, VID = 0.2V VOL Receiver Output Low Voltage IO = 4mA, VID = -0.2V 0.4 V IOZR Three-State Output Current at Receiver VCC = Max 0.4V VO 2.4V 1 A ICC Supply Current No Load; D = GND, or VCC Outputs Enabled 300 500 A Outputs Disabled 300 500 RIN Receiver Input Resistance - 7V VCM 12V IOSD1 Driver Short Circuit Current, VOUT = High VO = - 7V IOSD2 Driver Short Circuit Current, VOUT = Low VO = 12V IOSR Receiver Short Circuit Current 0V VO VCC IOZ Driver Three-State Output Current VO = - 7V to 12V - 0.2 0.2 70 V mV 3.5 V 12 A k 100 250 mA 100 250 mA 85 mA 200 A 7 2 491fa 2 LTC491 U SWITCHI G CHARACTERISTICS The denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. VCC = 5V 5% SYMBOL PARAMETER CONDITIONS tPLH Driver Input to Output RDIFF = 54, CL1 = CL2 = 100pF (Figures 2, 5) tPHL Driver Input to Output tSKEW tr, tf Driver Output to Output Driver Rise or Fall Time tZH Driver Enable to Output High tZL tLZ MIN TYP MAX 10 30 50 ns 10 30 50 ns 5 5 15 25 ns ns UNITS CL = 100pF (Figures 4, 6) S2 Closed 40 70 ns Driver Enable to Output Low CL = 100pF (Figures 4, 6) S1 Closed 40 70 ns Driver Disable Time From Low CL = 15pF (Figures 4, 6) S1 Closed 40 70 ns tHZ Driver Disable Time From High CL = 15pF (Figures 4, 6) S2 Closed 40 70 ns tPLH Receiver Input to Output 40 70 150 ns Receiver Input to Output RDIFF = 54, CL1 = CL2 = 100pF (Figures 2, 7) tPHL 40 70 150 ns tSKD tPLH - tPHL Differential Receiver Skew tZL Receiver Enable to Output Low tZH Receiver Enable to Output High tLZ tHZ 13 CL = 15pF (Figures 3, 8) S1 Closed 20 50 ns CL = 15pF (Figures 3, 8) S2 Closed 20 50 ns Receiver Disable From Low CL = 15pF (Figures 3, 8) S1 Closed 20 50 ns Receiver Disable From High CL = 15pF (Figures 3, 8) S2 Closed 20 50 ns Note 1: Absolute Maximum Ratings are those values beyond which the life of the device may be impaired. Note 2: All currents into device pins are positive; all currents out of device pins are negative. All voltages are referenced to device ground unless otherwise specified. ns Note 3: All typicals are given for VCC = 5V and temperature = 25C. U U U PI FU CTIO S NC (Pin 1): Not Connected. GND (Pin 6): Ground Connection. R (Pin 2): Receiver Output. If the receiver output is enabled (REB low), then if A > B by 200mV, R will be high. If A < B by 200mV, then R will be low. GND (Pin 7): Ground Connection. REB (Pin 3): Receiver Output Enable. A low enables the receiver output, R. A high input forces the receiver output into a high impedance state. DE (Pin 4): Driver Output Enable. A high on DE enables the driver outputs, Y and Z. A low input forces the driver outputs into a high impedance state. D (Pin 5): Driver Input. If the driver outputs are enabled (DE high), then a low on D forces the driver outputs Y low and Z high. A high on D will force Y high and Z low. NC (Pin 8): Not Connected. Y (Pin 9): Driver Output. Z (Pin 10): Driver Output. B (Pin 11): Receiver Input. A (Pin 12): Receiver Input. NC (Pin 13): Not Connected. VCC (Pin 14): Positive Supply; 4.75V VCC 5.25V. 491fa 3 LTC491 U W TYPICAL PERFOR A CE CHARACTERISTICS Driver Output High Voltage vs Output Current, TA = 25C Driver Differential Output Voltage vs Output Current, TA = 25C OUTPUT CURRENT (mA) OUTPUT CURRENT (mA) -72 - 48 -24 80 OUTPUT CURRENT (mA) 64 -96 48 32 16 0 0 0 1 2 3 1 2 3 4.0 1.59 1.57 3.0 1.0 -50 100 TEMPERATURE (C ) 0 50 330 320 310 -50 100 6.0 TEMPERATURE (C ) 5.0 3.0 -50 100 LTC491 * TPC06 0.8 0.6 0.4 0.2 0 50 100 TEMPERATURE (C ) LTC491 * TPC07 50 Receiver Output Low Voltage vs Temperature at I = 8mA 4.0 100 0 TEMPERATURE (C ) OUTPUT VOLTAGE (V) 2.1 TIME (ns) 7.0 50 340 Receiver tPLH tPHL vs Temperature 2.3 4 Supply Current vs Temperature LTC491 * TPC05 Driver Differential Output Voltage vs Temperature, RO = 54 0 3 LTC491 * TPC03 TEMPERATURE (C ) LTC491 * TPC04 1.7 2 350 2.0 1.9 1 OUTPUT VOLTAGE (V) SUPPLY CURRENT (A) 1.61 TIME (ns) INPUT THRESHOLD VOLTAGE (V) 5.0 1.5 -50 0 4 Driver Skew vs Temperature 1.63 50 20 LTC491 * TPC02 TTL Input Threshold vs Temperature 0 40 OUTPUT VOLTAGE (V) LTC491 * TPC01 1.55 -50 60 0 0 4 OUTPUT VOLTAGE (V) DIFFERENTIAL VOLTAGE (V) Driver Output Low Voltage vs Output Current, TA = 25C 0 -50 0 50 100 TEMPERATURE (C ) LTC491 * TPC08 LTC491 * TPC09 491fa 4 LTC491 TEST CIRCUITS Y R VOD2 R VOC Z LTC491 * F01 Figure 1. Driver DC Test Load CL1 Y D A RDIFF DRIVER RECEIVER Z CL2 B R 15pF LTC491 * F02 Figure 2. Driver/Receiver Timing Test Circuit S1 RECEIVER OUTPUT 1k VCC CL 1k S2 LTC491 * F03 Figure 3. Receiver Timing Test Load S1 VCC OUTPUT UNDER TEST 500 CL S2 LTC491 * F04 Figure 4. Driver Timing Test Load 491fa 5 LTC491 U W W SWITCHI G TI E WAVEFOR S 3V D f = 1MHz : tr 10ns : tf 10ns 1.5V 1.5V 0V tPHL tPLH VO 80% 50% 10% -VO 90% VDIFF = V(Y) - V(Z) tr 50% 20% tf Z VO Y tSKEW 1/2 VO tSKEW 1/2 VO LTC491 * F05 Figure 5. Driver Propagation Delays 3V DE f = 1MHz : tr 10ns : tr 10ns 1.5V 1.5V 0V tZL tLZ 5V A, B VOL 2.3V OUTPUT NORMALLY LOW 2.3V OUTPUT NORMALLY HIGH 0.5V VOH A, B 0.5V 0V tZH LTC491 * F06 tHZ Figure 6. Driver Enable and Disable Times INPUT VOD2 A-B -VOD2 f = 1MHz ; tr 10ns : tf 10ns 0V tPHL tPLH VOH R 0V OUTPUT 1.5V 1.5V VOL LTC491 * F07 Figure 7. Receiver Propagation Delays 3V REB f = 1MHz : tr 10ns : tf 10ns 1.5V 1.5V 0V tZL tLZ 5V R 1.5V VOL OUTPUT NORMALLY LOW 0.5V VOH R 1.5V 0V 0.5V OUTPUT NORMALLY HIGH tZH tHZ LTC491 * F08 Figure 8. Receiver Enable and Disable Times 491fa 6 LTC491 U W U U APPLICATIO S I FOR ATIO Typical Application A typical connection of the LTC491 is shown in Figure 9. Two twisted-pair wires connect up to 32 driver/receiver pairs for full duplex data transmission. There are no restrictions on where the chips are connected to the wires, and it isn't necessary to have the chips connected at the ends. However, the wires must be terminated only at the ends with a resistor equal to their characteristic impedance, typically 120. The input impedance of a receiver is typically 20k to GND, or 0.6 unit RS-485 load, so in practice 50 to 60 transceivers can be connected to the same wires. The optional shields around the twisted pair help reduce unwanted noise, and are connected to GND at one end. outputs of the driver are accidently shorted to a power supply or low impedance source, up to 250mA can flow through the part. The thermal shutdown circuit disables the driver outputs when the internal temperature reaches 150C and turns them back on when the temperature cools to 130C. If the outputs of two or more LTC491 drivers are shorted directly, the driver outputs can not supply enough current to activate the thermal shutdown. Thus, the thermal shutdown circuit will not prevent contention faults when two drivers are active on the bus at the same time. 12 2 RX The LTC491 can also be used as a line repeater as shown in Figure 10. If the cable length is longer than 4000 feet, the LTC491 is inserted in the middle of the cable with the receiver output connected back to the driver input. 3 120 RECEIVER DATA IN 11 4 10 5 DX Thermal Shutdown 120 DRIVER DATA OUT 9 LTC491 LTC491 * F10 The LTC491 has a thermal shutdown feature which protects the part from excessive power dissipation. If the Figure 10. Line Repeater 12 12 RX 2 3 RECEIVER 120 120 11 11 RECEIVER 4 DX 3 RX 4 10 5 2 DRIVER 10 120 120 9 9 LTC491 9 10 11 DRIVER 5 DX LTC491 12 LTC491 RECEIVER DRIVER LTC491 * F09 5 DX 4 3 2 RX Figure 9. Typical Connection 491fa 7 LTC491 W U U UO APPLICATI S I FOR ATIO Cables and Data Rate The transmission line of choice for RS485 applications is a twisted pair. There are coaxial cables (twinaxial) made for this purpose that contain straight pairs, but these are less flexible, more bulky, and more costly than twisted pairs. Many cable manufacturers offer a broad range of 120 cables designed for RS485 applications. Losses in a transmission line are a complex combination of DC conductor loss, AC losses (skin effect), leakage and AC losses in the dielectric. In good polyethylene cables such as the Belden 9841, the conductor losses and dielectric losses are of the same order of magnitude, leading to relatively low over all loss (Figure 11). LOSS PER 100 ft (dB) 10 1.0 When using low loss cables, Figure 12 can be used as a guideline for choosing the maximum line length for a given data rate. With lower quality PVC cables, the dielectric loss factor can be 1000 times worse. PVC twisted pairs have terrible losses at high data rates (>100kBs), and greatly reduce the maximum cable length. At low data rates however, they are acceptable and much more economical. Cable Termination The proper termination of the cable is very important. If the cable is not terminated with it's characteristic impedance, distorted waveforms will result. In severe cases, distorted (false) data and nulls will occur. A quick look at the output of the driver will tell how well the cable is terminated. It is best to look at a driver connected to the end of the cable, since this eliminates the possibility of getting reflections from two directions. Simply look at the driver output while transmitting square wave data. If the cable is terminated properly, the waveform will look like a square wave (Figure 13). If the cable is loaded excessively (47), the signal initially sees the surge impedance of the cable and jumps to an initial amplitude. The signal travels down the cable and is reflected back out of phase because of the mistermination. 0.1 0.1 1.0 10 100 PROBE HERE FREQUENCY (MHZ) LTC491 * F11 Figure 11. Attenuation vs Frequency for Belden 9481 Rt DX DRIVER RECEIVER RX 10k CABLE LENGTH (ft) Rt = 120 1k Rt = 47 100 Rt = 470 10 10k 100k 1M 2.5M 10M DATA RATE (bps) LTC491 * F13 LTC491 * F12 Figure 12. Cable Length vs Data Rate Figure 13. Termination Effects 491fa 8 LTC491 W U U UO APPLICATI S I FOR ATIO When the reflected signal returns to the driver, the amplitude will be lowered. The width of the pedestal is equal to twice the electrical length of the cable (about 1.5ns/foot). If the cable is lightly loaded (470), the signal reflects in phase and increases the amplitude at the driver output. An input frequency of 30kHz is adequate for tests out to 4000 feet of cable. AC Cable Termination Cable termination resistors are necessary to prevent unwanted reflections, but they consume power. The typical differential output voltage of the driver is 2V when the cable is terminated with two 120 resistors, causing 33mA of DC current to flow in the cable when no data is being sent. This DC current is about 60 times greater than the supply current of the LTC491. One way to eliminate the unwanted current is by AC coupling the termination resistors as shown in Figure 14. The coupling capacitor must allow high-frequency energy to flow to the termination, but block DC and low frequencies. The dividing line between high and low frequency depends on the length of the cable. The coupling capacitor must pass frequencies above the point where the line represents an electrical one-tenth wavelength. The value of the coupling capacitor should therefore be set at 16.3pF per foot of cable length for 120 cables. With the coupling capacitors in place, power is consumed only on the signal edges, and not when the driver output is idling at a 1 or 0 state. A 100nF capacitor is adequate for lines up to 4000 feet in length. Be aware that the power savings start to decrease once the data rate surpasses 1/(120 x C). Receiver Open-Circuit Fail-Safe Some data encoding schemes require that the output of the receiver maintains a known state (usually a logic 1) when the data is finished transmitting and all drivers on the line are forced into three-state. The receiver of the LTC491 has a fail-safe feature which guarantees the output to be in a logic 1 state when the receiver inputs are left floating (open-circuit). However, when the cable is terminated with 120, the differential inputs to the receiver are shorted together, not left floating. Because the receiver has about 70mV of hysteresis, the receiver output will tend to maintain the last data bit received, but this is not guaranteed. The termination resistors are used to generate a DC bias which forces the receiver output to a known state; in the case of Figure 15, a logic 0. The first method consumes about 208mW and the second about 8mW. The lowest power solution is to use an AC termination with a pull-up resistor. Simply swap the receiver inputs for data protocols ending in logic 1. 5V 110 130 130 110 RECEIVER RX RECEIVER RX RECEIVER RX 5V 1.5k 140 1.5k 100k 120 C 5V C RECEIVER 120 RX C = LINE LENGTH (ft) x 16.3pF LTC491 * F14 Figure 14. AC Coupled Termination LTC491 * F15 Figure 15. Forcing "O" When All Drivers are Off 491fa 9 LTC491 W U U UO APPLICATI S I FOR ATIO Fault Protection Y All of LTC's RS485 products are protected against ESD transients up to 2kV using the human body model (100pF, 1.5k). However, some applications need more protection. The best protection method is to connect a bidirectional TransZorb(R) from each line side pin to ground (Figure 16). A TransZorb is a silicon transient voltage suppressor that has exceptional surge handling capabilities, fast response time, and low series resistance. They are available from General Semiconductor Industries and come in a variety of breakdown voltages and prices. Be sure to pick a breakdown voltage higher than the common mode voltage 120 DRIVER Z LTC491 * F16 Figure 16. ESD Protection with TransZorbs required for your application (typically 12V). Also, don't forget to check how much the added parasitic capacitance will load down the bus. TransZorb is a registered trademark of General Instruments, GSI U PACKAGE DESCRIPTIO N Package 14-Lead PDIP (Narrow .300 Inch) (Reference LTC DWG # 05-08-1510) .770* (19.558) MAX 14 13 12 11 10 9 8 1 2 3 4 5 6 7 .255 .015* (6.477 0.381) .130 .005 (3.302 0.127) .300 - .325 (7.620 - 8.255) .045 - .065 (1.143 - 1.651) .020 (0.508) MIN .065 (1.651) TYP .008 - .015 (0.203 - 0.381) ( +.035 .325 -.015 +0.889 8.255 -0.381 NOTE: 1. DIMENSIONS ARE ) .120 (3.048) MIN .005 (0.125) .100 MIN (2.54) BSC INCHES MILLIMETERS *THESE DIMENSIONS DO NOT INCLUDE MOLD FLASH OR PROTRUSIONS. MOLD FLASH OR PROTRUSIONS SHALL NOT EXCEED .010 INCH (0.254mm) .018 .003 (0.457 0.076) N14 1002 491fa 10 LTC491 U PACKAGE DESCRIPTIO S Package 14-Lead Plastic Small Outline (Narrow .150 Inch) (Reference LTC DWG # 05-08-1610) .337 - .344 (8.560 - 8.738) NOTE 3 .045 .005 .050 BSC 14 N 12 11 10 9 8 N .245 MIN .160 .005 .150 - .157 (3.810 - 3.988) NOTE 3 .228 - .244 (5.791 - 6.197) 1 .030 .005 TYP 13 2 3 N/2 N/2 RECOMMENDED SOLDER PAD LAYOUT 1 .010 - .020 x 45 (0.254 - 0.508) .008 - .010 (0.203 - 0.254) 2 3 4 5 6 .053 - .069 (1.346 - 1.752) .004 - .010 (0.101 - 0.254) 0 - 8 TYP .016 - .050 (0.406 - 1.270) NOTE: 1. DIMENSIONS IN .014 - .019 (0.355 - 0.483) TYP 7 .050 (1.270) BSC INCHES (MILLIMETERS) 2. DRAWING NOT TO SCALE 3. THESE DIMENSIONS DO NOT INCLUDE MOLD FLASH OR PROTRUSIONS. MOLD FLASH OR PROTRUSIONS SHALL NOT EXCEED .006" (0.15mm) S14 0502 491fa 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 LTC491 U TYPICAL APPLICATIO S RS232 Receiver RS232 to RS485 Level Transistor with Hysteresis R = 220k RS232 IN 5.6k Y RX RECEIVER 10k 1/2 LTC491 LTC491 * TA02 RS232 IN 120 DRIVER 5.6k 1/2 LTC491 Z VY - VZ 19k HYSTERESIS = 10k * -------- -------- R R LTC491 * TA03 RELATED PARTS PART NUMBER DESCRIPTION COMMENTS LTC486/LTC487 Low Power Quad RS485 Drivers 110A Supply Current LTC488/LTC489 Low Power Quad RS485 Receivers 7mA Supply Current LTC1480 3.3V Supply RS485 Transceiver Lower Supply Voltage LTC1481 Low Power RS485 Transceiver with Shutdown Lowest Power LTC1482 RS485 Transceiver with Carrier Detect 15kV ESD, Fail-Safe LTC1483 Low Power, Low EMI RS485 Transceiver Slew Rate Limited Driver Outputs, Lowest Power LTC1484 RS485 Transceiver with Fail-Safe 15kV ESD, MSOP Package LTC1485 10Mbps RS485 Transceiver High Speed LTC1518/LTC1519 52Mbps Quad RS485 Receivers Higher Speed, LTC488/LTC489 Pin-Compatible LTC1520 LVDS-Compatible Quad Receiver 100mV Threshold, Low Channel-to-Channel Skew LTC1535 2500V Isolated RS485 Transceiver Full-Duplex, Self-Powered Using External Transformer LTC1685 52Mbps RS485 Transceiver Industry-Standard Pinout, 500ps Propagation Delay Skew LTC1686/LTC1687 52Mbps Full-Duplex RS485 Transceiver LTC490/LTC491 Pin Compatible LTC1688/LTC1689 100Mbps Quad RS485 Drivers Highest Speed, LTC486/LTC487 Pin Compatible LTC1690 Full-Duplex RS485 Transceiver with Fail-Safe 15kV ESD, LTC490 Pin Compatible LT1785/LTC1785A 60V Protected RS485 Transceivers 15kV ESD, Fail-Safe (LT1785A) LT1791/LTC1791A 60V Protected Full-Duplex RS485 Transceivers 15kV ESD, Fail-Safe (LT1791A), LTC491 Pin Compatible 491fa 12 Linear Technology Corporation LT/TP 0104 1K REV A * PRINTED IN USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 FAX: (408) 434-0507 www.linear.com LINEAR TECHNOLOGY CORPORATION 1992