LTC491 Differential Driver and Receiver Pair U DESCRIPTIO FEATURES 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 Pin Compatible with the SN75180 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. S Low Power RS485/RS422 Transceiver Level Translator UO The CMOS design offers significant power savings over its bipolar counterpart without sacrificing ruggedness against overload or ESD damage. Both AC and DC specifications are guaranteed from 0C to 70C and 4.75V to 5.25V supply voltage range. UO APPLICATI The LTC491 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 APPLICATI 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 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 TOP VIEW NC 1 R 2 REB 3 12 A DE 4 11 B D 5 GND 6 9 Y GND 7 8 NC ORDER PART NUMBER 14 VCC R 13 NC LTC491CN LTC491CS LTC491IN LTC491IS 10 Z D S PACKAGE N PACKAGE 14-LEAD PLASTIC DIP 14-LEAD PLASTIC SOIC LTC491 * POI01 Consult factory for Military grade parts. DC ELECTRICAL CHARACTERISTICS VCC = 5V 5% SYMBOL PARAMETER CONDITIONS VOD1 Differential Driver Output Voltage (Unloaded) IO = 0 MIN VOD2 Differential Driver Output Voltage (With load) R = 50; (RS422) 2 R = 27; (RS485) (Figure 1) 1.5 5 V VOD Change in Magnitude of Driver Differential Output Voltage for Complementary Output States R = 27 or R = 50 (Figure 1) 0.2 V 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 0.8 V lIN1 Input Current 2 A lIN2 Input Current (A, B) VCC = 0V or 5.25V VIN = 12V 1.0 mA VIN = -7V - 0.8 mA VTH Differential Input Threshold Voltage for Receiver - 7V VCM 12V - 0.2 0.2 V VTH Receiver Input Hysteresis VCM = 0V 70 VOH Receiver Output High Voltage IO = -4mA, VID = 0.2V 3.5 VOL Receiver Output Low Voltage 0.4 V IOZR Three-State Output Current at Receiver IO = 4mA, VID = -0.2V VCC = Max 0.4V VO 2.4V 1 A ICC Supply Current No Load; D = GND, Outputs Enabled 300 500 A or VCC Outputs Disabled 300 500 A D, DE, REB TYP MAX 5 UNITS V V 2.0 V mV V RIN Receiver Input Resistance - 7V VCM 12V IOSD1 Driver Short Circuit Current, VOUT = High VO = - 7V 100 250 mA IOSD2 Driver Short Circuit Current, VOUT = Low VO = 12V 100 250 mA IOSR Receiver Short Circuit Current 0V VO VCC 85 mA IOZ Driver Three-State Output Current VO = - 7V to 12V 200 A 2 12 k 7 2 LTC491 U SWITCHI G CHARACTERISTICS VCC = 5V 5% SYMBOL PARAMETER CONDITIONS tPLH Driver Input to Output tPHL Driver Input to Output RDIFF = 54, CL1 = CL2 = 100pF (Figures 2, 5) MIN TYP MAX 10 30 50 tSKEW tr, tf Driver Output to Output Driver Rise or Fall Time tZH Driver Enable to Output High CL = 100pF (Figures 4, 6) S2 Closed tZL Driver Enable to Output Low tLZ tHZ UNITS ns 10 30 50 ns 5 5 15 25 ns ns 40 70 ns 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 Driver Disable Time From High CL = 15pF (Figures 4, 6) S2 Closed 40 70 ns tPLH Receiver Input to Output 40 70 150 ns tPHL Receiver Input to Output RDIFF = 54, CL1 = CL2 = 100pF (Figures 2, 7) 40 70 150 tSKD tZL tPLH - tPHL Differential Receiver Skew Receiver Enable to Output Low tZH tLZ tHZ ns 13 CL = 15pF (Figures 3, 8) S1 Closed 20 50 ns Receiver Enable to Output High 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 The denotes specifications which apply over the full operating temperature range. Note 1: "Absolute Maximum Ratings" are those beyond which the safety of the device cannot be guaranteed. ns 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. Note 3: All typicals are given for VCC = 5V and Temperature = 25C. UO U U PI FU CTI 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. Y (Pin 9): Driver output. DE (Pin 4): Driver output enable. A high on DE enables the driver outputs, A and B. 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 A low and B high. A high on D will force A high and B low. NC (Pin 8): Not Connected. 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. 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 -72 - 48 -24 OUTPUT CURRENT (mA) OUTPUT CURRENT (mA) OUTPUT CURRENT (mA) 80 64 -96 48 32 16 0 1 2 3 0 4 1 2 3 5.0 1.61 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 0 -50 0 50 100 TEMPERATURE (C ) 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) DIFFERENTIAL VOLTAGE (V) 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) 1.63 1.5 -50 0 4 Driver Skew vs Temperature TIME (ns) INPUT THRESHOLD VOLTAGE (V) TTL Input Threshold vs Temperature 50 20 LTC491 * TPC02 LTC491 * TPC01 0 40 OUTPUT VOLTAGE (V) OUTPUT VOLTAGE (V) 1.55 -50 60 0 0 0 4 Driver Output Low Voltage vs Output Current TA = 25C LTC491 * TPC08 LTC491 * TPC09 LTC491 TEST CIRCUITS Y R VOD2 R VOC Z LTC491 * TA02 Figure 1. Driver DC Test Load CL1 Y D A RDIFF DRIVER Z RECEIVER CL2 B R 15pF LTC491 * TA03 Figure 2. Driver/Receiver Timing Test Circuit S1 RECEIVER OUTPUT S1 1k VCC CL VCC OUTPUT UNDER TEST 1k 500 CL S2 LTC491 * TA04 Figure 3. Receiver Timing Test Load S2 LTC491 * TA05 Figure 4. Driver Timing Test Load 5 LTC491 W U 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 * TA06 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 tHZ LTC491 * TA07 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 * TA08 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 tZH tHZ Figure 8. Receiver Enable and Disable Times 6 0.5V OUTPUT NORMALLY HIGH LTC491 * TA09 LTC491 W U U UO APPLICATI S I FOR ATIO Typical Application 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. 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 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. 12 RX 2 3 RECEIVER 12 120 120 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 RECEIVER LTC491 DRIVER 5 4 3 2 LTC491 * TA10 DX RX Figure 9. Typical Connection 12 RX 2 3 RECEIVER 120 11 DATA IN 4 10 DX 5 DRIVER 120 9 DATA OUT LTC491 LTC491 * TA11 Figure 10. Line Repeater 7 LTC491 W U U UO APPLICATI S I FOR ATIO Thermal Shutdown The LTC491 has a thermal shutdown feature which protects the part from excessive power dissipation. If the 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. Cables and Data Rate 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). 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. 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 10k CABLE LENGTH (ft) LOSS PER 100 ft (dB) 10 1.0 0.1 0.1 1.0 10 100 FREQUENCY (MHZ) 100 10 10k 100k 1M 2.5M 10M DATA RATE (bps) LTC491 * TA12 Figure 11. Attenuation vs Frequency for Belden 9481 8 1k LTC491 * TA13 Figure 12. Cable Length vs Data Rate LTC491 U W U UO APPLICATI S I FOR ATIO 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). PROBE HERE 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. Rt DX DRIVER RECEIVER RX 120 C Rt = 120 RECEIVER RX C = LINE LENGTH (ft) x 16.3pF LTC491 * TA15 Rt = 47 Figure 14. AC Coupled Termination Rt = 470 LTC491 * TA14 Figure 13. Termination Effects 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. 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). 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). 9 LTC491 U W U UO APPLICATI S I FOR ATIO Receiver Open-Circuit Fail-Safe Fault Protection 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 maintain the last data bit received. 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 from each line side pin to ground (Figure 16). Y DRIVER 120 Z +5V 110 130 130 110 LTC491 * TA17 RECEIVER RX +5V 1.5k 140 RECEIVER RX RECEIVER RX 1.5k 100k +5V C 120 LTC491 * TA16 Figure 15. Forcing "O" When All Drivers are Off The termination resistors are used to generate a DC bias which forces the receiver output to a known state, in this case 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. 10 Figure 16. ESD Protection with TransZorbs 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 required for your application (typically 12V). Also, don't forget to check how much the added parasitic capacitance will load down the bus. LTC491 UO TYPICAL APPLICATI S RS232 Receiver RS232 IN 5.6k RX RECEIVER 1/2 LTC491 LTC491 * TA18 RS232 to RS485 Level Transistor with Hysteresis R = 220k Y 10k RS232 IN DRIVER 5.6k 1/2 LTC491 120 Z 19k VY - VZ HYSTERESIS = 10k * -------- -------- R R LTC491 * TA19 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 PACKAGE DESCRIPTIO Dimensions in inches (millimeters) unless otherwise noted. N Package 14-Lead Plastic DIP 0.770 (19.558) MAX 14 13 11 12 10 9 8 0.260 0.010 (6.604 0.254) 1 0.300 - 0.325 (7.620 - 8.255) 3 2 5 4 JA 100C 90C/W 7 6 0.045 - 0.065 (1.143 - 1.651) 0.015 (0.380) MIN 0.130 0.005 (3.302 0.127) TJ MAX 0.065 (1.651) TYP 0.009 - 0.015 (0.229 - 0.381) +0.025 0.325 -0.015 ( 8.255 +0.635 -0.381 ) 0.075 0.015 (1.905 0.381) 0.018 0.003 (0.457 0.076) 0.100 0.010 (2.540 0.254) 0.125 (3.175) MIN N14 0392 S Package 14-Lead Plastic SOIC 0.337 - 0.344 (8.560 - 8.738) 14 13 12 11 10 9 8 0.228 - 0.244 (5.791 - 6.197) 2 3 4 5 12 6 7 0.004 - 0.010 (0.101 - 0.254) 0 - 8 TYP Linear Technology Corporation 110C/W 0.053 - 0.069 (1.346 - 1.752) 0.008 - 0.010 (0.203 - 0.254) 0.016 - 0.050 0.406 - 1.270 JA 100C 0.150 - 0.157 (3.810 - 3.988) 1 0.010 - 0.020 x 45 (0.254 - 0.508) TJ MAX 0.014 - 0.019 (0.355 - 0.483) 0.050 (1.270) TYP SO14 0392 BA/GP 0492 10K REV 0 1630 McCarthy Blvd., Milpitas, CA 95035-7487 (408) 432-1900 FAX: (408) 434-0507 TELEX: 499-3977 LINEAR TECHNOLOGY CORPORATION 1992