Final Electrical Specifications LTC1535 Isolated RS485 Transceiver August 1999 U DESCRIPTIO FEATURES ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ The LTC®1535 is an isolated RS485 full-duplex differential line transceiver. Isolated RS485 is ideal for systems where the ground loop is broken to allow for much larger common mode voltage ranges. An internal capacitive isolation barrier provides 2500VRMS of isolation between the line transceiver and the logic level interface. The powered side contains a 400kHz push-pull converter to power the isolated RS485 transceiver. Internal full-duplex communication occurs through the capacitive isolation barrier. The transceiver meets RS485 and RS422 requirements. UL Rated Isolated RS485: 2500VRMS Half- or Full-Duplex Eliminates Ground Loops 350kBd Maximum Data Rate Self-Powered with 400kHz Converter Fail-Safe Output High for Open or Shorted Receiver Inputs Short-Circuit Current Limit Slow Slew Rate Control 68kΩ Input Impedance Allows Up to 128 Nodes Thermal Shutdown 8kV ESD Protection On Driver Outputs and Receiver Inputs The driver and receiver feature three-state outputs, with the driver maintaining high impedance over the entire common mode range. The drivers have short-circuit current limits in both directions and a slow slew rate select to minimize EMI or reflections. The 68kΩ receiver input allows up to 128 node connections. A fail-safe feature defaults to a high output state when the receiver inputs are open or shorted. U APPLICATIO S ■ ■ ■ Isolated RS485 Receiver/Driver RS485 with Large Common Mode Voltage Breaking RS485 Ground Loops Multiple Unterminated Line Taps , LTC and LT are registered trademarks of Linear Technology Corporation. U ■ TYPICAL APPLICATIO ** CTX02-14659 1/2 BAT54C + 10µF 2 1/2 BAT54C 2 VCC 1 + VCC 3 ST1 ST2 2 11 14 GND2 VCC2 400kHz 10µF A 1 28 RO RO R B RO2 RE 27 DE 26 25 DI RE Y DE D DI SLO GND 4 Z 1 16 15 TWISTED-PAIR CABLE 17 13 12 18 1535 TA01 LOGIC COMMON FLOATING RS485 COMMON 1 2 ** TRANSFORMER COILTRONICS (561) 241-7876 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. 1 LTC1535 U W W W ABSOLUTE AXI U RATI GS U W U PACKAGE/ORDER I FOR ATIO (Note 1) TOP VIEW VCC to GND ................................................................ 6V VCC2 to GND2 ............................................................ 8V Control Input Voltage to GND ...... – 0.3V to (VCC + 0.3V) Driver Input Voltage to GND ........ – 0.3V to (VCC + 0.3V) Driver Output Voltage (Driver Disabled) to GND2 .............. (VCC2 – 13V) to 13V Driver Output Voltage (Driver Enabled) to GND2 ............... (VCC2 – 13V) to 10V Receiver Input Voltage to GND2 ............................ ±14V Receiver Output Voltage .............. – 0.3V to (VCC + 0.3V) Operating Temperature Range .............. 0°C ≤ TA ≤ 70°C Lead Temperature (Soldering, 10 sec).................. 300°C VCC 1 28 RO ST1 2 27 RE ST2 3 26 DE GND 4 25 DI ORDER PART NUMBER LTC1535CSW GND2 11 18 SLO Z 12 17 RO2 Y 13 16 A VCC2 14 15 B SW PACKAGE 28-LEAD PLASTIC SO WIDE TJMAX = 125°C, θJA = 125°C/W Consult factory for Industrial and Military grade parts. ELECTRICAL CHARACTERISTICS The ● denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. 0°C ≤ TA ≤ 70°C, VCC = 5V, VCC2 = 5V unless otherwise noted. SYMBOL PARAMETER VCC VCC Supply Range ● 4.5 5.5 V VCC2 VCC2 Supply Range ● 4.5 7.5 V ICC VCC Supply Current No Load ● 13 18 mA ICC2 VCC2 Supply Current R = 27Ω, Figure 1 No Load ● ● 63 7 73 12 mA mA VOD1 Differential Driver Output No Load ● VOD2 Differential Driver Output R = 50Ω (RS422) Note 2 R = 27Ω(RS485), Figure 1 ● ● 2 1.5 2 Driver Short-Circuit Current VOUT = HIGH VOUT = LOW –7V ≤ VCM ≤ 10V –7V ≤ VCM ≤ 10V ● ● 75 75 100 100 VIH Logic Input High Voltage DE, DI, RE VCC = 4.5V ● 2 VIL Logic Input Low Voltage DE, DI, RE VCC = 4.5V IIN Input Current (A, B) Note 3 IOSD1 CONDITIONS MIN TYP MAX 5 UNITS V V V 135 135 mA mA V ● 0.8 V VIN = 12V ● 0.25 mA VIN = – 7V ● –0.20 mA VTH Receiver Input Threshold –7V ≤ VCM ≤ 12V, Note 4 ● –200 –90 –10 mV ∆VTH Receiver Input Hysteresis –7V ≤ VCM ≤ 12V ● 10 30 70 mV RIN Receiver Input Impedance ● 50 68 85 kΩ VOH RO Output High Voltage IRO = – 4mA, VCC = 4.5V IRO = –10mA, VCC = 4.5V ● 3.7 4.0 3.4 VOL RO Output Low Voltage IRO = 4mA, VCC = 4.5V IRO = 10mA, VCC = 4.5V ● 2 0.4 0.9 V V 0.8 V V LTC1535 ELECTRICAL CHARACTERISTICS The ● denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. 0°C ≤ TA ≤ 70°C, VCC = 5V, VCC2 = 5V unless otherwise noted. SYMBOL PARAMETER IOZ Driver Output Leakage CONDITIONS MIN VOH2 RO2 Output High Voltage IRO2 = – 4mA, VCC = 4.5V IRO2 = –10mA, VCC = 4.5V ● VOL2 RO2 Output Low Voltage IRO2 = 4mA, VCC = 4.5V IRO2 = 10mA, VCC = 4.5V ● fSW DC Converter Frequency RSWH DC Converter R High VCC = 4.5V ● RSWL DC Converter R Low VCC = 4.5V ● IREL RE Output Low Current RE Sink Current, Fault = 0 ● IREH RE Output High Current RE Source Current, Fault = 1 ● VUVL Undervoltage Low Threshold RE Fault = 1, Note 5 ● VUVH Undervoltage High Threshold RE Fault = 0, Note 5 ● VISO Isolation Voltage 1 Minute, Note 6 1 Second ● 3.7 TYP MAX UNITS 1 µA 3.9 3.4 V V 0.4 0.9 0.8 V V 420 520 kHz 4 6 Ω 2.5 5 Ω – 40 – 50 – 80 µA 80 100 130 µA 3.90 4.00 4.25 V 4.05 4.20 4.40 290 2500 3000 V VRMS VRMS U SWITCHI G CHARACTERISTICS The ● denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. 0°C ≤ TA ≤ 70°C, VCC = 5V, VCC2 = 5V, R = 27Ω (RS485) unless otherwise noted. SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS tSJ Data Sample Jitter Figure 8, Note 7 ● 250 285 ns fMAX Max Baud Rate Jitter = 10% Max, SLO = 1, Note 8 ● tPLH Driver Input to Output DE = 1, SLO = 1, Figure 3, Figure 5 DE = 1, SLO = 0, Figure 3, Figure 5 ● ● 600 1300 855 1560 ns ns tPHL Driver Input to Output DE = 1, SLO = 1, Figure 3, Figure 5 DE = 1, SLO = 0, Figure 3, Figure 5 ● ● 600 1300 855 1560 ns ns tr, tf Driver Rise or Fall Time DE = 1, SLO = 1, Figure 3, Figure 5 DE = 1, SLO = 0 ● ● 20 500 50 1000 ns ns 350 150 kBd tZH Driver Enable to Output DI = 1, SLO = 1, Figure 4, Figure 6 ● 1000 1400 ns tZL Driver Enable to Output DI = 0, SLO = 1, Figure 4, Figure 6 ● 1000 1400 ns tLZ Driver Disable to Output DI = 0, SLO = 1, Figure 4, Figure 6 ● 700 1000 ns tHZ Driver Disable to Output DI = 1, SLO = 1, Figure 4, Figure 6 ● 700 1000 ns tPLH Receiver Input to RO RE = 0, Figure 2, Figure 7 ● 600 855 ns tPHL Receiver Input to RO RE = 0, Figure 2, Figure 7 ● 600 855 ns tPLH Receiver Input to RO2 RE = 0, Figure 2, Figure 7 30 ns tPHL Receiver Input to RO2 RE = 0, Figure 2, Figure 7 30 ns tr, tf Receiver Rise or Fall Time RE = 0, Figure 2, Figure 7 20 ns tLZ Receiver Disable to Output Figure 2, Figure 8 30 ns tHZ Receiver Disable to Output Figure 2, Figure 8 30 ns tSTART Initial Start-Up Time Note 9 1200 ns tTOF Data Time-Out Fault Note 9 1200 ns 3 LTC1535 ELECTRICAL CHARACTERISTICS Note 1: Absolute Maximum Ratings are those values beyond which the life of a device may be impaired. Note 2: RS422 50Ω specification based on RS485 27Ω test. Note 3: IIN is tested at VCC2 = 5V, guaranteed by design from VCC2 = GND2 ≤ 5.25V. Note 4: Input fault conditions on the RS485 receiver are detected with a fixed receiver offset. The offset is such that an input short or open will result in a high data output. Note 5: The low voltage detect faults when VCC2 or VCC drops below 4.2V and reenables when greater than 4.4V. The fault can be monitored through the weak driver output on RE. Note 6: Value derived from 1 second test. Note 7: The input signals are internally sampled and encoded. The internal sample rate determines the data output jitter since the internal sampling is asynchronous with respect to the external data. Nominally, a 4MHz internal sample rate gives 250ns of sampling uncertainty in the input signals. Note 8: The maximum baud rate is 350kBd with 10% sampling jitter. Lower baud rates have lower jitter. Note 9: Start-up time is the time for communication to recover after a fault condition. Data time-out is the time a fault is indicated on RE after data communication has stopped. U U U PI FU CTIO S POWER SIDE ISOLATED SIDE VCC (Pin 1): 5V Supply. Bypass to GND with 10µF capacitor. GND2 (Pin 11): Isolated Side Power Ground. ST1 (Pin 2): DC Converter Output 1 to DC Transformer. Y (Pin 13): Differential Driver Noninverting Output. ST2 (Pin 3): DC Converter Output 2 to DC Transformer. VCC2 (Pin 14): 5V to 7.5V Supply from DC Transformer. Bypass to GND with 10µF capacitor. GND (Pin 4): Ground. Z (Pin 12): Differential Driver Inverting Output. DI (Pin 25): Transmit Data TTL Input to the Isolated Side RS485 Driver. Do not float. B (Pin 15): Differential Receiver Inverting Input. DE (Pin 26): Transmit Enable TTL Input to the Isolated Side RS485 Driver. A high level enables the driver. Do not float. RO2 (Pin 17): Isolated Side Receiver TTL Output. RE (Pin 27): Receive Data Output Enable TTL Input. A low level enables the receiver. This pin also provides a fault output signal. (See Applications Information.) RO (Pin 28): Receive Data TTL Output. 4 A (Pin 16): Differential Receiver Noninverting Input. SLO (Pin 18): Slow Slew Rate Control of RS485 Driver. A low level forces the driver outputs into slow slew rate mode. LTC1535 W BLOCK DIAGRA POWER SIDE 1 ISOLATED SIDE 1.3 + 2 3 ST1 ST2 11 14 GND2 VCC2 A 400kHz 1 28 DECODE VCC ENCODE R RO B EN 27 RE EN RO2 FAULT Y ENCODE 26 25 4 16 DECODE D DE Z EN DI SLO EN 15 17 13 12 18 FAULT GND 1535 BD TEST CIRCUITS Y R VOD S1 TEST POINT RECEIVER OUTPUT 1k VCC VOC 1k CRL S2 R 1535 F02 Z 1535 F01 Figure 1. Driver DC Test Load Figure 2. Receiver Timing Test Load 3V DE Y R Z S1 CL1 DI R CL2 OUTPUT UNDER TEST VCC 500Ω S2 CL 1535 F03 1535 F04 Figure 3. Driver Timing Test Circuit Figure 4. Driver Timing Test Load 5 LTC1535 U W W SWITCHI G TI E WAVEFOR S 3V tr ≤ 10ns, tf ≤ 10ns 1.5V DI 1.5V 0V t PLH t PHL Z VO Y VO 0V –VO 80% tr 80% 20% VDIFF = V(Y) – V(Z) 20% t SJ 1535 F05 tf t SJ Figure 5. Driver Propagation Delays 3V tr ≤ 10ns, tf ≤ 10ns 1.5V DE 1.5V 0V t LZ t ZL 5V Y, Z 2.3V OUTPUT NORMALLY LOW 0.5V 2.3V OUTPUT NORMALLY HIGH 0.5V VOL VOH Y, Z 0V t HZ t ZH 1535 F06 t SJ t SJ Figure 6. Driver Enable and Disable Times t SJ t SJ VOH 1.5V RO 1.5V OUTPUT VOL tr ≤ 10ns, tf ≤ 10ns t PHL VOD2 A–B –VOD2 0V t PLH 0V INPUT 1535 F07 Figure 7. Receiver Propagation Delays 3V 1.5V RE 1.5V tr ≤ 10ns, tf ≤ 10ns 0V tZL 5V RO 1.5V t LZ OUTPUT NORMALLY LOW t SJ RO 1.5V 0.5V t SJ OUTPUT NORMALLY HIGH 0.5V 0V t HZ tZH t SJ Figure 8. Receiver Enable and Disable Times 6 1535 F08 t SJ LTC1535 U U W U APPLICATIO S I FOR ATIO Isolation Barrier and Sampled Communication Push-Pull DC/DC Converter The LTC1535 uses the SW-28 isolated lead frame package to provide capacitive isolation barrier between the logic interface and the RS485 driver/receiver pair. The barrier provides 2500VRMS of isolation. Communication between the two sides uses the isolation capacitors in a multiplexed way to communicate full-duplex data across this barrier. The data is sampled and encoded before transmitting across the isolation barrier, which will add sampling jitter and delay to the signals. The sampling jitter is approximately 250ns with a nominal delay of 600ns. At 250kBd rate, this represents 6.2% total jitter. The nominal DE signal to the driver output delay is 875ns ±125ns, which is longer due to the encoding. Communication start-up time is approximately 1µs to 2µs. A time-out fault will occur if communication from the isloated side fails. Faults can be monitored on the RE pin. The powered side contains a full-bridge open-loop driver, optimized for use with a single primary and center-tapped secondary transformer. Figure 9 shows the DC/DC converter in a configuration that can deliver up to to 100mA of current to the isolated side using a Coiltronics CTX0214659 transformer. Because the DC/DC converter is open-loop, care in choosing low impedance parts is important for good regulation. Care must also be taken to not exceed the VCC2 recommended maximum voltage of 7.5V when there is very light loading. The isolated side contains a low voltage detect circuit to ensure that communication across the barrier will only occur when there is sufficient isolated supply voltage. If the output of the DC/DC converter is overloaded, the supply voltage will trip the low voltage detection at 4.2V. For higher voltage stand-off, the Coiltronics CTX02-14608 transformer may be used. IEXT ILOAD ** CTX02-14659 VCC2 vs ILOAD 1/2 BAT54C IVCC2 + 8 10µF 2 1/2 BAT54C 2 VCC 1 + VCC 3 ST1 ST2 VCC2 (V) 6 2 11 14 GND2 VCC2 400kHz VCC = 5.5V VCC = 5V 4 VCC = 4.5V 2 10µF 1 GND 4 0 0 1 1535 F09 50 100 TOTAL LOAD CURRENT, ILOAD (mA) 150 1535 F09a LOGIC COMMON FLOATING RS485 COMMON 1 2 ** TRANSFORMER COILTRONICS (561) 241-7876 Figure 9 7 LTC1535 U W U U APPLICATIO S I FOR ATIO Driver Output and Slow Slew Rate Control Monitoring Faults on RE The LTC1535 uses a proprietary driver output stage that allows a common mode voltage range that extends beyond the power supplies. Thus, the high impedance state is maintained over the full RS485 common mode range. The output stage provides 100mA of short-circuit current limiting in both the positive and negative directions. Thus, even under short-circuit conditions, the supply voltage from the open-loop DC converter will not be pulled too low to prevent proper communication across the isolation barrier. The driver output will be disabled in the event of a thermal shutdown and a fault condition will be indicated through the RE weak output. The RE pin can be used to monitor the following fault conditions: low supply voltages, thermal shutdown or a time-out fault when there is no data communication across the barrier. Open circuit or short-circuit conditions on the twisted pair do not cause a fault indication. However, the RS485 receiver defaults to a high output state when the receiver input is open or short-circuited. The CMOS level SLO pin selects slow or fast slew rates on the RS485 driver output. The SLO input has an internal 100k pull-up resistor. When SLO is low, the driver outputs are slew rate limited to reduce high frequency edges. Left open or tied high, SLO defaults to fast edges. The part draws more current during slow slew rate edges. The RE pin has a weak current drive output mode for indicating fault conditions. This fault state can be polled using the circuit in Figure 10 where the control to RE is three-stated and the fault condition read back from the RE pin. The weak drive has 100µA pull-up current to indicate a fault and 50µA pull-down current for no fault. This allows the RE pin to be polled without disabling RE on nonfault conditions. Both sides contain a low voltage detect circuit. A voltage less than 4.2V on the isolated side disables communication. VCC RO RE VCC RE LTC1535 DI POLL DE FAULT FAULT GND BUFFER POLL FAULT FAULT INDICATED WHEN RE IS THREE-STATED Figure 10. Detecting Fault Conditions 8 1535 F10 LTC1535 U W U U APPLICATIO S I FOR ATIO High Voltage Considerations The LTC1535 eliminates ground loops on data communication lines. However, such isolation can bring potentially dangerous voltages onto the circuit board. An example would be accidental faulting to 117V AC at some point on the cable which is then conducted to the PC board. Figure␣ 11 shows how to detect and warn the user or installer that a voltage fault condition exists on the twisted pair or its shield. A small (3.2mm) glow lamp is connected between GND2 (the isolated ground) and the equipment’s safety “earth” ground. If a potential of more than 75V AC is present on the twisted pair or shield, B1 will light, indicating a wiring fault. Resistors R3 and R4 are used to ballast the current in B1. Two resistors are necessary because they can only stand off 200V each, as well as for power dissipation. As shown, the circuit can withstand a direct fault to a 440V 3∅ system. Other problems introduced by floating the twisted pair include the collection of static charge on the twisted pair, its shield and the attached circuitry. Resistors R1 and R2 provide a path to shunt static charge safely to ground. Again, two resisitors are necessary to withstand high voltage faults. Electrostatic spikes and transients can be limited by the addition of capacitor C1 and discharged through R1–R4. A Y TWISTED-PAIR NETWORK LTC1535 B GND2 Z 2 2 2 R1* 470k R2* 470k C1*** 10nF R3** 100k R4** 100k B1 CN2R (JKL) EQUIPMENT SAFETY GROUND EARTH GROUND * IRC WCR1206 ** IRC WCR1210 *** PANASONIC ECQ-U2A103MV FLOATING RS485 COMMON 2 1535 F11 Figure 11. Detecting Wiring Faults 9 LTC1535 U W U U APPLICATIO S I FOR ATIO DI DI Y–Z Y–Z Figure 12. Driver Propagation Delay with Sample Jitter. SLO = VCC2 Figure 13. Driver Propagation Delay with Sample Jitter. SLO = 0V Z Z Y Y Figure 14. Driver Output. R = 27Ω, VCC2 = 5V, SLO = VCC2 Y–Z Y–Z Figure 16. Driver Differential Output. R = 27Ω, VCC2 = 5V, SLO = VCC2 10 Figure 15. Driver Output. R = 27Ω, VCC2 = 5V, SLO = 0V Figure 17. Driver Differential Output. R = 27Ω, VCC2 = 5V, SLO = 0V LTC1535 U PACKAGE DESCRIPTIO Dimensions in inches (millimeters) unless otherwise noted. SW Package 28-Lead Plastic Small Outline Isolation Barrier (Wide 0.300) (LTC DWG # 05-08-1690) 0.697 – 0.712* (17.70 – 18.08) 28 27 26 25 18 17 16 15 0.394 – 0.419 (10.007 – 10.643) NOTE 1 1 2 3 11 4 12 13 14 0.291 – 0.299** (7.391 – 7.595) 0.005 (0.127) RAD MIN 0.037 – 0.045 (0.940 – 1.143) 0.093 – 0.104 (2.362 – 2.642) 0.010 – 0.029 × 45° (0.254 – 0.737) 0° – 8° TYP 0.009 – 0.013 (0.229 – 0.330) NOTE 1 0.050 (1.270) BSC 0.016 – 0.050 (0.406 – 1.270) NOTE: 1. PIN 1 IDENT, NOTCH ON TOP AND CAVITIES ON THE BOTTOM OF PACKAGES ARE THE MANUFACTURING OPTIONS THE PART MAY BE SUPPLIED WITH OR WITHOUT ANY OF THE OPTIONS. 0.014 – 0.019 (0.356 – 0.482) TYP 0.004 – 0.012 (0.102 – 0.305) SW28 (ISO) 1098 *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 11 LTC1535 U TYPICAL APPLICATIO ** CTX02-14659 1/2 BAT54C + 10µF 2 1/2 BAT54C 2 VCC 1 + VCC 3 ST1 ST2 2 11 14 GND2 VCC2 400kHz 10µF A 1 28 RO RO R 120Ω B RO2 RE 27 DE 26 25 DI RE Y DE D DI 1 15 17 13 120Ω Z SLO GND 4 16 12 18 1535 TA02 LOGIC COMMON FLOATING RS485 COMMON 1 2 ** TRANSFORMER COILTRONICS (561) 241-7876 Figure 18. Full-Duplex Connection RELATED PARTS PART NUMBER DESCRIPTION COMMENTS LTC1177 Isolated MOSFET Driver No Secondary Supply, 2500VRMS Isolation LT1424-5 Isolated Flyback Switching Regulator ±5% Accurate with No Optoisolator Required LTC1485 High Speed RS485 Transceiver 10Mbps, Pin Compatible with LTC485 LTC1531 Self-Powered Isolated Comparator 2.5V Isolated Reference, 3000VRMS Isolation LT1785/LT1791 ±60V Fault Protected RS485 Transceiver, Half/Full-Duplex ±15kV ESD Protection, Industry Standard Pinout LTC1690 Full-Duplex RS485 Transceiver ±15kV ESD Protection, Fail-Safe Receiver 12 Linear Technology Corporation 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408)432-1900 ● FAX: (408) 434-0507 ● www.linear-tech.com 1535i LT/TP 0899 4K • PRINTED IN USA LINEAR TECHNOLOGY CORPORATION 1999