LTC1535 Isolated RS485 Transceiver 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 420kHz 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 UL Recognized File #E151738 Eliminates Ground Loops 250kBd Maximum Data Rate Self-Powered with 420kHz Converter Half- or Full-Duplex 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 Available in 28-Lead SW Package ® ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ 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 ST1 3 ST2 2 11 14 GND2 VCC2 420kHz 10µF A 1 LOGIC COMMON 28 RO RO R B 1 FLOATING RS485 COMMON 2 ** TRANSFORMER COOPER (561) 241-7876 RO2 RE 27 DE 26 25 DI 4 1 RE Y DE D DI GND Z SLO 16 15 TWISTED-PAIR CABLE 17 13 12 18 1535 TA01 1535fa 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 LTC1535C ........................................ 0°C ≤ TA ≤ 70°C LTC1535I ..................................... – 40°C ≤ TA ≤ 85°C Storage Temperature Range ................. – 65°C to 150°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 LTC1535ISW GND2 11 18 SLO Z 12 17 RO2 Y 13 16 A VCC2 14 15 B SW PACKAGE 28-LEAD PLASTIC SO TJMAX = 125°C, θJA = 125°C/W Consult LTC Marketing for parts specified with wider operating temperature ranges. ELECTRICAL CHARACTERISTICS The ● denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VCC = 5V, VCC2 = 5V unless otherwise noted. SYMBOL PARAMETER VCC VCC Supply Range CONDITIONS MIN VCC2 VCC2 Supply Range 7.5 V ICC VCC Supply Current Transformer Not Driven (Note 10) ● 13 28 mA ICC2 VCC2 Supply Current R = 27Ω, Figure 2 No Load ● ● 63 7 73 12 mA mA VOD1 Differential Driver Output No Load ● VOD2 Differential Driver Output R = 50Ω (RS422) (Note 2), VCC2 = 4.5V R = 27Ω(RS485), Figure 2, VCC2 = 4.5V ● ● 2 1.5 2 ● 4.5 ● 4.5 TYP MAX UNITS 5.5 V 5 V V V VOC Driver Output Common Mode Voltage DC Level, R = 50Ω, Figure 2 ● 2.0 2.5 3.0 V IOSD1 Driver Short-Circuit Current VOUT = HIGH VOUT = LOW Driver Enabled (DE = 1) –7V ≤ VCM ≤ 10V –7V ≤ VCM ≤ 10V ● ● 60 60 100 100 150 150 mA mA VIH Logic Input High Voltage DE, DI, RE SLO ● ● 2 4 1.7 2.2 VIL Logic Input Low Voltage DE, DI, RE SLO ● ● IIN Input Current (A, B) (Note 3) VTH Receiver Input Threshold –7V ≤ VCM ≤ 12V, (Note 4) ∆VTH Receiver Input Hysteresis –7V ≤ VCM ≤ 12V RIN Receiver Input Impedance 1.7 1.8 V V 0.8 1 V V VIN = 12V ● 0.25 mA VIN = – 7V ● –0.20 mA ● –200 –90 –10 mV 0°C ≤ TA ≤ 0°C ● 10 30 70 mV – 40°C ≤ TA ≤ 85°C ● 5 30 70 mV ● 50 68 100 kΩ 1535fa 2 LTC1535 ELECTRICAL CHARACTERISTICS The ● denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VCC = 5V, VCC2 = 5V unless otherwise noted. SYMBOL PARAMETER VIOC Receiver Input Open Circuit Voltage CONDITIONS MIN VOH RO Output High Voltage IRO = – 4mA, VCC = 4.5V ● VOL RO Output Low Voltage IRO = 4mA, VCC = 4.5V ● IOZ Driver Output Leakage Driver Disabled (DE = 0) VOH2 RO2 Output High Voltage IRO2 = – 4mA, VCC = 4.5V ● VOL2 RO2 Output Low Voltage IRO2 = 4mA, VCC = 4.5V ● fSW DC Converter Frequency ● RSWH DC Converter Impedance High ● RSWL DC Converter Impedance Low 2.5 5 Ω IREL RE Output Low Current RE Sink Current, Fault = 0 ● – 40 – 50 – 80 µA IREH RE Output High Current RE Source Current, Fault = 1 ● 80 100 130 µA VUVL Undervoltage Low Threshold RE Fault = 1, (Note 5) ● 3.70 4.00 4.25 V VUVH Undervoltage High Threshold RE Fault = 0, (Note 5) ● 4.05 4.20 4.40 V VISO Isolation Voltage 1 Minute, (Note 6) 1 Second 3.7 TYP MAX UNITS 3.4 V 4.0 V 0.4 0.8 V 1 µA 3.7 3.9 V 0.4 0.8 V 290 420 590 kHz 4 6 ● 2500 3000 Ω 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. VCC = 5V, VCC2 = 5V, R = 27Ω (RS485) unless otherwise noted. SYMBOL PARAMETER CONDITIONS MIN TYP MAX 250 285 250 410 tSJ Data Sample Jitter Figure 8, (Note 7) ● fMAX Max Baud Rate Jitter = 10% Max, SLO = 1, (Note 8) ● tPLH Driver Input to Output DE = 1, SLO = 1, Figure 4, Figure 6 DE = 1, SLO = 0, Figure 4, Figure 6 ● ● 600 1300 855 1560 ns ns tPHL Driver Input to Output DE = 1, SLO = 1, Figure 4, Figure 6 DE = 1, SLO = 0, Figure 4, Figure 6 ● ● 600 1300 855 1560 ns ns tr, tf Driver Rise or Fall Time DE = 1, SLO = 1, Figure 4, Figure 6 DE = 1, SLO = 0, VCC = VCC2 = 4.5V ● ● 20 500 100 1000 ns ns 150 UNITS ns kBd tZH Driver Enable to Output DI = 1, SLO = 1, Figure 5, Figure 7 ● 1000 1400 ns tZL Driver Enable to Output DI = 0, SLO = 1, Figure 5, Figure 7 ● 1000 1400 ns tLZ Driver Disable to Output DI = 0, SLO = 1, Figure 5, Figure 7 ● 700 1300 ns tHZ Driver Disable to Output DI = 1, SLO = 1, Figure 5, Figure 7 ● 700 1300 ns tPLH Receiver Input to RO RE = 0, Figure 3, Figure 8 ● 600 855 ns tPHL Receiver Input to RO RE = 0, Figure 3, Figure 8 ● 600 855 ns tPLH Receiver Input to RO2 RE = 0, Figure 3, Figure 8 30 ns tPHL Receiver Input to RO2 RE = 0, Figure 3, Figure 8 30 ns tr, tf Receiver Rise or Fall Time RE = 0, Figure 3, Figure 8 20 ns tLZ Receiver Disable to Output Figure 3, Figure 9 30 ns tHZ Receiver Disable to Output Figure 3, Figure 9 30 ns tSTART Initial Start-Up Time (Note 9) 1200 ns tTOF Data Time-Out Fault (Note 9) 1200 ns ST1, ST2 Duty Cycle 0°C ≤ TA ≤ 70°C – 40°C ≤ TA ≤ 85°C ● ● 56 57 % % 1535fa 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 GND2 ≤ VCC2 ≤ 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 VUVL and reenables when greater than VUVH. 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 250kBd 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. Note 10. ICC measured with no load, ST1 and ST2 floating. U W TYPICAL PERFOR A CE CHARACTERISTICS VCC Supply Current vs Temperature VCC2 Supply Current vs Temperature 90 VCC = 5V COOPER CTX02-14659 TRANSFORMER 120 80 VCC2 CURRENT (mA) VCC CURRENT (mA) 110 RL = 54Ω 100 90 RL = 120Ω 80 70 RL = OPEN 60 50 –50 –25 0 6.5 VCC2 = 6V 70 6.0 VCC2 = 5V 60 50 VCC2 = 4.5V 40 30 20 0 55 25 50 75 100 125 150 TEMPERATURE (°C) Driver Differential Output Rise/ Fall Time vs Temperature 800 VCC2 = 5V, 4.5V SLO = VCC2 RL = 54Ω 700 SLO = 0V RL = 54Ω VCC2 = 5V 600 50 TIME (ns) TIME (ns) 400 300 0 1535 G03 65 60 fMAX (kHz) 4.5 –50 –25 25 50 75 100 125 150 TEMPERATURE (°C) Driver Differential Output Rise/ Fall Time vs Temperature 500 45 40 500 VCC2 = 4.5V 400 35 VCC = VCC2 = 4.5V SLO = VCC2 RL = 54Ω 0 RL = 54Ω, VCC = 4.5V 1535 G02 Maximum Baud Rate vs Temperature 100 –50 –25 5.5 COOPER CTX02-14659 TRANSFORMER 1535 G01 200 RL = 54Ω, VCC = 5V 5.0 fDI = fMAX SLO = 0V RL = 54Ω 10 –50 –25 25 50 75 100 125 150 TEMPERATURE (°C) fDI = 250kHz SLO = 0V RL = OPEN, VCC = 5V VCC2 VOLTAGE (V) 130 VCC2 Supply Voltage vs Temperature 300 30 25 50 75 100 125 150 TEMPERATURE (°C) 1535 G04 25 –50 –25 0 25 50 75 100 125 150 TEMPERATURE (°C) 1535 G05 200 –50 –25 0 25 50 75 100 125 150 TEMPERATURE (°C) 1535 G06 1535fa 4 LTC1535 U W TYPICAL PERFOR A CE CHARACTERISTICS Switcher Frequency vs Temperature Driver Differential Output Voltage vs Temperature 600 Receiver Output Low Voltage vs Temperature 4 1.0 VCC = 5V 400 300 VCC2 = 5V 2 VCC2 = 4.5V 1 0 25 50 75 100 125 150 TEMPERATURE (°C) 0 5 TA = 25°C VCC = 5.5V TA = 25°C OUTPUT VOLTAGE (V) OUTPUT VOLTAGE (V) OUTPUT VOLTAGE (V) 4 VCC = 5V 3 2 VCC = 4.5V 0 25 50 75 100 125 150 TEMPERATURE (°C) 0 10 20 30 40 50 60 70 OUTPUT CURRENT (mA) VCC = 4.5V 1 VCC = 5.5V 0 10 20 30 40 50 60 70 80 90 100 110 OUTPUT CURRENT (mA) 1535 G12 Receiver Output Voltage vs Load Current 5.0 TA = 25°C VCC = 5V 4.5 4 OUTPUT VOLTAGE (V) OUTPUT VOLTAGE (V) OUTPUT VOLTAGE (V) 0 90 TA = 25°C RL = 60Ω 4 0 80 5 TA = 25°C VCC = 5V VCC = 5V 2 Driver Differential Output Voltage vs VCC2 Supply Voltage 5 2 VCC = 4.5V 1535 G11 Driver Output Low Voltage vs Output Current 3 3 1 1535 G10 VCC = 6V 25 50 75 100 125 150 TEMPERATURE (°C) Driver Output High Voltage vs Output Current 1 0 0 1535 G09 4 3.0 –50 –25 0.3 0 –50 –25 25 50 75 100 125 150 TEMPERATURE (°C) 5 I = 8mA 3.5 0.4 Driver Differential Output Voltage vs Output Current 4.5 VCC = 4.5V 0.5 1535 G08 Receiver Output High Voltage vs Temperature 4.0 VCC = 5V 0.6 0.1 1535 G07 VCC = 5V 0.7 0.2 SLO = VCC2 RL = 54Ω 0 –50 –25 VCC = 4.5V 0.8 3 OUTPUT VOLTAGE (V) OUTPUT VOLTAGE (V) FREQUENCY (kHz) 500 200 –50 –25 I = 8mA 0.9 VCC2 = 6V 3 2 OUTPUT HIGH, SOURCING 4.0 1.0 OUTPUT LOW, SINKING 0.5 0 10 20 30 40 50 60 70 80 90 100 110 OUTPUT CURRENT (mA) 1535 G13 1 4.5 5 5.5 6 6.5 7 VCC2 SUPPLY VOLTAGE (V) 7.5 1535 G14 0 0 1 2 3 4 5 6 7 LOAD CURRENT (mA) 8 9 1535 G15 1535fa 5 LTC1535 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 GND2 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. This output is always enabled and is unaffected by RE. RE (Pin 27): Receive Data Output Enable TTL Input. A low level enables the receiver. This pin also provides a fault output signal. (See Figure 11.) 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. RO (Pin 28): Receive Data TTL Output. W BLOCK DIAGRA POWER SIDE 1 ISOLATED SIDE 1.3 + 2 3 ST1 ST2 11 14 GND2 VCC2 12.75k 63.5k A 420kHz 16 27.25k 1 28 VCC DECODE ENCODE R RO 12.75k B 27.25k 27 RE RO2 FAULT Y ENCODE 26 25 4 DE DECODE D Z EN DI SLO EN GND FAULT 15 63.5k 17 13 12 18 100k VCC2 1535 BD 1535fa 6 LTC1535 TEST CIRCUITS ILOAD IEXT ** CTX02-14659 VCC2 1/2 BAT54C + IVCC2 10µF 2 1/2 BAT54C 2 VCC 1 + ST1 VCC 2 3 ST2 11 14 GND2 VCC2 420kHz 10µF A 1 28 RO RO R fRO = MAX BAUD RATE B RO2 27 26 25 RE Y DE D DI SLO GND 4 Z 1 16 15 17 Y 13 C1 50pF 18 LOGIC COMMON FLOATING RS485 COMMON 2 C2 50pF 2 1535 F01 1 RL Z 12 2 SLOW SLEW RATE JUMPER ** TRANSFORMER COOPER (561) 241-7876 2 Figure 1. Self-Oscillation at Maximum Data Rate (Test Configuration for the First Six Typical Performance Characteristics Curves) Y R VOD S1 TEST POINT RECEIVER OUTPUT 1k VCC VOC 1k CRL S2 R 1535 F03 Z 1535 F02 Figure 2. Driver DC Test Load Figure 3. Receiver Timing Test Load 3V DE Y R DI Z S1 CL1 R CL2 OUTPUT UNDER TEST VCC 500Ω S2 CL 1535 F04 1535 F05 Figure 4. Driver Timing Test Circuit Figure 5. Driver Timing Test Load 1535fa 7 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 F06 tf t SJ Figure 6. 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 F07 t SJ t SJ Figure 7. 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 F08 Figure 8. 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 1535 F09 t SJ Figure 9. Receiver Enable and Disable Times 1535fa 8 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 (see Figure 20 and Block Diagram). The data is sampled and encoded before transmitting across the isolation barrier, which will add sampling jitter and delay to the signals (see Figures 13 and 14). 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 isolated 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 10 shows the DC/DC converter in a configuration that can deliver up to 100mA of current to the isolated side using a Cooper CTX02-14659 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 Cooper CTX02-14608 transformer may be used. The maximum baud rate can be determined by connecting in self-oscillation mode as shown in Figure 1. In this configuration, with SLO = VCC2, the oscillation frequency is set by the internal sample rate. With SLO = 0V, the frequency is reduced by the slower output rise and fall times. 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 420kHz VCC = 5.5V VCC = 5V 4 VCC = 4.5V 2 10µF 1 GND 4 0 0 1 1535 F10 50 100 TOTAL LOAD CURRENT, ILOAD (mA) 150 1535 F10a LOGIC COMMON FLOATING RS485 COMMON 1 2 ** TRANSFORMER COOPER (561) 241-7876 Figure 10 1535fa 9 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 remain high enough for 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. During a fault, the receiver output, RO, defaults to a high state (see Table 2). 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 shortcircuited. The CMOS level SLO pin selects slow or fast slew rates on the RS485 driver output (see Figures 15, 16, 17, 18 for typical waveforms). 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 a bidirectional microcontroller I/O line or by using the circuit in Figure 11, where the control to RE is threestated 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 1535 F11 Figure 11. Detecting Fault Conditions 1535fa 10 LTC1535 U U W U APPLICATIO S I FOR ATIO Table 1. List of Transformers Designed for LTC1535 DC ISOLATION VOLTAGE PHONE (1 Second) NUMBER PART NUMBER MANUFACTURER Cooper CTX02-14659 Cooper CTX02-14608 500V Epcos AG (Germany) B78304-A1477-A3 (USA) Midcom 500V 31160R 1.25kV P1597 500V Pulse FEE (France) (561) 241-7876 3.75kVAC (561) 241-7876 (0 89) 636-2 80 00 (800) 888-7724 (605) 886-4385 (33) 3 84 35 04 04 Sumida (Japan) S-167-5779 100V 03-3667-3320 Transpower TTI7780-SM 500V (775) 852-0140 Table 2. Fault Mode Behavior VCC > VUVH VCC2 > VUVH FUNCTION (PINS) DC/DC Converter (2, 3) RO (28) VCC < VUVL VCC2 > VUVH VCC > VUVH VCC2 < VUVL VCC < VUVL VCC2 > VUVL THERMAL SHUTDOWN On On On On Off RE = 0V Active Forced High Forced High Forced High Forced High RE = VCC Hi-Z Hi-Z Hi-Z Hi-Z Hi-Z Active Hi-Z Hi-Z Hi-Z Hi-Z RO2 (17) RE = Floating Active Active Active Active Active Driver Outputs Y and Z (13,12) Active Hi-Z Hi-Z Hi-Z Hi-Z Communication Across Isolation Barrier Active Disabled Disabled Disabled Disabled Low High High High High Fault Indicator on RE (27) Table 3. Driver Function Table Table 4. Receiver Function Table INPUTS OUTPUTS INPUTS OUTPUTS RE DE DI A B RE DE A–B RO RO2 X 1 1 1 0 0 X ≥ VTH(MAX) 1 1 X 1 0 0 1 0 X ≤ VTH(MIN) 0 0 X 0 X Z Z 0 X Inputs Open 1 1 0 X Inputs Shorted 1 1 1 X ≥ VTH(MAX) Z 1 1 X ≤ VTH(MIN) Z 0 1 X Inputs Open Z 1 1 X Inputs Shorted Z 1 Note: Z = high impedance, X = don’t care Note: Z = high impedance, X = don’t care 1535fa 11 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␣ 12 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, electromagnetically induced transients and radio frequency pickup are shunted by addition capacitor C1. Receiver Inputs Fail-Safe The LTC1535 features an input common mode range covering the entire RS485 specified range of –7V to 12V. Differential signals of greater than ±200mV within the specified input common mode range will be converted to TTL compatible signals at the receiver outputs, RO and RO2. A small amount of input hyteresis is included to minimize the effects of noise on the line signals. If the receiver inputs are floating or shorted, a designed-in receiver offset guarantees a fail-safe logic high at the receiver outputs. If a fail-safe logic low is desired, connect as shown in Figure 19. 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 F12 Figure 12. Detecting Wiring Faults 1535fa 12 LTC1535 U W U U APPLICATIO S I FOR ATIO DI DI Y–Z Y–Z 1535 F13.tif 1535 F14.tif Figure 13. Driver Propagation Delay with Sample Jitter. SLO = VCC2 Figure 14. Driver Propagation Delay with Sample Jitter. SLO = 0V Z Z Y Y 1535 F15.tif 1535 F16.tif Figure 15. Driver Output. R = 27Ω, VCC2 = 5V, SLO = VCC2 Figure 16. Driver Output. R = 27Ω, VCC2 = 5V, SLO = 0V Y–Z Y–Z 1535 F17.tif Figure 17. Driver Differential Output. R = 27Ω, VCC2 = 5V, SLO = VCC2 1535 F18.tif Figure 18. Driver Differential Output. R = 27Ω, VCC2 = 5V, SLO = 0V 1535fa 13 LTC1535 U TYPICAL APPLICATIO S 3V DE Y R Z R CL1 DI CL2 1535 F04 Figure 19. Fail-Safe Logic “0” RO RE DE DI A B Y Z LTC1535 RO RE DE DI TTL INPUT 30k A B Y Z LTC1535 TTL INPUT 30k 1535 TA05 (20a) Noninverting (20b) Inverting Figure 20. Configuring Receiver for TTL Level Input. Y and Z Outputs Are TTL Compatible with No Modification Full-Duplex Connection ** CTX02-14659 1/2 BAT54C + 10µF 2 1/2 BAT54C 2 VCC 1 + VCC 3 ST1 ST2 2 11 14 GND2 VCC2 420kHz 10µF A 1 28 RO RO B RO2 27 RE 1 VCC 26 DI 25 RE Y DE Z SLO GND 4 15 17 13 120Ω D DI 16 120Ω R 1 12 18 1535 TA02 LOGIC COMMON FLOATING RS485 COMMON 1 2 ** TRANSFORMER COOPER (561) 241-7876 1535fa 14 LTC1535 U PACKAGE DESCRIPTIO SW Package 28-Lead Plastic Small Outline Isolation Barrier (Wide .300 Inch) (Reference LTC DWG # 05-08-1690) .697 – .712* (17.70 – 18.08) 28 27 26 25 18 17 16 15 .394 – .419 (10.007 – 10.643) NOTE 1 1 2 3 11 4 12 13 14 .291 – .299** (7.391 – 7.595) .005 (0.127) RAD MIN .037 – .045 (0.940 – 1.143) .093 – .104 (2.362 – 2.642) .010 – .029 × 45° (0.254 – 0.737) 0° – 8° TYP .009 – .013 (0.229 – 0.330) .050 (1.270) BSC NOTE 1 .016 – .050 (0.406 – 1.270) NOTE: 1. DIMENSIONS IN .014 – .019 (0.356 – 0.482) TYP INCHES (MILLIMETERS) 2. 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. *DIMENSION DOES NOT INCLUDE MOLD FLASH. MOLD FLASH SHALL NOT EXCEED .006" (0.152mm) PER SIDE **DIMENSION DOES NOT INCLUDE INTERLEAD FLASH. INTERLEAD FLASH SHALL NOT EXCEED .010" (0.254mm) PER SIDE .004 – 0.012 (0.102 – 0.305) SW28 (ISO) 0502 1535fa 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. 15 LTC1535 U TYPICAL APPLICATIO Complete, Isolated 24-Bit Data Acquisition System 1/2 BAT54C LT1761-5 + T1 10µF 16V TANT IN OUT SHDN BYP 10µF + GND 1µF 10µF 10V TANT 2 + 1/2 BAT54C RO ST1 RE DE DI VCC1 “SDO” “SCK” LOGIC 5V 1 10µF 10V TANT + ST2 VCC2 LTC1535 G1 G2 1 1 2 ISOLATION BARRIER 1 A B Y Z = LOGIC COMMON 2 10µF CERAMIC 10µF 10V TANT LTC2402 FO SCK SDO CS GND 1k 2 1535 TA03 = FLOATING COMMON 2 2 VCC FSSET CH1 CH0 ZSSET 2 T1 = COOPER CTX02-14659 RELATED PARTS PART NUMBER DESCRIPTION COMMENTS 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 1535fa 16 Linear Technology Corporation LT/TP 1103 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 1999