19-1778; Rev 3; 11/01 High CMRR RS-485 Transceiver with ±50V Isolation Features ♦ ±50V Isolated Data Interface ♦ +5V Single Supply ♦ Low-Cost Replacement for Opto-Isolated Transceivers ♦ True Fail-Safe Receiver While Maintaining EIA/TIA-485 Compliance ♦ Pin-Selectable Full/Half-Duplex Operation ♦ Phase Controls to Correct for Twisted-Pair Reversal ♦ 25µA Low-Power Shutdown Mode ♦ Thermal Shutdown for Driver Overload Protection ♦ 28-Pin SSOP Package ♦ Slew Rate Limited Reduced EMI Ordering Information ________________________Applications PART TEMP. RANGE MAX3157CAI 0°C to +70°C PIN-PACKAGE 28 SSOP 28 SSOP MAX3157EAI -40°C to +85°C Industrial Controls MAX3157CPI 0°C to +70°C 28 PDIP Level Translators MAX3157EPI -40°C to +85°C 28 PDIP Telecommunications Pin Configurations appear at end of data sheet. Local Area Networks Typical Operating Circuit VCC 0.1µF MICRO DE DI RE RO MAX3157 GND C3 1µF A B Y Z YR ZR AR BR RT = 100Ω RS-485 NODE REMOTE MICRO ISOVCC VCC H/F TXP RXP RG C1LO C1HI C2LO C2HI ISOCOM ±50V C1 0.047µF C2 0.047µF C4 10nF GND OFFSET REMOTE GND ________________________________________________________________ Maxim Integrated Products For pricing, delivery, and ordering information, please contact Maxim/Dallas Direct! at 1-888-629-4642, or visit Maxim’s website at www.maxim-ic.com. 1 MAX3157 General Description The MAX3157 is a high CMRR RS-485/RS-422 data-communications interface providing ±50V isolation in a hybrid microcircuit. A single +5V supply on the logic side powers both sides of the interface, with external ±50V capacitors transferring power from the logic side to the isolated side. Each MAX3157 contains one transmitter and one receiver and is guaranteed to operate at data rates up to 250kbps. Drivers are short-circuit current limited and protected against excessive power dissipation by thermal shutdown circuitry that places the driver outputs into a high-impedance state. The receiver input has a fail-safe feature that guarantees a logic high receiver output if the inputs are open, shorted, or connected to a terminated transmission line with all drivers disabled. The MAX3157 typically draws 25mA of supply current when unloaded or when fully loaded with the driver disabled. Supply current drops to 25µA when the device is placed in shutdown mode. The device is pin selectable between half- and full-duplex mode and also features independently programmable receiver and transmitter output phase through separate pins. The MAX3157 is a low-cost replacement for opto-isolated transceivers. For fully isolated RS-485/RS-422 transceivers, refer to the MAX1480 family data sheet. MAX3157 High CMRR RS-485 Transceiver with ±50V Isolation ABSOLUTE MAXIMUM RATINGS All voltages referenced to GND unless otherwise noted. Supply Voltage (VCC).............................................................+7V Cable Ground (ISOCOM) ....................................................±75V Isolated Supply ISOVCC Relative to Cable Ground (ISOCOM) ............................................................................+7V Digital Input, Output Voltage (DI, DE, RE, TXP, RXP, RO)......................-0.3V to (VCC + 0.3V) Digital Inputs (H/F) Relative to Cable Ground (ISOCOM) ..........................................-0.3V to (ISOVCC + 0.3V) Driver Output Voltage (Y, Z), Relative to Cable Ground (ISOCOM).............................................................-8V to +12.5V Receiver Input Voltage (A, B), Relative to Cable Ground (ISOCOM).............................................................-8V to +12.5V Termination Connections (YR, ZR, AR, BR) Relative to Cable Ground (ISOCOM)................................................-8V to +12.5V Charge-Pump Capacitance Low (C1LO, C2LO) .......................................-0.3V to (VCC + 0.3V) Charge-Pump Capacitance High (C1HI, C2HI) Relative to Cable Ground (ISOCOM) ...........-0.3V to (ISOVCC + 0.3V) Continuous Power Dissipation 28-Pin SSOP (derate 10.8mW/°C above +70°C) ........860mW 28-Pin PDIP (derate 14.3mW/°C above +70°C)........1143mW Operating Temperature Ranges MAX3157C_I .....................................................0°C to +70°C MAX3157E_I ..................................................-40°C to +85°C Junction Temperature ......................................................+150°C Storage Temperature Range ............................-65°C to +160°C Lead Temperature (soldering, 10s) ................................+300°C Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability. ELECTRICAL CHARACTERISTICS (VCC = +5V ±5%, YR = ZR = AR = BR = ISOCOM, C1 = C2 = 0.047µF, C3 = C4 = 1µF, TA = TMIN to TMAX, unless otherwise noted. Typical values are at VCC = +5V and TA = +25°C.) (Note 1) PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS 6 V DRIVER Differential Driver Output, No Load VOD1 Differential Driver Output, Loaded VOD2 Change in Magnitude of Differential Output Voltage (Note 2) ∆VOD Figure 1, R = 50Ω or R = 27Ω 0.2 V VOC Figure 1, R = 50Ω or R = 27Ω, VOC relative to ISOCOM 3.5 V ∆VOC Figure 1, R = 50Ω or R = 27Ω 0.2 V Driver Common-Mode Voltage Change in Magnitude of Common-Mode Voltage (Note 2) Figure 1, R = 50Ω (RS-422) 2.0 Figure 1, R = 27Ω (RS-485) 1.5 Input High Voltage VIH DE, DI, RE, TXP, RXP, relative to GND, H/F relative to ISOCOM Input Low Voltage VIL DE, DI, RE, TXP, RXP, relative to GND, H/F relative to ISOCOM DI Input Hysteresis VHYS Input Current Input Current (A and B) 2 Figure 1, R = 10MΩ 2.0 V 0.8 100 IIN1 DE, DI, RE IIN2 H/F, TXP, RXP internal pulldown IIN3 V V mV ±2 10 40 DE = GND, VCC = GND or 5.25V, -7V ≤ (VIN VISOCOM) ≤ 12V, AR = BR = ISOCOM -5.0 8.0 DE = GND, VCC = GND or 5.25V, -7V ≤ (VIN VISOCOM) ≤ 12V, AR open, BR open -0.58 1.0 µA mA _______________________________________________________________________________________ High CMRR RS-485 Transceiver with ±50V Isolation (VCC = +5V ±5%, YR = ZR = AR = BR = ISOCOM, C1 = C2 = 0.047µF, C3 = C4 = 1µF, TA = TMIN to TMAX, unless otherwise noted. Typical values are at VCC = +5V and TA = +25°C.) (Note 1) PARAMETER Output Leakage (Y and Z) Full Duplex Driver Short-Circuit Output Current (Note 3) SYMBOL CONDITIONS MIN DE = GND, VCC = GND or 5.25V, -7V ≤ (VIN VISOCOM) ≤ 12V, YR = ZR = ISOCOM -5.0 8.0 DE = GND, VCC = GND or 5.25V, -7V ≤ (VIN VISOCOM) ≤ 12V, YR open, ZR open -0.58 1.0 IOSD1 VISOCOM - 7V ≤ VOUT ≤ VISOVCC -250 IOSD2 VISOCOM - 7V ≤ VOUT ≤ VISOCOM +12V IO TYP MAX UNITS mA 250 mA RECEIVER Receiver Differential Threshold Voltage VTH -7V ≤ VCM - VISOCOM ≤ 12V -200 -125 -50 mV Receiver Input Hysteresis ∆VTH Receiver Output High Voltage VOH IO = -4mA, VID = -50mV Receiver Output Low Voltage VOL IO = 4mA, VID = -200mV 0.4 V Three-State Output Current at Receiver IOZR 0.4V ≤ VO ≤ 2.4V ±1 µA Receiver Input Resistance Receiver Output Short-Circuit Current 25 V -7V ≤ VCM - VISOCOM ≤ 12V, AR = BR = ISOCOM 1.5 -7V ≤ VCM - VISOCOM ≤ 12V, AR open, BR open 12 IOSR 0V ≤ VRO ≤ VCC ±7 ICC No load, RE = DE = DI = GND or VCC 25 DE = GND, RE = VCC, VISOCOM = GND 25 RIN mV V CC - 1.5 kΩ ±100 mA 85 mA SUPPLY CURRENT Supply Current Supply Current in Shutdown Mode ISHDN Maximum Ground Differential ∆VGND DE = GND, RE = VCC, VISOCOM = ±50V DE = GND, RE = VCC, | ISOCOM leakage | ≤ 600µA 60 ±600 ±50 µA V SWITCHING CHARACTERISTICS (VCC = +5V ±5%, YR = ZR = AR = BR = ISOCOM, C1 = C2 = 0.047µF, C3 = C4 = 1µF. Typical values are at VCC = +5V and TA = +25°C.) PARAMETER SYMBOL CONDITIONS MIN TYP MAX tDPLH Figures 3 and 5, RDIFF = 54Ω, CL1 = CL2 = 100pF 400 1400 2000 tDPHL Figures 3 and 5, RDIFF = 54Ω, CL1 = CL2 = 100pF 400 Driver Output Skew | tDPLH - tDPHL | tDSKEW Figures 3 and 5, RDIFF = 54Ω, CL1 = CL2 = 100pF Driver Rise or Fall Time tDR, tDF Figures 3 and 5, RDIFF = 54Ω, CL1 = CL2 =100pF Driver Input to Output Maximum Data Rate fMAX UNITS ns 200 250 1400 1200 2000 250 ns 2200 ns kbps _______________________________________________________________________________________ 3 MAX3157 ELECTRICAL CHARACTERISTICS (continued) MAX3157 High CMRR RS-485 Transceiver with ±50V Isolation SWITCHING CHARACTERISTICS (continued) (VCC = +5V ±5%, YR = ZR = AR = BR = ISOCOM, C1 = C2 = 0.047µF, C3 = C4 = 1µF. Typical values are at VCC = +5V and TA = +25°C.) PARAMETER MAX UNITS tDZH Figures 4 and 6, CL = 100pF, S2 closed 3500 ns Driver Enable to Output Low tDZL Figures 4 and 6, CL = 100pF, S1 closed 3500 ns Driver Disable Time from Low tDLZ Figures 4 and 6, CL = 15pF, S1 closed 300 ns Driver Disable Time from High tDHZ Figures 4 and 6, CL = 15pF, S2 closed 300 ns 600 ns Driver Enable to Output High SYMBOL CONDITIONS MIN TYP Receiver Input to Output tRPLH, tRPHL Figures 7 and 9; | VID | ≥ 2.0V 440 Differential Receiver Skew |tRPLH - tRPHL| tRSKEW Figures 7 and 9; | VID | ≥ 2.0V 20 ns Receiver Enable to Output Low tRZL Figures 2 and 8, CL = 100pF, S1 closed 20 50 Receiver Enable to Output High tRZH Figures 2 and 8, CL = 100pF, S2 closed 20 50 ns Receiver Disable Time from Low tRLZ Figures 2 and 8, CL = 100pF, S1 closed 200 500 ns tRHZ Figures 2 and 8, CL = 100pF, S2 closed 200 500 ns 200 700 ns Receiver Disable Time from High Time to Shutdown tSHDN (Note 4) 50 ns Driver Enable from Shutdown to Output High tDZH (SHDN) Figures 4 and 6, CL = 15pF, S2 closed 0.2 1 ms Driver Enable from Shutdown to Output Low tDZL (SHDN) Figures 4 and 6, CL = 15pF, S1 closed 0.2 1 ms Receiver Enable from Shutdown to Output High tRZH (SHDN) Figures 2 and 8, CL = 100pF, S2 closed 0.2 1 ms Receiver Enable from Shutdown to Output Low tRZL (SHDN) Figures 2 and 8, CL = 100pF, S1 closed 0.2 1 ms Charge-Pump Oscillating Frequency fOSC 1.3 MHz Note 1: All currents into the device are positive; all currents out of the device are negative. All voltages are referred to device ground unless otherwise noted. Note 2: ∆VOD and ∆VOC are the changes in VOD and VOC, respectively, when the DI input changes state. Note 3: Current level applies to peak current just prior to foldback-current limiting. Note 4: The device is put into shutdown by bringing RE high and DE low. If the enable inputs are in this state for less than 50ns, the device is guaranteed not to enter shutdown. If the enable inputs are in this state for at least 700ns, the device is guaranteed to have entered shutdown. 4 _______________________________________________________________________________________ High CMRR RS-485 Transceiver with ±50V Isolation SHUTDOWN SUPPLY CURRENT vs. TEMPERATURE 100 80 60 40 100 0 VISOCOM = 0 -100 VISOCOM = +50V NO LOAD 30 25 20 15 5 -300 0 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90 TEMPERATURE (°C) TEMPERATURE (°C) RECEIVER OUTPUT CURRENT vs. RECEIVER OUTPUT HIGH VOLTAGE 14 12 10 8 6 IRO = 8mA 0.40 0.30 0.25 0.20 0.15 4.25 4.20 4.15 4.10 0.10 4.05 2 0.05 4.00 0 3.95 -40 -20 OUTPUT HIGH VOLTAGE (V) 0 20 40 60 80 100 -40 -20 TEMPERATURE (°C) 80 60 40 60 80 100 100 MAX3157toc08 0 -20 -30 -40 -50 OUTPUT CURRENT (mA) 100 40 DRIVER OUTPUT CURRENT vs. DIFFERENTIAL OUTPUT VOLTAGE -10 OUTPUT CURRENT (mA) 120 20 TEMPERATURE (°C) DRIVER OUTPUT CURRENT vs. DRIVER OUTPUT HIGH VOLTAGE MAX3157toc07 140 0 -60 -70 -80 -90 MAX3157toc09 DRIVER OUTPUT CURRENT vs. DRIVER OUTPUT LOW VOLTAGE 160 6 4.30 0.35 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 5 IRO = -8mA 4.35 4 0 4 4.40 OUTPUT VOLTAGE (V) OUTPUT LOW VOLTAGE (V) 16 3 RECEIVER OUTPUT HIGH VOLTAGE vs. TEMPERATURE 0.50 0.45 2 OUTPUT LOW VOLTAGE (V) MAX3157toc05 18 1 RECEIVER OUTPUT LOW VOLTAGE vs. TEMPERATURE MAX3157toc04 20 0 MAX3157toc06 0 OUTPUT CURRENT (mA) 35 10 -200 20 OUTPUT CURRENT (mA) 40 OUTPUT CURRENT (mA) SUPPY CURRENT (µA) RL = 54Ω 120 VISOCOM = -50V 200 45 MAX1357toc02 140 SUPPY CURRENT (mA) 300 MAX1357toc01 160 RECEIVER OUTPUT CURRENT vs. RECEIVER OUTPUT LOW VOLTAGE MAX3157toc03 SUPPLY CURRENT vs. TEMPERATURE 10 1 0.1 -100 20 -110 -120 0 0 2 4 6 8 10 OUTPUT VOLTAGE (V) 12 14 0.01 -8 -6 -4 -2 0 2 OUTPUT HIGH VOLTAGE (V) 4 6 0 1 2 3 4 5 6 DIFFERENTIAL OUTPUT VOLTAGE (V) _______________________________________________________________________________________ 5 MAX3157 Typical Operating Characteristics (VCC = +5V, YR = ZR = AR = BR = ISOCOM, C1 = C2 = 0.047µF, C3 = 1µF, C4 = 10nF, TA = +25°C, unless otherwise noted.) Typical Operating Characteristics (continued) (VCC = +5V, YR = ZR = AR = BR = ISOCOM, C1 = C2 = 0.047µF, C3 = 1µF, C4 = 10nF, TA = +25°C, unless otherwise noted.) RECEIVER PROPAGATION DELAY vs. TEMPERATURE 1.92 1.90 1.88 1.86 470 460 450 440 1.84 430 1.82 420 1.80 410 RL = 54Ω 1.50 PROPAGATION DELAY (µs) 480 PROPAGATION DELAY (ns) OUTPUT VOLTAGE (V) 1.94 1.52 MAX3157toc11 RL = 54Ω 1.96 490 MAX3157toc10 1.98 DRIVER PROPAGATION DELAY vs. TEMPERATURE MAX3157toc12 DRIVER DIFFERENTIAL OUTPUT VOLTAGE vs. TEMPERATURE 1.48 1.46 1.44 1.42 1.40 1.38 1.36 -40 -20 0 20 40 60 80 1.34 1.32 -40 -20 0 TEMPERATURE (°C) DRIVER ENABLE TIME vs. TEMPERATURE DRIVER DISABLE TIME (ns) 1.450 1.425 1.400 1.375 1.350 40 60 80 -40 -20 0 20 40 60 80 TEMPERATURE (°C) TEMPERATURE (°C) DRIVER DISABLE TIME vs. TEMPERATURE RECEIVER PROPAGATION DELAY MAX3157toc15 MAX3157toc14 1.475 20 250 MAX3157toc13 1.500 DRIVER ENABLE TIME (µs) MAX3157 High CMRR RS-485 Transceiver with ±50V Isolation 200 VA - VB 0 5V/div 150 0 RO 100 50 1.325 1.300 0 -40 -20 0 20 40 60 80 100 -40 -20 0 20 40 60 TEMPERATURE (°C) TEMPERATURE (°C) DRIVER PROPAGATION DELAY POWER-UP DELAY (VISOCOM = 0) MAX3157toc16 80 100 2µs/div POWER-UP DELAY (VISOCOM = -50V) MAX3157toc17 MAX3157toc18 DE 5V/div DI DE 10V/div 0 2V/div 0 0 VY -50V 5V/div VY - VZ VY 10V/div RDIFF = 54Ω CL1 = CL2 = 100pF 2µs/div 6 40µs/div 100µs/div _______________________________________________________________________________________ High CMRR RS-485 Transceiver with ±50V Isolation MAX3157 Typical Operating Characteristics (continued) (VCC = +5V, YR = ZR = AR = BR = ISOCOM, C1 = C2 = 0.047µF, C3 = 1µF, C4 = 10nF, TA = +25°C, unless otherwise noted.) POWER-UP DELAY (VISOCOM = +50V) DE 5V/div 0 +50V VY 10V/div 60 MAXIMUM COMMON PEAK VOLTAGE TO ISOCOM 50 40 COMMON-MODE VOLTAGE TO ISOCOM = 7V PEAK 30 20 10 0 0.1 100µs/div MAX3157 toc20 MAX3157toc19 COMMON-MODE PEAK VOLTAGE TO GROUND (V) MAXIMUM COMMON-MODE PEAK VOLTAGE TO GROUND vs. COMMON-MODE FREQUENCY 1 10 COMMON-MODE FREQUENCY (kHz) Test Circuits and Timing Diagrams Y 3V DE CL1 R Y DI RDIFF VID VOD Z R VOC CL2 Z Figure 3. Driver Timing Test Circuit Figure 1. Driver DC Test Load RECEIVER OUTPUT 1kΩ TEST POINT CL 100pF VCC S1 500Ω S1 VCC OUTPUT UNDER TEST 1kΩ CL S2 S2 Figure 2. Receiver Enable/Disable Timing Test Load Figure 4. Driver Enable/Disable Timing Test Load _______________________________________________________________________________________ 7 High CMRR RS-485 Transceiver with ±50V Isolation MAX3157 Test Circuits and Timing Diagrams (continued) 3V 3V DI 1.5V RE 1.5V 0 1.5V 1/2 VO tRZL(SHDN), tRZL Z VO 1/2 VO 10% 1.5V OUTPUT NORMALLY LOW RO 1.5V tDR VOL + 0.5V OUTPUT NORMALLY HIGH VDIFF = V (Y) - V (Z) 90% 90% tRLZ VCC RO Y VO VDIFF 0 -VO 1.5V 0 tDPHL tDPLH 10% VOH - 0.5V 0 tRZH(SHDN), tRZH tDF tDSKEW = | tDPLH - tDPHL | Figure 5. Driver Propagation Delays tRHZ Figure 8. Receiver Enable and Disable Times 3V DE 1.5V 1.5V 0 tDLZ tDZL(SHDN), tDZL B Y, Z 2.3V OUTPUT NORMALLY LOW VOL VOL + 0.5V ATE VID RR A OUTPUT NORMALLY HIGH Y, Z VOH - 0.5V 2.3V 0 tDZH(SHDN), tDZH tDHZ Figure 6. Driver Enable and Disable Times RO Figure 9. Receiver Propagation Delay Test Circuit VOH VOL 1V A -1V B 1.5V OUTPUT tRPHL 1.5V tRPLH INPUT tRSKEW = | tRPLH - tRPHL | Figure 7. Receiver Propagation Delays 8 _______________________________________________________________________________________ RECEIVER OUTPUT High CMRR RS-485 Transceiver with ±50V Isolation PIN NAME FUNCTION SSOP PDIP 1 2 RO Receiver Output. When RE is low and A - B > -50mV, RO will be high; if A - B ≤ -200mV, RO will be low. 2 3 RE Receiver Output Enable. Drive RE low to enable RO. 3 4 DE Driver Output Enable. Drive DE high to enable driver outputs. 4 5 DI Driver Input. With DE high, a low on DI forces the noninverting output low and the inverting output high; with DI high, outputs reverse. 5 6 C1LO 6, 7, 12, 22, 23 1, 12, 14, 15, 28 N.C. No Connection 8 7 C1HI Connect to the positive terminal of C1 (0.047µF, 50V). 9 8 ISOVCC 10 9 H/F Half/Full-Duplex Selector Pin. Leave open, or connect to ISOCOM to select Full Duplex, or connect to ISOVCC to select Half Duplex. 11 10 ZR Connect to ISOCOM for a 1.5kΩ input impedance on Z. Leave open for a 12kΩ input impedance on Z. 13 11 YR Connect to ISOCOM for a 1.5kΩ input impedance on Y. Leave open for a 12kΩ input impedance on Y. 14 13 ISOCOM 15 16 Z Inverting Driver Output (and Inverting Receiver Input in Half-Duplex Mode) 16 17 Y Noninverting Driver Output (and Noninverting Receiver Input in Half-Duplex Mode) 17 18 B Inverting Receiver Input in Full-Duplex Mode 18 19 A Noninverting Receiver Input in Full-Duplex Mode 19 20 BR Connect to ISOCOM for a 1.5kΩ input impedance on B. Leave open for a 12kΩ input impedance on B. 20 21 AR Connect to ISOCOM for a 1.5kΩ input impedance on A. Leave open for a 12kΩ input impedance on A. 21 22 C2HI 24 23 C2LO Connect to the negative terminal of C2 (0.047µF, 50V). 25 24 GND Ground 26 25 TXP Transmitter Phase. Leave open, or connect to GND for normal transmitter polarity, or connect to VCC to invert the transmitter polarity. 27 26 RXP Receiver Phase. Leave open, or connect to GND for normal receiver polarity, or connect to VCC to invert the receiver polarity. 28 27 VCC +4.75V to +5.25V Positive Supply. Connect a 0.1µF capacitor to GND. Connect to the negative terminal of C1 (0.047µF, 50V). Internally generated power-supply voltage, referenced to the cable ground (ISOCOM). Connect a 1µF capacitor to ISOCOM. Cable Ground Connect to the positive terminal of C2 (0.047µF, 50V). _______________________________________________________________________________________ 9 MAX3157 Pin Description MAX3157 High CMRR RS-485 Transceiver with ±50V Isolation Detailed Description The MAX3157 is a high CMRR RS-485/RS-422 datacommunications interface providing ±50V isolation in a hybrid microcircuit. A single +5V supply on the logic side powers both sides of the interface, with external ±50V capacitors transferring power from the logic side to the isolated side (see Block Diagram). The MAX3157 typically draws 25mA of supply current when unloaded or when fully loaded with the driver disabled. Supply current drops to 25µA when the device is placed in shutdown mode (see Low-Power Shutdown Mode). The MAX3157 transceiver for RS-485/RS-422 communication contains one driver and one receiver. This device features fail-safe circuitry, which guarantees a logic-high receiver output when the receiver inputs are open or shorted, or when they are connected to a terminated transmission line with all drivers disabled (see Fail-Safe). The MAX3157 is selectable between half- and full-duplex communication by connecting a selector pin to ISOVCC or ISOCOM, respectively. Drivers are output short-circuit current limited. Thermal shutdown circuitry protects drivers against excessive power dissipation. When activated, the thermal shutdown circuitry places the driver outputs into a high-impedance state. The device also features independently programmable receiver and transmitter output phase via separate pins. The MAX3157 is a low-cost replacement for opto-isolated transceivers. connect it to VCC. To invert the receiver phase, drive RXP high or connect it to VCC. Note that the receiver threshold is positive when RXP is high. The MAX3157 can operate in full- or half-duplex mode. Connect H/F to ISOCOM for full-duplex mode, and connect it to ISOVCC for half-duplex operation. In halfduplex mode, the receiver inputs are switched to the driver outputs, connecting outputs Y and Z to inputs A and B, respectively. In half-duplex mode, the internal full-duplex receiver input resistors are still connected to pins A and B. Low-Power Shutdown Mode The low-power shutdown mode is initiated by bringing both RE high and DE low. In shutdown, this device typically draws only 25µA of supply current, and no power is transferred across the isolation capacitors in this mode. RE and DE may be driven simultaneously; the parts are guaranteed not to enter shutdown if RE is high and DE is low for less than 50ns. If the inputs are in this state for at least 700ns, the parts are guaranteed to enter shutdown. Enable times t_ZH and t_ZL in the Switching Characteristics table assume the part was not in a low-power shutdown state. Enable times t_ZH(SHDN) and t_ZL(SHDN) assume the parts were shut down. It takes drivers and receivers longer to become enabled from low-power shutdown mode t_ZH(SHDN), t_ ZL(SHDN) than from driver/receiver-disable mode (t_ZH, t_ZL). Fail-Safe Driver Output Protection The MAX3157 guarantees a logic-high receiver output when the receiver inputs are shorted or open, or when they are connected to a terminated transmission line with all drivers disabled. The receiver threshold is fixed between -50mV and -200mV. If the differential receiver input voltage (A - B) is greater than or equal to -50mV, RO is logic high. If A - B is less than or equal to -200mV, RO is logic low. In the case of a terminated bus with all transmitters disabled, the receiver’s differential input voltage is pulled to 0V by the termination. With the receiver thresholds of the MAX3157, this results in a logic high with a 50mV minimum noise margin. Unlike competitor’s fail-safe devices, the -50mV to -200mV threshold complies with the ±200mV EIA/TIA-485 standard. Two mechanisms prevent excessive output current and power dissipation caused by faults or by bus contention. The first, a foldback current limit on the output stage, provides immediate protection against short circuits over the whole common-mode voltage range (see Typical Operating Characteristics). The second, a thermal shutdown circuit, forces the driver outputs into a high-impedance state if the die temperature becomes excessive—typically around +150°C. MAX3157 Programming The MAX3157 has several programmable operating modes. Occasionally, twisted-pair lines are reversed. The MAX3157 has two pins that invert the phase of the driver and the receiver to correct for this problem. For normal operation, drive TXP and RXP low, connect them to ground, or leave them unconnected (internal pulldown). To invert the driver phase, drive TXP high or 10 Applications Information Capacitor and Grounding Resistor Selection The value for the charge-pump capacitors C1 and C2 should be between 47nF and 100nF. Smaller values will result in insufficient supply voltage on the isolated side. Larger values are allowed but will not result in better charge-pump capacity. The values for C1 and C2, as well as that of C4, determine the maximum frequency and amplitude of the voltage difference (under operating conditions) between the local and isolated ground. Besides the capacitor values, this maximum frequency ______________________________________________________________________________________ High CMRR RS-485 Transceiver with ±50V Isolation Y DI Z TXP YR DE ZR RO A B RE RXP AR BR H/F VCC ISCOVCC POWER OSC GND C1L C2L and amplitude are also determined by the resistance between the remote ground and the ISOCOM pin. The receiver input resistors will cause the isolated common voltage to go to the mean voltage of the receiver inputs, which will be a direct function of the remote ground potential. The receiver input resistance and the capacitors C1, C2, and C4 set up a time constant that limits how fast the ISOCOM pin can follow variations in the remote ground voltage. Connecting YR and ZR in halfduplex operation, or AR and BR in full-duplex operation, to ISOCOM results in a relatively low input impedance of the MAX3157 receiver inputs (2kΩ). This allows for a 60Hz sine wave with a 50V maximum amplitude (see Typical Operating Characteristics). If YR, ZR, AR, and BR are left open, the receiver input impedance is 12kΩ allowing up to 32 transceivers on the bus. To guarantee the same low time constant under those conditions, use a shielded cable with a 1kΩ resistor connected between the shield and ISOCOM. Using a lower value for this resistor is not recommended because this could trigger C1H C2H ISOCOM a holding current in the internal ESD protection device if the ±75V isolation limit is exceeded. A single point hard-ground connection for the shield is recommended. Communication Between Two MAX3157s If two MAX3157 devices are used to communicate with each other, one of the devices must have ISOCOM connected to local ground. Failure to do so will result in floating ISOCOM pins, with both devices trying to adapt to the isolated ground of the other. Chip Information TRANSISTOR COUNT: 1309 ______________________________________________________________________________________ 11 MAX3157 Block Diagram High CMRR RS-485 Transceiver with ±50V Isolation MAX3157 Pin Configurations TOP VIEW TOP VIEW RO 1 28 VCC N.C. 1 28 N.C. RE 2 27 RXP RO 2 27 VCC DE 3 26 TXP RE 3 26 RXP DI 4 25 GND DE 4 25 TXP 24 C2LO DI 5 C1LO 5 N.C. 6 MAX3157 23 N.C. C1LO 6 N.C. 7 22 N.C. C1HI 7 C1HI 8 21 C2HI 23 C2LO 22 C2HI ISOVCC 8 21 AR ISOVCC 9 20 AR H/F 9 20 BR H/F 10 19 BR ZR 10 19 A ZR 11 18 A YR 11 18 B N.C. 12 17 B N.C. 12 17 Y YR 13 16 Y ISOCOM 13 16 Z ISOCOM 14 15 Z N.C. 14 28 SSOP 12 24 GND MAX3157 15 N.C. 28 PDIP ______________________________________________________________________________________ High CMRR RS-485 Transceiver with ±50V Isolation SSOP.EPS PDIPN.EPS Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are implied. Maxim reserves the right to change the circuitry and specifications without notice at any time. Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 ____________________ 13 © 2001 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products. MAX3157 Package Information