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19-1778; Rev 3; 11/01
High CMRR RS-485 Transceiver with 50V Isolation
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 25A 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. o 50V Isolated Data Interface o +5V Single Supply o Low-Cost Replacement for Opto-Isolated Transceivers o True Fail-Safe Receiver While Maintaining EIA/TIA-485 Compliance o Pin-Selectable Full/Half-Duplex Operation o Phase Controls to Correct for Twisted-Pair Reversal o 25A Low-Power Shutdown Mode o Thermal Shutdown for Driver Overload Protection o 28-Pin SSOP Package o Slew Rate Limited Reduced EMI
Features
MAX3157
Ordering Information
PART MAX3157CAI MAX3157EAI MAX3157CPI MAX3157EPI TEMP. RANGE 0C to +70C -40C to +85C 0C to +70C -40C to +85C PIN-PACKAGE 28 SSOP 28 SSOP 28 PDIP 28 PDIP
________________________Applications
Industrial Controls Level Translators Telecommunications Local Area Networks
VCC 0.1F VCC H/F TXP RXP MICRO DE DI RE RO GND ISOVCC A B Y Z YR ZR AR BR
Pin Configurations appear at end of data sheet.
Typical Operating Circuit
MAX3157
RT = 100
RS-485 NODE
RG
C1LO C1HI C2LO C2HI ISOCOM 50V
C1 0.047F
C2 0.047F
C4 10nF
GND OFFSET
REMOTE GND
________________________________________________________________ Maxim Integrated Products
REMOTE MICRO
C3 1F
1
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.
High CMRR RS-485 Transceiver with 50V Isolation MAX3157
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 +70C) ........860mW 28-Pin PDIP (derate 14.3mW/C above +70C)........1143mW Operating Temperature Ranges MAX3157C_I .....................................................0C to +70C MAX3157E_I ..................................................-40C to +85C Junction Temperature ......................................................+150C Storage Temperature Range ............................-65C to +160C Lead Temperature (soldering, 10s) ................................+300C
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 = 0.047F, C3 = C4 = 1F, TA = TMIN to TMAX, unless otherwise noted. Typical values are at VCC = +5V and TA = +25C.) (Note 1)
PARAMETER DRIVER Differential Driver Output, No Load Differential Driver Output, Loaded Change in Magnitude of Differential Output Voltage (Note 2) Driver Common-Mode Voltage Change in Magnitude of Common-Mode Voltage (Note 2) Input High Voltage Input Low Voltage DI Input Hysteresis Input Current VOD1 VOD2 Figure 1, R = 10M Figure 1, R = 50 (RS-422) Figure 1, R = 27 (RS-485) Figure 1, R = 50 or R = 27 Figure 1, R = 50 or R = 27, VOC relative to ISOCOM Figure 1, R = 50 or R = 27 DE, DI, RE, TXP, RXP, relative to GND, H/F relative to ISOCOM DE, DI, RE, TXP, RXP, relative to GND, H/F relative to ISOCOM 100 DE, DI, RE H/F, TXP, RXP internal pulldown DE = GND, VCC = GND or 5.25V, -7V (VIN VISOCOM) 12V, AR = BR = ISOCOM DE = GND, VCC = GND or 5.25V, -7V (VIN VISOCOM) 12V, AR open, BR open 10 -5.0 -0.58 2 40 8.0 mA 1.0 2.0 1.5 0.2 6 V V SYMBOL CONDITIONS MIN TYP MAX UNITS
VOD
V
VOC
3.5
V
VOC
0.2
V
VIH VIL VHYS IIN1 IIN2
2.0 0.8
V V mV A
Input Current (A and B)
IIN3
2
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High CMRR RS-485 Transceiver with 50V Isolation
ELECTRICAL CHARACTERISTICS (continued)
(VCC = +5V 5%, YR = ZR = AR = BR = ISOCOM, C1 = C2 = 0.047F, C3 = C4 = 1F, TA = TMIN to TMAX, unless otherwise noted. Typical values are at VCC = +5V and TA = +25C.) (Note 1)
PARAMETER SYMBOL CONDITIONS DE = GND, VCC = GND or 5.25V, -7V (VIN VISOCOM) 12V, YR = ZR = ISOCOM DE = GND, VCC = GND or 5.25V, -7V (VIN VISOCOM) 12V, YR open, ZR open VISOCOM - 7V VOUT VISOVCC VISOCOM - 7V VOUT VISOCOM +12V MIN -5.0 -0.58 -250 250 TYP MAX 8.0 mA 1.0 mA UNITS
MAX3157
Output Leakage (Y and Z) Full Duplex
IO
Driver Short-Circuit Output Current (Note 3) RECEIVER Receiver Differential Threshold Voltage Receiver Input Hysteresis Receiver Output High Voltage Receiver Output Low Voltage Three-State Output Current at Receiver
IOSD1 IOSD2
VTH VTH VOH VOL IOZR
-7V VCM - VISOCOM 12V
-200
-125 25
-50
mV mV V
IO = -4mA, VID = -50mV IO = 4mA, VID = -200mV 0.4V VO 2.4V -7V VCM - VISOCOM 12V, AR = BR = ISOCOM
V CC - 1.5 0.4 1 1.5
V A
Receiver Input Resistance
RIN
-7V VCM - VISOCOM 12V, AR open, BR open 0V VRO VCC
k 12 7 100 mA
Receiver Output Short-Circuit Current SUPPLY CURRENT Supply Current Supply Current in Shutdown Mode Maximum Ground Differential
IOSR
ICC ISHDN VGND
No load, RE = DE = DI = GND or VCC DE = GND, RE = VCC, VISOCOM = GND DE = GND, RE = VCC, VISOCOM = 50V DE = GND, RE = VCC, | ISOCOM leakage | 600A 50
25 25
85 60 600
mA A V
SWITCHING CHARACTERISTICS
(VCC = +5V 5%, YR = ZR = AR = BR = ISOCOM, C1 = C2 = 0.047F, C3 = C4 = 1F. Typical values are at VCC = +5V and TA = +25C.)
PARAMETER SYMBOL tDPLH Driver Input to Output tDPHL Driver Output Skew | tDPLH - tDPHL | Driver Rise or Fall Time Maximum Data Rate tDSKEW tDR, tDF fMAX CONDITIONS Figures 3 and 5, RDIFF = 54, CL1 = CL2 = 100pF Figures 3 and 5, RDIFF = 54, CL1 = CL2 = 100pF Figures 3 and 5, RDIFF = 54, CL1 = CL2 = 100pF Figures 3 and 5, RDIFF = 54, CL1 = CL2 =100pF 200 250 1200 MIN 400 400 TYP 1400 1400 MAX 2000 ns 2000 250 2200 ns ns kbps UNITS
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3
High CMRR RS-485 Transceiver with 50V Isolation MAX3157
SWITCHING CHARACTERISTICS (continued)
(VCC = +5V 5%, YR = ZR = AR = BR = ISOCOM, C1 = C2 = 0.047F, C3 = C4 = 1F. Typical values are at VCC = +5V and TA = +25C.)
PARAMETER Driver Enable to Output High Driver Enable to Output Low Driver Disable Time from Low Driver Disable Time from High Receiver Input to Output Differential Receiver Skew |tRPLH - tRPHL| Receiver Enable to Output Low Receiver Enable to Output High Receiver Disable Time from Low Receiver Disable Time from High Time to Shutdown Driver Enable from Shutdown to Output High Driver Enable from Shutdown to Output Low Receiver Enable from Shutdown to Output High Receiver Enable from Shutdown to Output Low Charge-Pump Oscillating Frequency SYMBOL tDZH tDZL tDLZ tDHZ tRPLH, tRPHL tRSKEW tRZL tRZH tRLZ tRHZ tSHDN tDZH (SHDN) tDZL (SHDN) tRZH (SHDN) tRZL (SHDN) fOSC CONDITIONS Figures 4 and 6, CL = 100pF, S2 closed Figures 4 and 6, CL = 100pF, S1 closed Figures 4 and 6, CL = 15pF, S1 closed Figures 4 and 6, CL = 15pF, S2 closed Figures 7 and 9; | VID | 2.0V Figures 7 and 9; | VID | 2.0V Figures 2 and 8, CL = 100pF, S1 closed Figures 2 and 8, CL = 100pF, S2 closed Figures 2 and 8, CL = 100pF, S1 closed Figures 2 and 8, CL = 100pF, S2 closed (Note 4) Figures 4 and 6, CL = 15pF, S2 closed Figures 4 and 6, CL = 15pF, S1 closed Figures 2 and 8, CL = 100pF, S2 closed Figures 2 and 8, CL = 100pF, S1 closed 50 440 20 20 20 200 200 200 0.2 0.2 0.2 0.2 1.3 50 50 500 500 700 1 1 1 1 MIN TYP MAX 3500 3500 300 300 600 UNITS ns ns ns ns ns ns ns ns ns ns ns ms ms ms ms 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
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High CMRR RS-485 Transceiver with 50V Isolation
Typical Operating Characteristics
(VCC = +5V, YR = ZR = AR = BR = ISOCOM, C1 = C2 = 0.047F, C3 = 1F, C4 = 10nF, TA = +25C, unless otherwise noted.)
SHUTDOWN SUPPLY CURRENT vs. TEMPERATURE
MAX1357toc01
MAX1357toc02
MAX3157
SUPPLY CURRENT vs. TEMPERATURE
160 140 SUPPY CURRENT (mA) 120 100 80 60 40 20 0 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90 TEMPERATURE (C) NO LOAD RL = 54 300 200
SUPPY CURRENT (A)
RECEIVER OUTPUT CURRENT vs. RECEIVER OUTPUT LOW VOLTAGE
40 OUTPUT CURRENT (mA) 35 30 25 20 15 10 5
MAX3157toc03
45
VISOCOM = -50V
100 0 VISOCOM = 0 -100 VISOCOM = +50V -200 -300 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90 TEMPERATURE (C)
0 0 1 2 3 4 5 6 OUTPUT LOW VOLTAGE (V)
RECEIVER OUTPUT CURRENT vs. RECEIVER OUTPUT HIGH VOLTAGE
MAX3157toc04
RECEIVER OUTPUT LOW VOLTAGE vs. TEMPERATURE
MAX3157toc05
RECEIVER OUTPUT HIGH VOLTAGE vs. TEMPERATURE
4.35 4.30 OUTPUT VOLTAGE (V) 4.25 4.20 4.15 4.10 4.05 4.00 3.95 IRO = -8mA
MAX3157toc06
20 18 16 OUTPUT CURRENT (mA) 14 12 10 8 6 4 2 0
0.50 0.45 OUTPUT LOW VOLTAGE (V) 0.40 0.35 0.30 0.25 0.20 0.15 0.10 0.05 0 IRO = 8mA
4.40
0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 OUTPUT HIGH VOLTAGE (V)
-40
-20
0
20
40
60
80
100
-40
-20
0
20
40
60
80
100
TEMPERATURE (C)
TEMPERATURE (C)
DRIVER OUTPUT CURRENT vs. DRIVER OUTPUT LOW VOLTAGE
MAX3157toc07
DRIVER OUTPUT CURRENT vs. DRIVER OUTPUT HIGH VOLTAGE
MAX3157toc08
DRIVER OUTPUT CURRENT vs. DIFFERENTIAL OUTPUT VOLTAGE
MAX3157toc09
160 140 OUTPUT CURRENT (mA) 120 100 80 60 40 20 0 0 2 4 6 8 10 12
0 -10 OUTPUT CURRENT (mA) -20 -30 -40 -50 -60 -70 -80 -90 -100 -110 -120
100
OUTPUT CURRENT (mA) -8 -6 -4 -2 0 2 4 6
10
1
0.1
0.01 0 1 2 3 4 5 6 OUTPUT HIGH VOLTAGE (V) DIFFERENTIAL OUTPUT VOLTAGE (V)
14
OUTPUT VOLTAGE (V)
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5
High CMRR RS-485 Transceiver with 50V Isolation MAX3157
Typical Operating Characteristics (continued)
(VCC = +5V, YR = ZR = AR = BR = ISOCOM, C1 = C2 = 0.047F, C3 = 1F, C4 = 10nF, TA = +25C, unless otherwise noted.)
DRIVER DIFFERENTIAL OUTPUT VOLTAGE vs. TEMPERATURE
MAX3157toc10
RECEIVER PROPAGATION DELAY vs. TEMPERATURE
MAX3157toc11
DRIVER PROPAGATION DELAY vs. TEMPERATURE
1.50 PROPAGATION DELAY (s) 1.48 1.46 1.44 1.42 1.40 1.38 1.36 RL = 54
MAX3157toc12
1.98 1.96 1.94 OUTPUT VOLTAGE (V) 1.92 1.90 1.88 1.86 1.84 1.82 1.80 -40 -20 0 20 40 60 80 TEMPERATURE (C) RL = 54
490 480 PROPAGATION DELAY (ns) 470 460 450 440 430 420 410 -40 -20 0 20 40 60 80 TEMPERATURE (C)
1.52
1.34 1.32 -40 -20 0 20 40 60 80 TEMPERATURE (C)
DRIVER ENABLE TIME vs. TEMPERATURE
MAX3157toc13
DRIVER DISABLE TIME vs. TEMPERATURE
MAX3157toc14
RECEIVER PROPAGATION DELAY
MAX3157toc15
1.500 1.475 DRIVER ENABLE TIME (s) 1.450 1.425 1.400 1.375 1.350 1.325 1.300 -40 -20 0 20 40 60 80
250
DRIVER DISABLE TIME (ns)
200
VA - VB 0 5V/div 0
150
100
RO
50
0 100 -40 -20 0 20 40 60 80 100 2s/div TEMPERATURE (C) TEMPERATURE (C)
DRIVER PROPAGATION DELAY
MAX3157toc16
POWER-UP DELAY (VISOCOM = 0)
MAX3157toc17
POWER-UP DELAY (VISOCOM = -50V)
MAX3157toc18
DI 0 2V/div 0 DE 10V/div 0
DE 5V/div
VY - VZ
VY -50V 5V/div
VY 10V/div
RDIFF = 54 CL1 = CL2 = 100pF
2s/div
40s/div
100s/div
6
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High CMRR RS-485 Transceiver with 50V Isolation
Typical Operating Characteristics (continued)
(VCC = +5V, YR = ZR = AR = BR = ISOCOM, C1 = C2 = 0.047F, C3 = 1F, C4 = 10nF, TA = +25C, unless otherwise noted.)
POWER-UP DELAY (VISOCOM = +50V)
COMMON-MODE PEAK VOLTAGE TO GROUND (V)
MAX3157toc19
MAX3157
MAXIMUM COMMON-MODE PEAK VOLTAGE TO GROUND vs. COMMON-MODE FREQUENCY
MAXIMUM COMMON PEAK VOLTAGE TO ISOCOM
MAX3157 toc20
60 50 40 30 20 10 0 0.1 1 COMMON-MODE FREQUENCY (kHz)
0
DE 5V/div
COMMON-MODE VOLTAGE TO ISOCOM = 7V PEAK
+50V
VY 10V/div
100s/div
10
Test Circuits and Timing Diagrams
Y R DI VOD Z R Z VOC CL2 Y VID 3V DE
CL1 RDIFF
Figure 1. Driver DC Test Load
Figure 3. Driver Timing Test Circuit
RECEIVER OUTPUT
TEST POINT S1 1k
1k VCC 500 OUTPUT UNDER TEST CL S2 S2 S1 VCC
CL 100pF
Figure 2. Receiver Enable/Disable Timing Test Load
Figure 4. Driver Enable/Disable Timing Test Load 7
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High CMRR RS-485 Transceiver with 50V Isolation MAX3157
Test Circuits and Timing Diagrams (continued)
3V DI 0 1.5V tDPLH tDPHL 1.5V 1/2 VO tRZL(SHDN), tRZL VO Y 1/2 VO VO VDIFF 0 -VO 10% tDR VDIFF = V (Y) - V (Z) 90% 90% RO 10% 0 tRZH(SHDN), tRZH tRHZ 1.5V VCC RO tRLZ VOL + 0.5V RE 0 3V 1.5V 1.5V
Z
1.5V OUTPUT NORMALLY LOW OUTPUT NORMALLY HIGH
VOH - 0.5V
tDF tDSKEW = | tDPLH - tDPHL |
Figure 5. Driver Propagation Delays
Figure 8. Receiver Enable and Disable Times
3V DE 0 tDZL(SHDN), tDZL Y, Z VOL Y, Z 2.3V 0 tDZH(SHDN), tDZH tDHZ 2.3V OUTPUT NORMALLY LOW OUTPUT NORMALLY HIGH VOH - 0.5V VOL + 0.5V ATE VID A tDLZ B RR RECEIVER OUTPUT 1.5V 1.5V
Figure 6. Driver Enable and Disable Times
Figure 9. Receiver Propagation Delay Test Circuit
RO
VOH VOL A B 1.5V tRPHL INPUT tRSKEW = | tRPLH - tRPHL | OUTPUT tRPLH 1.5V
1V -1V
Figure 7. Receiver Propagation Delays 8 _______________________________________________________________________________________
High CMRR RS-485 Transceiver with 50V Isolation
Pin Description
PIN SSOP 1 2 3 4 5 6, 7, 12, 22, 23 8 9 10 11 13 14 15 16 17 18 19 20 21 24 25 26 27 28 PDIP 2 3 4 5 6 1, 12, 14, 15, 28 7 8 9 10 11 13 16 17 18 19 20 21 22 23 24 25 26 27 NAME RO RE DE DI C1LO FUNCTION Receiver Output. When RE is low and A - B > -50mV, RO will be high; if A - B -200mV, RO will be low. Receiver Output Enable. Drive RE low to enable RO. Driver Output Enable. Drive DE high to enable driver outputs. Driver Input. With DE high, a low on DI forces the noninverting output low and the inverting output high; with DI high, outputs reverse. Connect to the negative terminal of C1 (0.047F, 50V).
MAX3157
N.C.
No Connection
C1HI ISOVCC H/F ZR YR ISOCOM Z Y B A BR AR C2HI C2LO GND TXP RXP VCC
Connect to the positive terminal of C1 (0.047F, 50V). Internally generated power-supply voltage, referenced to the cable ground (ISOCOM). Connect a 1F capacitor to ISOCOM. Half/Full-Duplex Selector Pin. Leave open, or connect to ISOCOM to select Full Duplex, or connect to ISOVCC to select Half Duplex. Connect to ISOCOM for a 1.5k input impedance on Z. Leave open for a 12k input impedance on Z. Connect to ISOCOM for a 1.5k input impedance on Y. Leave open for a 12k input impedance on Y. Cable Ground Inverting Driver Output (and Inverting Receiver Input in Half-Duplex Mode) Noninverting Driver Output (and Noninverting Receiver Input in Half-Duplex Mode) Inverting Receiver Input in Full-Duplex Mode Noninverting Receiver Input in Full-Duplex Mode Connect to ISOCOM for a 1.5k input impedance on B. Leave open for a 12k input impedance on B. Connect to ISOCOM for a 1.5k input impedance on A. Leave open for a 12k input impedance on A. Connect to the positive terminal of C2 (0.047F, 50V). Connect to the negative terminal of C2 (0.047F, 50V). Ground Transmitter Phase. Leave open, or connect to GND for normal transmitter polarity, or connect to VCC to invert the transmitter polarity. Receiver Phase. Leave open, or connect to GND for normal receiver polarity, or connect to VCC to invert the receiver polarity. +4.75V to +5.25V Positive Supply. Connect a 0.1F capacitor to GND.
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9
High CMRR RS-485 Transceiver with 50V Isolation MAX3157
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 25A 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 25A 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
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.
Driver Output Protection
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 +150C.
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
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
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High CMRR RS-485 Transceiver with 50V Isolation
Block Diagram
MAX3157
Y DI TXP YR DE ZR Z
RO RE RXP
A B
AR BR H/F VCC ISCOVCC
OSC
POWER
GND
C1L
C2L
C1H C2H
ISOCOM
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 (2k). 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 12k allowing up to 32 transceivers on the bus. To guarantee the same low time constant under those conditions, use a shielded cable with a 1k resistor connected between the shield and ISOCOM. Using a lower value for this resistor is not recommended because this could trigger
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
High CMRR RS-485 Transceiver with 50V Isolation MAX3157
Pin Configurations
TOP VIEW
RO 1 RE 2 DE 3 DI 4 C1LO 5 N.C. 6 N.C. 7 C1HI 8 ISOVCC 9 H/F 10 ZR 11 N.C. 12 YR 13 ISOCOM 14 28 VCC 27 RXP 26 TXP 25 GND 24 C2LO
TOP VIEW
N.C. 1 RO 2 RE 3 DE 4 DI 5 C1LO 6 C1HI 7 ISOVCC 8 H/F 9 ZR 10 YR 11 N.C. 12 ISOCOM 13 N.C. 14 28 N.C. 27 VCC 26 RXP 25 TXP 24 GND
MAX3157
23 N.C. 22 N.C. 21 C2HI 20 AR 19 BR 18 A 17 B 16 Y 15 Z
MAX3157
23 C2LO 22 C2HI 21 AR 20 BR 19 A 18 B 17 Y 16 Z 15 N.C.
28 SSOP
28 PDIP
12
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High CMRR RS-485 Transceiver with 50V Isolation
Package Information
SSOP.EPS
MAX3157
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 (c) 2001 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.
PDIPN.EPS

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