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
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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
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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
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