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19-0453; Rev 0; 1/96
Complete, Isolated, 3.3V RS-485/RS-422 Data Interface
_______________General Description
The MAX3480A/MAX3480B are electrically isolated RS-485/RS-422 data-communications interfaces. Transceivers, optocouplers, and a transformer are all included in one low-cost, 28-pin DIP package. A single +3.3V supply on the logic side powers both sides of the interface. The MAX3480B features reduced-slew-rate drivers that minimize EMI and reduce reflections caused by improperly terminated cables, allowing error-free data transmission at data rates up to 250kbps. The MAX3480A's driver slew rate is not limited, allowing transmission rates up to 2.5Mbps. These devices typically draw 180mA of quiescent supply current. The MAX3480B provides a low-power shutdown mode in which it consumes only 0.2A. Drivers are short-circuit current limited and are 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 output if the input is open circuit. The MAX3480A/MAX3480B typically withstand 1600VRMS (1 minute) or 2000VRMS (1 second). Their isolated inputs and outputs meet RS-485/RS-422 specifications.
____________________________Features
o Isolated Data Interface to 1600VRMS (1 minute) o Slew-Rate-Limited Data Transmission (MAX3480B) o High-Speed, Isolated, 2.5Mbps RS-485 Interface (MAX3480A) o -7V to +12V Common-Mode Input Voltage Range with Respect to Isolated Ground o Single +3.3V Supply o Current Limiting and Thermal Shutdown for Driver Overload Protection o Standard 28-Pin DIP Package
MAX3480A/MAX3480B
__________________Pin Configuration
TOP VIEW
________________________Applications
Isolated RS-485/RS-422 Data Interface Transceivers for EMI-Sensitive Applications Industrial-Control Local Area Networks Automatic Test Equipment HVAC/Building Control Networks
VCC1 1 VCC2 D1 D2 GND1 FS 2 3 4 5 6 7 8 9
MAX3480A/B
28 AC1 27 AC2 26 ISO VCC1 25 B 24 ISO RO DRV
MAX845
MAX485 MAX487
23 A 22 ISO DI IN 21 ISO DE IN 20 ISO COM1 19 ISO DI DRV 18 ISO VCC2 17 ISO DE DRV 16 ISO COM2 15 ISO RO LED
______________Ordering Information
PART MAX3480ACPI MAX3480AEPI MAX3480BCPI MAX3480BEPI TEMP. RANGE PIN-PACKAGE 0C to +70C -40C to +85C 0C to +70C -40C to +85C 28 Plastic DIP 28 Plastic DIP 28 Plastic DIP 28 Plastic DIP DATA RATE (kbps) 2500 2500 250 250
SD VCC3 DI
VCC4 10 DE 11 GND2 12 RO 13 VCC5 14
ISOLATION BARRIER
DIP
________________________________________________________________ Maxim Integrated Products 1
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Complete, Isolated, 3.3V RS-485/RS-422 Data Interface MAX3480A/MAX3480B
ABSOLUTE MAXIMUM RATINGS
With Respect to GND: Supply Voltage (VCC1, VCC2, VCC4, VCC5) .........-0.3V to +3.8V Supply Voltage (VCC3) ..........................................-0.3V to +7V Control Input Voltage (SD, FS) ................-0.3V to (VCC + 0.3V) Receiver Output Voltage (RO).................-0.3V to (VCC + 0.3V) With Respect to ISO COM: Control Input Voltage (ISO DE _) .........-0.3V to (ISO VCC + 0.3V) Driver Input Voltage (ISO DI _).........-0.3V to (ISO VCC + 0.3V) Receiver Output Voltage (ISO RO _) ......-0.3V to (ISO VCC + 0.3V) Driver Output Voltage (A, B)................................-8V to +12.5V Receiver Input Voltage (A, B)..............................-8V to +12.5V LED Forward Current (DI, DE, ISO RO LED) ......................50mA Continuous Power Dissipation (TA = +70C) Plastic DIP (derate 9.09mW/C above +70C) ...............727mW Operating Temperature Ranges MAX3480_CPI ......................................................0C to +70C MAX3480_EPI ...................................................-40C to +85C Storage Temperature Range .............................-65C to +160C Lead Temperature (soldering, 10sec) .............................+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 = VCC1 = VCC2 = VCC4 = VCC5 = 3.0V to 3.6V, FS = 0V, TA = TMIN to TMAX, unless otherwise noted. Typical values are at VCC = VCC1 = VCC2 = VCC4 = VCC5 = 3.3V and TA = +25C.) (Notes 1, 2, 3) PARAMETER Supply Voltage Switch Frequency SYMBOL VS fSWL fSWH FS = 0V FS = VCC or open MAX3480A, Operating Supply Current ICC DE = VCC or open MAX3480B, DE = VCC or open Shutdown Supply Current (Note 3) FS Input Threshold FS Input Pull-Up Current FS Input Leakage Current Input High Voltage Input Low Voltage Isolation Resistance Isolation Capacitance Differential Driver Output (no load) Differential Driver Output (with load) Change in Magnitude of Driver Output Voltage for Complementary Output States Driver Common-Mode Output ISHDN VFSH VFSL IFSL IFSM VIH VIL RISO CISO VOD1 R = 50 (RS-422) VOD2 R = 27 (RS-485), Figure 3 R = 27 or 50, Figure 3 Differential Common Mode 2 1.5 5 0.3 V 0.3 4 V V SD = VCC3 High Low FS low FS high DE, DI (Figure 1) DE, DI (Figure 1) TA = +25C, VISO = 50VDC TA = +25C, VISO = 50VDC VCC - 0.4 0.4 100 10,000 10 8 10 2.4 0.8 50 RL = RL = 54 RL = RL = 54 CONDITIONS MIN 3.0 TYP 3.3 600 900 180 280 120 240 0.2 A V A pA V V M pF V 200 260 mA MAX 3.6 UNITS V kHz
VOD VOC
R = 27 or 50, Figure 4
2
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Complete, Isolated, 3.3V RS-485/RS-422 Data Interface
ELECTRICAL CHARACTERISTICS (continued)
(VCC = VCC1 = VCC2 = VCC4 = VCC5 = 3.0V to 3.6V, FS = 0V, TA = TMIN to TMAX, unless otherwise noted. Typical values are at VCC = VCC1 = VCC2 = VCC4 = VCC5 = 3.3V and TA = +25C.) (Notes 1, 2, 3) PARAMETER SYMBOL CONDITIONS DE = 0V, VCC = 0V or 3.6V VIN = 12V VIN = 12V MAX3480A VIN = -7V MAX3480B MAX3480A MAX3480B -0.2 70 0.4 250 100 VIN = 12V VIN = -7V 12 48 0.2 MIN TYP MAX 1.0 0.8 -0.8 0.25 -0.2 k V mV V A mA mA UNITS
MAX3480A/MAX3480B
Input Current (A, B)
ISO IIN
Receiver Input Resistance Receiver Differential Threshold Receiver Input Hysteresis Receiver Output/Receiver Output Low Voltage Receiver Output/Receiver Output High Current Driver Short-Circuit Current
RIN VTH VTH VOL IOH ISO IOSD
-7V VCM 12V -7V VCM 12V VCM = 0V DI = VCC
VOUT = 3.6V, DI = 0V -7V VO 12V (Note 4)
SWITCHING CHARACTERISTICS--MAX3480A
(VCC = VCC1 = VCC2 = VCC4 = VCC5 = 3.0V to 3.6V, FS = 0V, TA = TMIN to TMAX, unless otherwise noted. Typical values are at VCC = VCC1 = VCC2 = VCC4 = VCC5 = 3.3V and TA = +25C.) PARAMETER Driver Input to Output Propagation Delay Driver Output Skew (Note 5) Driver Rise or Fall Time Driver Enable to Output High Driver Enable to Output Low Driver Disable Time from High Driver Disable Time from Low Receiver Input to Output Propagation Delay SYMBOL tPLH tPHL tSKEW tR, tF tZH tZL tHZ tLZ tPLH tPHL tSKD fMAX CONDITIONS Figures 4, 6; RDIFF = 54, CL1 = CL2 = 100pF Figures 4, 6; RDIFF = 54, CL1 = CL2 = 100pF Figures 4, 6; RDIFF = 54, CL1 = CL2 = 100pF Figures 5, 7; CL = 100pF, S2 closed Figures 5, 7; CL = 100pF, S1 closed Figures 5, 7; CL = 15pF, S2 closed Figures 5, 7; CL = 15pF, S1 closed Figures 4, 8; RDIFF = 54, CL1 = CL2 = 100pF Figures 4, 8; RDIFF = 54, CL1 = CL2 = 100pF tPLH, tPHL < 50% of data period 2.5 MIN TYP 100 100 25 15 0.2 0.2 0.3 0.3 100 100 20 MAX 275 275 90 40 1.5 1.5 1.5 1.5 225 225 UNITS ns ns ns s s s s ns ns Mbps
tPLH - tPHL Differential
Receiver Skew Maximum Data Rate
_______________________________________________________________________________________
3
Complete, Isolated, 3.3V RS-485/RS-422 Data Interface MAX3480A/MAX3480B
SWITCHING CHARACTERISTICS--MAX3480B
(VCC = VCC1 = VCC2 = VCC4 = VCC5 = 3.0V to 3.6V, FS = 0V, TA = TMIN to TMAX, unless otherwise noted. Typical values are at VCC = VCC1 = VCC2 = VCC4 = VCC5 = 3.3V and TA = +25C.) PARAMETER Driver Input to Output Propagation Delay Driver Output Skew Driver Rise or Fall Time Driver Enable to Output High Driver Enable to Output Low Driver Disable Time from Low Driver Disable Time from High Receiver Input to Output Propagation Delay SYMBOL tPLH tPHL tSKEW tR, tF tZH tZL tLZ tHZ tPLH tPHL tSKD fMAX CONDITIONS Figures 4, 6; RDIFF = 54, CL1 = CL2 = 100pF Figures 4, 6; RDIFF = 54, CL1 = CL2 = 100pF Figures 4, 6; RDIFF = 54, CL1 = CL2 = 100pF Figures 5, 7; CL = 100pF, S2 closed Figures 5, 7; CL = 100pF, S1 closed Figures 5, 7; CL = 15pF, S1 closed Figures 5, 7; CL = 15pF, S2 closed Figures 4, 8; RDIFF = 54, CL1 = CL2 = 100pF Figures 4, 8; RDIFF = 54, CL1 = CL2 = 100pF tPLH, tPHL < 50% of data period 0.25 MIN TYP 1.0 1.0 100 1.0 50 50 13 13 0.8 0.8 50 MAX 2.0 2.0 800 2.0 100 100 50 50 2.0 2.0 UNITS s ns s s s s s s ns Mbps
tPLH - tPHL Differential
Receiver Skew Maximum Data Rate
Note 1: All currents into device pins are positive; all currents out of device pins are negative. All voltages are referenced to logic-side ground (GND1, GND2), unless otherwise specified. Note 2: For DE and DI pin descriptions, see Detailed Block Diagram and Typical Application Circuit (Figure 1 for MAX3480A/MAX3480B). Note 3: Shutdown supply current is the current at VCC1 when shutdown is enabled. Note 4: Applies to peak current. See Typical Operating Characteristics and Applications Information.
4
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Complete, Isolated, 3.3V RS-485/RS-422 Data Interface
__________________________________________Typical Operating Characteristics
(VCC_ = 3.3V, TA = +25C, unless otherwise noted.)
MAX3480A/MAX3480B
DRIVER DIFFERENTIAL OUTPUT VOLTAGE vs. TEMPERATURE
DIFFERENTIAL OUTPUT VOLTAGE (V) 2.9 2.8 2.7 2.6 2.5 2.4 2.3 2.2 2.1 2.0 -40 -20 0 20 40 60 80 TEMPERATURE (C) DI = HIGH OR OPEN RL = 54
MAX3480A/B-08
OUTPUT CURRENT vs. DRIVER OUTPUT HIGH VOLTAGE
-90 OUTPUT CURRENT (mA) -80 -70 -60 -50 -40 -30 -20 -10 0 -6 -4 -2 0 2 4 6 OUTPUT HIGH VOLTAGE (V)
MAX3480 A/B-06
OUTPUT CURRENT vs. DRIVER OUTPUT LOW VOLTAGE
160 OUTPUT CURRENT (mA) 140 120 100 80 60 40 20 0 0 2 4 6 8 10 12 OUTPUT LOW VOLTAGE (V)
MAX3480 A/B-05
3.0
-100
180
RECEIVER OUTPUT LOW VOLTAGE vs. TEMPERATURE
MAX3480 A/B-04
RECEIVER OUTPUT HIGH VOLTAGE vs. TEMPERATURE
4.75 OUTPUT HIGH VOLTAGE (V) 4.50 4.25 4.00 3.75 3.50 3.25 3.00 MEASURED AT ISO RO DRV IRO = 8mA
MAX3480 A/B-03
0.8 0.7 OUTPUT LOW VOLTAGE (V) 0.6 0.5 0.4 0.3 0.2 0.1 0 -40
MEASURED AT ISO RO DRV IRO = 8mA
5.00
-20
0
20
40
60
80
-40
-20
0
20
40
60
80
TEMPERATURE (C)
TEMPERATURE (C)
OUTPUT CURRENT vs. RECEIVER OUTPUT LOW VOLTAGE
MAX3480 A/B-01
OUTPUT CURRENT vs. RECEIVER OUTPUT HIGH VOLTAGE
MEASURED AT ISO RO DRV
MAX3480 A/B-02
80 70 OUTPUT CURRENT (mA) 60 50 40 30 20 10 0 0
-30 -25 OUTPUT CURRENT (mA) -20 -15 -10 -5 0
MEASURED AT ISO RO DRV
1.0
2.0
3.0
4.0
5.0
0
1.0
2.0
3.0
4.0
5.0
OUTPUT LOW VOLTAGE (V)
OUTPUT HIGH VOLTAGE (V)
_______________________________________________________________________________________
5
Complete, Isolated, 3.3V RS-485/RS-422 Data Interface MAX3480A/MAX3480B
____________________________Typical Operating Characteristics (continued)
(VCC_ = 3.3V, TA = +25C, unless otherwise noted.)
MAX3480A DRIVER INPUT (AB) AND RECEIVER OUTPUT (RO)
DE INPUT, 2V/div
MAX3480A DRIVER ENABLE (AB) AND RECEIVER OUTPUT (RO)
350 DE INPUT, 1V/div SUPPLY CURRENT (mA)
MAX3480A SUPPLY CURRENT vs. SUPPLY VOLTAGE
DE HIGH, 50 LOAD 300 250 200 DE LOW, DI LOW 150 100 50 0 DE LOW, DI HIGH
MAX3480A-11
A 2V/div B RO, 74HC240, DI, 2V/div
DE HIGH, 100 LOAD
A, 1V/div
B, 2V/div
100ns/div CIRCUIT OF FIGURE 2, TERMINATION: 100
100ns/div CIRCUIT OF FIGURE 2, TERMINATION: 100
3.0
3.2
3.4
3.6
3.8
SUPPLY VOLTAGE (V)
MAX3480B DRIVER INPUT (AB) AND RECEIVER OUTPUT (RO)
DE INPUT, 2V/div
MAX3480B DRIVER ENABLE (AB) AND RECEIVER OUTPUT (RO)
DE INPUT, 1V/div SUPPLY CURRENT (mA) 300
MAX3480B SUPPLY CURRENT vs. SUPPLY VOLTAGE
DE HIGH, 50 LOAD 250 200 150 DE LOW, DI LOW 100 DE LOW, DI HIGH 50 0 DE HIGH, 100 LOAD
MAX3480B-12
A B
A, 1V/div 2V/div
RO, 74HC240, DI, 2V/div
B, 2V/div
1s/div CIRCUIT OF FIGURE 2, TERMINATION: 100
25s/div CIRCUIT OF FIGURE 2, TERMINATION: 100
3.0
3.2
3.4
3.6
3.8
SUPPLY VOLTAGE (V)
DRIVER ENABLE TIME vs. TEMPERATURE
MAX3480A/B-10
DRIVER DISABLE TIME vs. TEMPERATURE
MAX3480A/B-9
100
100
MAX3480B 10 RL = 54, DI = 0V MEASURED FROM DE TO VALID OUTPUT 1 MAX3480A 0.1 -40 -20 0 20 40 60 80 TEMPERATURE (C)
DRIVER DISABLE TIME (s)
DRIVER ENABLE TIME (s)
MAX3480B 10 RL = 54, DI = 0V MEASURED FROM DE TO VALID OUTPUT 1 MAX3480A
0.1 -40 -20 0 20 40 60 80 TEMPERATURE (C)
6
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Complete, Isolated, 3.3V RS-485/RS-422 Data Interface
______________________________________________________________Pin Description
PIN NAME FUNCTION PINS ON THE NON-ISOLATED SIDE 1 VCC1 Logic-Side (non-isolated side) +3.3V Supply Voltage Input. Connect to pins 2, 10, and 14. 2 3, 4 5, 12 6 7 8 9 10 11 VCC2 D1, D2 GND1, GND2 FS SD VCC3 DI VCC4 DE Logic-Side (non-isolated side) +3.3V Supply Voltage Input. Connect to pins 1, 10, and 14. Boost-Voltage Generator Outputs. See Figures 1 and 2. Logic-Side Ground Inputs. Must be connected; not internally connected. Frequency Switch Input. If FS = VCC, switch frequency is high; if FS = 0V, switch frequency is low (normal connection). Power-Supply Shutdown Input. Must be connected to logic ground. Boosted V+ Voltage Input. Must be connected as shown in Figures 1 and 2. Driver Input. With DE high, a low on DI forces output A low and output B high. Similarly, a high on DI forces output A high and output B low. Drives internal LED cathode through R1 (Table 1 of Figure 2). Logic-Side (non-isolated side) +3.3V Supply Voltage Input. Connect to pins 1, 2, and 14. Driver-Enable Input. The driver outputs, A and B, are enabled by bringing DE high. The driver outputs are high impedance when DE is low. If the driver outputs are enabled, the device functions as a line driver. While the driver outputs are high impedance, the device functions as a line receiver. Drives internal LED cathode through R2 (Table 1 of Figure 2). Receiver Output. If A > B by 200mV, RO will be low; if A < B by 200mV, RO will be high. Open collector; must have pull-up (R3) to VCC (Table 1 of Figure 2). Logic-Side (non-isolated side) +3.3V Supply Voltage Input. Connect to pins 1, 2, and 10.
MAX3480A/MAX3480B
13 14
RO VCC5
_______________________________________________________________________________________
7
Complete, Isolated, 3.3V RS-485/RS-422 Data Interface MAX3480A/MAX3480B
_________________________________________________Pin Description (continued)
PIN NAME ISO RO LED ISO COM2 FUNCTION Isolated Receiver-Output LED Anode (input). If A > B by 200mV, ISO RO LED will be high; if A < B by 200mV, ISO RO LED will be low. Isolated-Supply Common Input. Connect to ISO COM1. Isolated Driver-Enable Drive Input. The driver outputs, A and B, are enabled by bringing DE high. The driver outputs are high impedance when DE is low. If the driver outputs are enabled, the device functions as a line driver. While the driver outputs are high impedance, the device functions as a line receiver. Open collector output; must have pull-up (R4) to ISO VCC and be connected to ISO DE IN for normal operation (Table 1 of Figure 2). Isolated-Supply Positive Input Voltage. Connect to ISO VCC1. Isolated Driver-Input Drive. With DE high, a low on DI forces output A low and output B high. Similarly, a high on DI forces output A high and output B low. Open-collector output; must have pull-up (R5) to ISO VCC and be connected to ISO DI IN for normal operation (Table 1 of Figure 2). Isolated-Supply Common Output. Connect to ISO COM2. If RS-485 wires have a shield, connect ISO COM1 to shield via 100 resistor. Isolated Driver-Enable Input. Connect to ISO DE DRV for normal operation. Isolated Driver Input. Connect to ISO DI DRV for normal operation. Noninverting Driver Output and Noninverting Receiver Input. Isolated Receiver-Output Drive. Connect to ISO RO LED through R6 (Table 1 of Figure 2). Inverting Driver Output and Inverting Receiver Input Isolated Supply Positive Output Voltage. Connect to ISO VCC2. Internal Connections. Leave these pins unconnected. PINS ON THE ISOLATED RS-485/RS-422 SIDE 15 16
17
ISO DE DRV
18 19
ISO VCC2 ISO DI DRV
20 21 22 23 24 25 26 27, 28
ISO COM1 ISO DE IN ISO DI IN A ISO RO DRV B ISO VCC1 AC2, AC1
Note: For DE and DI pin descriptions, see Detailed Block Diagram and Typical Application Circuit.
8
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Complete, Isolated, 3.3V RS-485/RS-422 Data Interface
_______________Detailed Description
The MAX3480A/MAX3480B are electrically isolated, RS-485/RS-422 data-communications interface solutions. Transceivers, optocouplers, a power driver, and a transformer are in one standard 28-pin DIP package. Signals and power are internally transported across the isolation barrier (Figure 1). Power is transferred from the logic side (non-isolated side) to the isolated side of the barrier through a center-tapped transformer. Signals cross the barrier through high-speed optocouplers. A single +3.3V supply on the logic side powers both sides of the interface. The MAX3480B features reduced-slew-rate drivers that minimize EMI and reduce reflections caused by improperly terminated cables, allowing error-free transmission at data rates up to 250kbps. The MAX3480A's driver slew rates are not limited, allowing transmission rates up to 2.5Mbps. The frequency-select FS is connected to GND_ in normal operation, which selects a switching frequency of approximately 600kHz. Connect to high for a higher 900kHz switching frequency. Drivers are short-circuit current limited and are protected against excessive power dissipation by thermal
MAX3480A/MAX3480B
VCC3
ISO VCC1
MAX845
Q FS OSC 1.1MHz/ 1.6MHz T F/F N
D1 ISO DI IN D D2 ISO DE IN ISO RO DRV RE R
MAX3480A: MAX485 MAX3480B: MAX487
B
A
Q
N
SD
GND1
ISO COM1
EXTERNAL RS-485/RS-422 WIRING TERMINATING RESISTOR (ONE RESISTOR ON EACH END) 28 AC1 (MAKE NO CONNECTION) 27 AC2 (MAKE NO CONNECTION) 26 ISO VCC1 B 25 ISO RO DRV 24 B R7 A R6 200 R4 R5 NOTE: RESISTOR R8 PROTECTS THE MAX3480 FROM TRANSIENT CURRENTS BETWEEN SHIELD AND "A" & "B". SH SHIELD (OPTIONAL) TWISTED PAIR TO OTHER TRANSCEIVERS RL SHIELD (OPTIONAL) TWISTED PAIR TO OTHER TRANSCEIVERS RL
VIN 3.0V TO 3.6V C1 22F C2 0.1F D1, D2 1N914
VCC1 VCC2
MAX3480A/B
1 2
D1 3 D2 4 5
BOOSTED V+ GND1 C3 0.01uF
FS 6 SD VCC3 7 8 9 10 11 12 13 14
MAX845
MAX485 MAX487
23 A ISO DI IN 22 21 ISO DE IN
R1 DI LOGIC I/O R2 DE
DI VCC4 DE GND2
20 ISO COM1 19 ISO DI DRV 18 ISO VCC2 17 ISO DE DRV 16 15 ISOLATION BARRIER ISO COM2 ISO RO LED
R8 100
RO
RO R3 VCC5
ISOLATION COMMON
LOGIC GROUND
Figure 1. Detailed Block Diagram
_______________________________________________________________________________________ 9
Complete, Isolated, 3.3V RS-485/RS-422 Data Interface MAX3480A/MAX3480B
shutdown circuitry that puts the driver outputs into a high-impedance state. The receiver input has a fail-safe feature that guarantees a logic-high output if the input is open circuit. The driver outputs are enabled by bringing DE high. Driver-enable times are typically 200ns for the MAX3480A and 50s for the MAX3480B. Allow time for the devices to be enabled before sending data. When enabled, driver outputs function as line drivers. Driver outputs are high impedance when DE is low. While outputs are high impedance, they function as line receivers. The MAX3480A/MAX3480B typically withstand 1600VRMS (1 minute) or 2000VRMS (1 second). The isolated outputs of these devices meet all RS-485/RS-422 specifications. The logic inputs can be driven from any TTL/CMOS-logic family with a series resistor, and the received data output can directly drive any of the TTL/CMOS-logic families with only a resistive pull-up.
Boost Voltage
The MAX3480 requires external diodes on the primary of the transformer to develop the boost voltage for the power oscillator. In normal operation, whenever one of the oscillator outputs (D1 and D2) goes low, the other
EXTERNAL RS-485/RS-422 WIRING TERMINATING RESISTOR (ONE RESISTOR ON EACH END) 28 AC1 (MAKE NO CONNECTION) 27 AC2 (MAKE NO CONNECTION) 26 ISO VCC1 B 25 24 ISO RO DRV B R7 A ISO DI IN ISO DE IN R4 R5 NOTE: RESISTOR R8 PROTECTS THE MAX3480 FROM TRANSIENT CURRENTS BETWEEN SHIELD AND "A" & "B". SHIELD (OPTIONAL) R6 200 SH SHIELD (OPTIONAL) TWISTED PAIR TO OTHER TRANSCEIVERS RL TWISTED PAIR TO OTHER TRANSCEIVERS RL
VIN 3.0V TO 3.6V C1 22F 6V C2 0.1F BOOSTED V+ C3 0.01F 74HC240 4 6 DI DE 2 17 18 3 8 15 13 20 RO 9 11 10 LOGIC GROUND RECEIVER OUTPUT R3 16 14 12 5 DRIVER ENABLE 7 R2 DRIVER INPUT R1 D1, D2 1N914
VCC1 VCC2
MAX3480A/B
1 2
D1 3 D2 4 GND1 5 FS 6 SD VCC3 DI VCC4 DE GND2 RO VCC5 7 8 9 10 11 12 13 14 ISOLATION BARRIER
MAX845
MAX485 MAX487
23 A 22 21
20 ISO COM1 19 ISO DI DRV 18 17 16 15 ISO VCC2 ISO DE DRV ISO COM2 ISO RO LED
R8 100
ISOLATION COMMON
Table 1. Pull-Up and LED Drive Resistors
PART MAX3480A MAX3480B R1 () 150 130 R2 () 130 330 R3 () 680 2000 R4 () 3000 2200 R5 () 1000 3000 R6 () 200 200 R7 () Open 470
Figure 2. Typical Application Circuit
10 ______________________________________________________________________________________
Complete, Isolated, 3.3V RS-485/RS-422 Data Interface
_________________________________________________________________Test Circuits
ISOLATION BARRIER A R VOD R B VOC 3.3V DE ISOLATION BARRIER 3.3V CL1 A DI RDIFF VID B CL2 B
MAX3480A/MAX3480B
D
A R
R3 RO
Figure 3. Driver DC Test Load
Figure 4. Driver/Receiver Timing Test Circuit
500 OUTPUT UNDER TEST CL
S1
ISO VCC1, ISO VCC2
S2
Figure 5. Driver Timing Test Load
_______________________________________________________Switching Waveforms
VCC_ - 0.4V DI VCC_ - 0.4V VCC_ - 0.4V 2 tPLH tPHL VCC_ - 0.4V 2 1/2 VO tZL A, B VO A 1/2 VO VO VDIFF 0V -VO 10% tR 90% VDIFF = VA - VB 90% tF tSKEW = | tPLH - tPHL | 10% VOL A, B 2.3V 0V tZH tHZ 2.3V OUTPUT NORMALLY LOW OUTPUT NORMALLY HIGH VOH - 0.5V VOL + 0.5V tLZ DE
VCC_ - 0.4V 2
VCC_ - 0.4V 2
0V
0V
B
Figure 6. Driver Propagation Delays and Transition Times
Figure 7. Driver Enable and Disable Times
______________________________________________________________________________________
11
Complete, Isolated, 3.3V RS-485/RS-422 Data Interface MAX3480A/MAX3480B
__Switching Waveforms (continued)
goes to approximately double the supply voltage. Since the circuit is symmetrical, the two outputs can be combined with diodes, filtered, and used to power the oscillator itself. The diodes on the primary side may be any fast-switching, small-signal diodes, such as the 1N914, 1N4148, or CMPD2838. The nominal value of the primary filter capacitor C3 is 0.01F.
RO
VOH VOL VID -VID 0V 1.5V tPHL INPUT OUTPUT tPLH 0V 1.5V
Driver Output Protection
There are two mechanisms to prevent excessive output current and power dissipation caused by faults or by bus contention. 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). In addition, a thermal shutdown circuit forces the driver outputs into a high-impedance state if the die temperature rises excessively. Resistor R8 provides additional protection by current limiting between the shield and the two signal wires. In the event that shielded cable is used and an external voltage or transient is inadvertently applied between the shield and the signal wires, the MAX3480 can be damaged. Although unlikely, this condition can occur during installation. The MAX3480 provides electrical isolation between logic ground and signal paths; it does not provide isolation from external shields and the signal paths. When in doubt, do not connect the shield. The MAX3480 can be damaged if resistor R8 is shorted out.
VB - VA
Figure 8. Receiver Propagation Delays
____________________Function Tables
Table 2. Transmitting
INPUTS DE 1 1 0 DI 1 0 X B 0 1 High-Z OUTPUTS A 1 0 High-Z
X = Don't care High-Z = High impedance
Table 3. Receiving
INPUTS DE 0 0 0 A-B +0.2V -0.2V Inputs open OUTPUT -- -- RO 0 1 0
12
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Complete, Isolated, 3.3V RS-485/RS-422 Data Interface MAX3480A/MAX3480B
__________Applications Information
The MAX3480A/MAX3480B are designed for bidirectional data communications on multipoint bus-transmission lines. Figure 9 shows a typical network application circuit. To minimize reflections, terminate the line at both ends with its characteristic impedance, and keep stub lengths off the main line as short as possible. The slewrate-limited MAX3480B is more tolerant of imperfect termination and stubs off the main line. The MAX3480A/MAX3480B are specified and characterized using the resistor values shown in Table 1 of Figure 2. Altering the recommended values can degrade performance. The DI and DE inputs are the cathodes of LEDs whose anodes are connected to VCC. These points are best driven by a 3.3V CMOS-logic gate with a series resistor to limit the current. The resistor values shown in Table 1 are recommended when the 74HC240 gate or equivalent is used. These values may need to be adjusted if a driving gate with dissimilar series resistance is used. DI and DE are intended to be driven through a series current-limiting resistor. Directly grounding these pins destroys the device.
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Complete, Isolated, 3.3V RS-485/RS-422 Data Interface MAX3480A/MAX3480B
TERMINATING RESISTOR (ONE RESISTOR ON EACH END)
B 120 A A B A B R D
DI DE RO
RE RE R D RE R D
RO DE
DI
RO DE
DI
VIN 3.0V to 3.6V C1 22F 6V C2 0.1F D1, D2 1N914
VCC1 VCC2 D1 D2
BOOSTED V+ C3 0.01F
MAX3480A/B
1 2 3 4
TERMINATING RESISTOR (ONE RESISTOR ON EACH END) 28 AC1 (MAKE NO CONNECTION) 27 AC2 (MAKE NO CONNECTION) 26 ISO VCC1 25 B 24 ISO RO DRV R7 A R6, 200 R4 SH SHIELD (OPTIONAL) 120 B
GND1 5 FS 6 SD 7 VCC3 DI VCC4
MAX845
74HC240
4 6
16 14 12 5 DRIVER ENABLE R2 7 DRIVER INPUT R1 130
MAX485 MAX487
23 A 22 ISO DI IN 21 ISO DE IN
DI DE
2 17
18 3
8 9 10 11 12 13 14 ISOLATION BARRIER
8 15 13 20
20 ISO COM1 19 ISO DI DRV 18 ISO VCC2 17 ISO DE DRV 16 15 ISO COM2 ISO RO LED NOTE: RESISTOR R8 PROTECTS THE MAX3480 FROM TRANSIENT CURRENTS BETWEEN SHIELD AND "A" & "B". R8 100 R5
DE GND2 RO
RO
9 10
11
RECEIVER OUTPUT
R3
VCC5
LOGIC GROUND
ISOLATED COMMON
Figure 9. Typical RS-485/RS-422 Network
14
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Complete, Isolated, 3.3V RS-485/RS-422 Data Interface MAX3480A/MAX3480B
Table 4. Maxim's Isolated RS-485 Product Family
PART MAX1480A MAX1480B MAX1490A MAX1490B MAX3480A MAX3480B No. OF Tx/Rx 1/1 1/1 1/1 1/1 1/1 1/1 GUARANTEED DATA RATE (Mbps) 2.50 0.25 2.50 0.25 2.50 0.25 FULL/HALF DUPLEX Half Half Full Full Half Half SLEW-RATE LIMITED No Yes No Yes No Yes No. OF Tx/Rx ON BUS 32 32 32 32 32 128 SUPPLY VOLTAGE (V) 5.0 5.0 5.0 5.0 3.3 3.3
________________________________________________________Package Information
DIM INCHES MAX MIN 0.200 - - 0.015 0.175 0.125 0.080 0.055 0.020 0.016 0.065 0.045 0.012 0.008 0.090 0.050 0.625 0.600 0.575 0.525 - 0.100 - 0.600 0.700 - 0.150 0.120 MILLIMETERS MIN MAX - 5.08 0.38 - 3.18 4.45 1.40 2.03 0.41 0.51 1.14 1.65 0.20 0.30 1.27 2.29 15.24 15.88 13.34 14.61 2.54 - 15.24 - - 17.78 3.05 3.81
E A2 A D A3 E1
A A1 A2 A3 B B1 C D1 E E1 e eA eB L
0-15 A1 L B D1 e B1 eA eB C
Plastic DIP PLASTIC DUAL-IN-LINE PACKAGE (0.600 in.)
PKG. DIM PINS P P P D D D 24 28 40
INCHES MILLIMETERS MIN MAX MIN MAX 1.230 1.270 31.24 32.26 1.430 1.470 36.32 37.34 2.025 2.075 51.44 52.71
21-0044A
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Complete, Isolated, 3.3V RS-485/RS-422 Data Interface MAX3480A/MAX3480B
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.
16 __________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 (408) 737-7600 (c) 1996 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

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