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LTC491 Differential Driver and Receiver Pair
FEATURES
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DESCRIPTIO
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Low Power: ICC = 300A Typical Designed for RS485 or RS422 Applications Single +5V Supply -7V to +12V Bus Common Mode Range Permits 7V Ground Difference Between Devices on the Bus Thermal Shutdown Protection Power-Up/Down Glitch-Free Driver Outputs Permit Live Insertion or Removal of Package Driver Maintains High Impedance in Three-State or with the Power Off Combined Impedance of a Driver Output and Receiver Allows up to 32 Transceivers on the Bus 70mV Typical Input Hysteresis 28ns Typical Driver Propagation Delays with 5ns Skew Pin Compatible with the SN75180
The LTC491 is a low power differential bus/line transceiver designed for multipoint data transmission standard RS485 applications with extended common mode range (+12V to -7V). It also meets the requirements of RS422. The CMOS design offers significant power savings over its bipolar counterpart without sacrificing ruggedness against overload or ESD damage. The driver and receiver feature three-state outputs, with the driver outputs maintaining high impedance over the entire common mode range. Excessive power dissipation caused by bus contention or faults is prevented by a thermal shutdown circuit which forces the driver outputs into a high impedance state. The receiver has a fail safe feature which guarantees a high output state when the inputs are left open. Both AC and DC specifications are guaranteed from 0C to 70C and 4.75V to 5.25V supply voltage range.
APPLICATI
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Low Power RS485/RS422 Transceiver Level Translator
TYPICAL APPLICATI
DE 4
9 D 5 DRIVER 120 10 4000 FT 24 GAUGE TWISTED PAIR LTC491 12 R 2 RECEIVER 120 11 4000 FT 24 GAUGE TWISTED PAIR 3 REB 120 DRIVER D LTC491 120 RECEIVER R
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DE REB
LTC491 * TA01
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LTC491 ABSOLUTE
(Note 1)
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RATI GS
PACKAGE/ORDER I FOR ATIO
TOP VIEW NC R REB DE D GND GND 1 2 3 4 5 D 6 7 9 8 Y NC R 14 VCC 13 NC 12 A 11 B 10 Z
Supply Voltage (VCC) ............................................... 12V Control Input Voltages ..................... -0.5V to VCC +0.5V Control Input Currents .......................... -50mA to 50mA Driver Input Voltages ....................... -0.5V to VCC +0.5V Driver Input Currents ............................ -25mA to 25mA Driver Output Voltages .......................................... 14V Receiver Input Voltages ......................................... 14V Receiver Output Voltages ................ -0.5V to VCC +0.5V Operating Temperature Range LTC491C.................................................. 0C to 70C LTC491I.............................................. - 40C to 85C Storage Temperature Range ................. - 65C to 150C Lead Temperature (Soldering, 10 sec.)................. 300C
ORDER PART NUMBER LTC491CN LTC491CS LTC491IN LTC491IS
S PACKAGE N PACKAGE 14-LEAD PLASTIC DIP 14-LEAD PLASTIC SOIC
LTC491 * POI01
Consult factory for Military grade parts.
VCC = 5V 5%
SYMBOL VOD1 VOD2 VOD VOC VOC VIH VIL lIN1 lIN2 VTH VTH VOH VOL IOZR ICC RIN IOSD1 IOSD2 IOSR IOZ
DC ELECTRICAL CHARACTERISTICS
PARAMETER Differential Driver Output Voltage (Unloaded) Differential Driver Output Voltage (With load) Change in Magnitude of Driver Differential Output Voltage for Complementary Output States Driver Common Mode Output Voltage Change in Magnitude of Driver Common Mode Output Voltage for Complementary Output States Input High Voltage Input Low Voltage Input Current Input Current (A, B) Differential Input Threshold Voltage for Receiver Receiver Input Hysteresis Receiver Output High Voltage Receiver Output Low Voltage Three-State Output Current at Receiver Supply Current Receiver Input Resistance Driver Short Circuit Current, VOUT = High Driver Short Circuit Current, VOUT = Low Receiver Short Circuit Current Driver Three-State Output Current VCC = 0V or 5.25V - 7V VCM 12V VCM = 0V IO = -4mA, VID = 0.2V IO = 4mA, VID = -0.2V VCC = Max 0.4V VO 2.4V No Load; D = GND, or VCC - 7V VCM 12V VO = - 7V VO = 12V 0V VO VCC VO = - 7V to 12V Outputs Enabled Outputs Disabled VIN = 12V VIN = -7V D, DE, REB CONDITIONS IO = 0 R = 50; (RS422) R = 27; (RS485) (Figure 1) R = 27 or R = 50 (Figure 1)
q q q q q q q q q q q q q q q q q q q q q q q
MIN 2 1.5
TYP
MAX 5 5 0.2 3 0.2
UNITS V V V V V V V
2.0 0.8 2 1.0 - 0.8 - 0.2 70 3.5 0.4 1 300 300 12 100 100 7 2 250 250 85 200 500 500 0.2
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V A mA mA V mV V V A A A k mA mA mA A
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LTC491
VCC = 5V 5%
SYMBOL tPLH tPHL tSKEW tr, tf tZH tZL tLZ tHZ tPLH tPHL tSKD tZL tZH tLZ tHZ
SWITCHI G CHARACTERISTICS
PARAMETER Driver Input to Output Driver Input to Output Driver Output to Output 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 Receiver Input to Output tPLH - tPHL Differential Receiver Skew Receiver Enable to Output Low Receiver Enable to Output High Receiver Disable From Low Receiver Disable From High CL = 100pF (Figures 4, 6) S2 Closed CL = 100pF (Figures 4, 6) S1 Closed CL = 15pF (Figures 4, 6) S1 Closed CL = 15pF (Figures 4, 6) S2 Closed RDIFF = 54, CL1 = CL2 = 100pF (Figures 2, 7) CL = 15pF (Figures 3, 8) S1 Closed CL = 15pF (Figures 3, 8) S2 Closed CL = 15pF (Figures 3, 8) S1 Closed CL = 15pF (Figures 3, 8) S2 Closed CONDITIONS RDIFF = 54, CL1 = CL2 = 100pF (Figures 2, 5)
q q q q q q q q q q q q q q q
The q denotes specifications which apply over the full operating temperature range. Note 1: "Absolute Maximum Ratings" are those beyond which the safety of the device cannot be guaranteed.
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NC (Pin 1): Not Connected. R (Pin 2): Receiver output. If the receiver output is enabled (REB low), then if A > B by 200mV, R will be high. If A < B by 200mV, then R will be low. REB (Pin 3): Receiver output enable. A low enables the receiver output, R. A high input forces the receiver output into a high impedance state. DE (Pin 4): Driver output enable. A high on DE enables the driver outputs, A and B. A low input forces the driver outputs into a high impedance state. D (Pin 5): Driver input. If the driver outputs are enabled (DE high), then A low on D forces the driver outputs A low and B high. A high on D will force A high and B low.
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MIN 10 10 5
TYP 30 30 5 15 40 40 40 40
MAX 50 50 25 70 70 70 70 150 150 50 50 50 50
UNITS ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns
40 40
70 70 13 20 20 20 20
Note 2: All currents into device pins are positive; all currents out of device pins are negative. All voltages are referenced to device ground unless otherwise specified. Note 3: All typicals are given for VCC = 5V and Temperature = 25C.
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GND (Pin 6): Ground Connection. GND (Pin 7): Ground Connection. NC (Pin 8): Not Connected. Y (Pin 9): Driver output. Z (Pin 10): Driver output. B (Pin 11): Receiver input. A (Pin 12): Receiver input. NC (Pin 13): Not Connected. VCC (Pin 14): Positive supply; 4.75V VCC 5.25V.
3
LTC491
TYPICAL PERFOR A CE CHARACTERISTICS
Driver Output High Voltage vs Output Current TA = 25C
-96
OUTPUT CURRENT (mA)
OUTPUT CURRENT (mA)
OUTPUT CURRENT (mA)
-72
- 48
-24
0 0 1 2 3 4
LTC491 * TPC01
OUTPUT VOLTAGE (V)
TTL Input Threshold vs Temperature
1.63
INPUT THRESHOLD VOLTAGE (V)
1.61
TIME (ns)
4.0
SUPPLY CURRENT (A)
1.59
1.57
1.55 -50
0
50
TEMPERATURE (C )
LTC491 * TPC04
Driver Differential Output Voltage vs Temperature RO = 54
2.3
DIFFERENTIAL VOLTAGE (V) 7.0
2.1
OUTPUT VOLTAGE (V)
1.9
TIME (ns)
1.7
1.5 -50
0
50
TEMPERATURE (C )
LTC491 * TPC07
4
UW
100 100
Driver Differential Output Voltage vs Output Current TA = 25C
64 80
Driver Output Low Voltage vs Output Current TA = 25C
48
60
32
40
16
20
0 0 1 2 3 4
LTC491 * TPC02
0 0 1 2 3 4
LTC491 * TPC03
OUTPUT VOLTAGE (V)
OUTPUT VOLTAGE (V)
Driver Skew vs Temperature
5.0
350
Supply Current vs Temperature
340
3.0
330
2.0
320
1.0 -50
0
50
100
LTC491 * TPC05
310 -50
0
50
100
LTC491 * TPC06
TEMPERATURE (C )
TEMPERATURE (C )
Receiver tPLH tPHL vs Temperature
0.8
Receiver Output Low Voltage vs Temperature at I = 8mA
6.0
0.6
5.0
0.4
4.0
0.2
3.0 -50
0
50
100
LTC491 * TPC08
0 -50
0
50
100
LTC491 * TPC09
TEMPERATURE (C )
TEMPERATURE (C )
LTC491
TEST CIRCUITS
Y R VOD2 R Z VOC
LTC491 * TA02
Figure 1. Driver DC Test Load
Y D DRIVER Z RDIFF
CL1
A RECEIVER R 15pF
CL2
B
LTC491 * TA03
Figure 2. Driver/Receiver Timing Test Circuit
RECEIVER OUTPUT CL 1k
S1
1k VCC OUTPUT UNDER TEST 500
S1 VCC
S2
CL
S2
LTC491 * TA04
LTC491 * TA05
Figure 3. Receiver Timing Test Load
Figure 4. Driver Timing Test Load
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LTC491
SWITCHI G TI E WAVEFOR S
3V D 0V tPLH VO -VO 50% 10% tr Z VO Y 1/2 VO tSKEW 1/2 VO tSKEW
LTC491 * TA06
1.5V
f = 1MHz : tr 10ns : tf 10ns
80%
Figure 5. Driver Propagation Delays
3V DE 0V tZL 5V A, B VOL VOH A, B 0V tZH 2.3V 2.3V 1.5V
Figure 6. Driver Enable and Disable Times
VOD2 A-B -VOD2 VOH R VOL 1.5V 0V tPLH
f = 1MHz ; tr 10ns : tf 10ns
Figure 7. Receiver Propagation Delays
3V REB 0V tZL 5V R VOL VOH R 0V tZH 1.5V 1.5V 1.5V
Figure 8. Receiver Enable and Disable Times
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1.5V tPHL
VDIFF = V(Y) - V(Z)
90% tf
50% 20%
f = 1MHz : tr 10ns : tr 10ns
1.5V tLZ
OUTPUT NORMALLY LOW
0.5V
OUTPUT NORMALLY HIGH tHZ
0.5V
LTC491 * TA07
INPUT 0V tPHL OUTPUT 1.5V
LTC491 * TA08
f = 1MHz : tr 10ns : tf 10ns
1.5V tLZ
OUTPUT NORMALLY LOW
0.5V
OUTPUT NORMALLY HIGH tHZ
0.5V
LTC491 * TA09
LTC491
APPLICATI S I FOR ATIO U
typically 20k to GND, or 0.6 unit RS-485 load, so in practice 50 to 60 transceivers can be connected to the same wires. The optional shields around the twisted pair help reduce unwanted noise, and are connected to GND at one end. The LTC491 can also be used as a line repeater as shown in Figure 10. If the cable length is longer than 4000 feet, the LTC491 is inserted in the middle of the cable with the receiver output connected back to the driver input.
12 120 11 120 11 RECEIVER 2 3 4 10 DX 5 DRIVER 120 9 120 9 10 DRIVER 5 DX RX 9 10 11 12 LTC491 RECEIVER LTC491 DRIVER 5 DX 4 32
LTC491 * TA10
Typical Application A typical connection of the LTC491 is shown in Figure 9. Two twisted pair wires connect up to 32 driver/receiver pairs for full duplex data transmission. There are no restrictions on where the chips are connected to the wires, and it isn't necessary to have the chips connected at the ends. However, the wires must be terminated only at the ends with a resistor equal to their characteristic impedance, typically 120. The input impedance of a receiver is
12 RX 2 3 4 RECEIVER
LTC491
RX
DX
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Figure 9. Typical Connection
12 2 3 4 10 5 DRIVER 120 9 DATA OUT RECEIVER 120 11 DATA IN
LTC491
LTC491 * TA11
Figure 10. Line Repeater
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LTC491
APPLICATI S I FOR ATIO U
less flexible, more bulky, and more costly than twisted pairs. Many cable manufacturers offer a broad range of 120 cables designed for RS485 applications. Losses in a transmission line are a complex combination of DC conductor loss, AC losses (skin effect), leakage and AC losses in the dielectric. In good polyethylene cables such as the Belden 9841, the conductor losses and dielectric losses are of the same order of magnitude, leading to relatively low over all loss (Figure 11). When using low loss cables, Figure 12 can be used as a guideline for choosing the maximum line length for a given data rate. With lower quality PVC cables, the dielectric loss factor can be 1000 times worse. PVC twisted pairs have terrible losses at high data rates (>100kBs), and greatly reduce the maximum cable length. At low data rates however, they are acceptable and much more economical.
10k CABLE LENGTH (ft) 1k 100 10 10k 100k 1M 2.5M 10M DATA RATE (bps)
LTC491 * TA12 LTC491 * TA13
Thermal Shutdown The LTC491 has a thermal shutdown feature which protects the part from excessive power dissipation. If the outputs of the driver are accidently shorted to a power supply or low impedance source, up to 250mA can flow through the part. The thermal shutdown circuit disables the driver outputs when the internal temperature reaches 150C and turns them back on when the temperature cools to 130C. If the outputs of two or more LTC491 drivers are shorted directly, the driver outputs can not supply enough current to activate the thermal shutdown. Thus, the thermal shutdown circuit will not prevent contention faults when two drivers are active on the bus at the same time. Cables and Data Rate The transmission line of choice for RS485 applications is a twisted pair. There are coaxial cables (twinaxial) made for this purpose that contain straight pairs, but these are
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LOSS PER 100 ft (dB)
1.0
0.1 0.1 1.0 10 100 FREQUENCY (MHZ)
Figure 11. Attenuation vs Frequency for Belden 9481
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Figure 12. Cable Length vs Data Rate
LTC491
APPLICATI
S I FOR ATIO
Cable Termination The proper termination of the cable is very important. If the cable is not terminated with it's characteristic impedance, distorted waveforms will result. In severe cases, distorted (false) data and nulls will occur. A quick look at the output of the driver will tell how well the cable is terminated. It is best to look at a driver connected to the end of the cable, since this eliminates the possibility of getting reflections from two directions. Simply look at the driver output while transmitting square wave data. If the cable is terminated properly, the waveform will look like a square wave (Figure 13).
PROBE HERE
Rt DX DRIVER RECEIVER RX
Rt = 120
Rt = 47
Rt = 470
LTC491 * TA14
Figure 13. Termination Effects
If the cable is loaded excessively (47), the signal initially sees the surge impedance of the cable and jumps to an initial amplitude. The signal travels down the cable and is reflected back out of phase because of the mistermination. When the reflected signal returns to the driver, the amplitude will be lowered. The width of the pedestal is equal to twice the electrical length of the cable (about 1.5ns/foot).
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If the cable is lightly loaded (470), the signal reflects in phase and increases the amplitude at the driver output. An input frequency of 30kHz is adequate for tests out to 4000 feet of cable. AC Cable Termination Cable termination resistors are necessary to prevent unwanted reflections, but they consume power. The typical differential output voltage of the driver is 2V when the cable is terminated with two 120 resistors, causing 33mA of DC current to flow in the cable when no data is being sent. This DC current is about 60 times greater than the supply current of the LTC491. One way to eliminate the unwanted current is by AC coupling the termination resistors as shown in Figure 14.
120 C RECEIVER RX C = LINE LENGTH (ft) x 16.3pF
LTC491 * TA15
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Figure 14. AC Coupled Termination
The coupling capacitor must allow high-frequency energy to flow to the termination, but block DC and low frequencies. The dividing line between high and low frequency depends on the length of the cable. The coupling capacitor must pass frequencies above the point where the line represents an electrical one-tenth wavelength. The value of the coupling capacitor should therefore be set at 16.3pF per foot of cable length for 120 cables. With the coupling capacitors in place, power is consumed only on the signal edges, and not when the driver output is idling at a 1 or 0 state. A 100nF capacitor is adequate for lines up to 4000 feet in length. Be aware that the power savings start to decrease once the data rate surpasses 1/(120 x C).
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LTC491
APPLICATI
S I FOR ATIO
Receiver Open-Circuit Fail-Safe Some data encoding schemes require that the output of the receiver maintains a known state (usually a logic 1) when the data is finished transmitting and all drivers on the line are forced into three-state. The receiver of the LTC491 has a fail-safe feature which guarantees the output to be in a logic 1 state when the receiver inputs are left floating (open-circuit). However, when the cable is terminated with 120, the differential inputs to the receiver are shorted together, not left floating. Because the receiver has about 70mV of hysteresis, the receiver output will maintain the last data bit received.
+5V 110 130 130 110
RECEIVER
+5V 1.5k 140 RECEIVER RX
1.5k
100k +5V C 120 RECEIVER RX
LTC491 * TA16
Figure 15. Forcing "O" When All Drivers are Off
The termination resistors are used to generate a DC bias which forces the receiver output to a known state, in this case a logic 0. The first method consumes about 208mW and the second about 8mW. The lowest power solution is to use an AC termination with a pull-up resistor. Simply swap the receiver inputs for data protocols ending in logic 1.
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Fault Protection All of LTC's RS485 products are protected against ESD transients up to 2kV using the human body model (100pF, 1.5k). However, some applications need more protection. The best protection method is to connect a bidirectional TransZorb from each line side pin to ground (Figure 16).
Y DRIVER 120 Z
LTC491 * TA17
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RX
Figure 16. ESD Protection with TransZorbs
A TransZorb is a silicon transient voltage suppressor that has exceptional surge handling capabilities, fast response time, and low series resistance. They are available from General Semiconductor Industries and come in a variety of breakdown voltages and prices. Be sure to pick a breakdown voltage higher than the common mode voltage required for your application (typically 12V). Also, don't forget to check how much the added parasitic capacitance will load down the bus.
LTC491
TYPICAL APPLICATI
Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
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RS232 Receiver
RS232 IN 5.6k RECEIVER 1/2 LTC491
LTC491 * TA18
RX
RS232 to RS485 Level Transistor with Hysteresis
R = 220k Y 10k RS232 IN 5.6k DRIVER 1/2 LTC491 120 Z
19k VY - VZ HYSTERESIS = 10k * -------- -------- R R
LTC491 * TA19
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LTC491
PACKAGE DESCRIPTIO
0.300 - 0.325 (7.620 - 8.255)
0.009 - 0.015 (0.229 - 0.381) +0.025 0.325 -0.015 8.255 +0.635 -0.381
(
)
0.010 - 0.020 x 45 (0.254 - 0.508) 0.008 - 0.010 (0.203 - 0.254)
0 - 8 TYP 0.016 - 0.050 0.406 - 1.270 0.014 - 0.019 (0.355 - 0.483)
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Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7487
(408) 432-1900 q FAX: (408) 434-0507 q TELEX: 499-3977
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Dimensions in inches (millimeters) unless otherwise noted. N Package 14-Lead Plastic DIP
0.770 (19.558) MAX 14 13 12 11 10 9 8
TJ MAX 100C JA 90C/W
0.260 0.010 (6.604 0.254)
1
2
3
4
5
6
7 0.065 (1.651) TYP
0.015 (0.380) MIN 0.130 0.005 (3.302 0.127)
0.045 - 0.065 (1.143 - 1.651)
0.075 0.015 (1.905 0.381)
0.018 0.003 (0.457 0.076) 0.100 0.010 (2.540 0.254)
0.125 (3.175) MIN
N14 0392
S Package 14-Lead Plastic SOIC
0.337 - 0.344 (8.560 - 8.738) 14 13 12 11 10 9 8 TJ MAX 100C 0.228 - 0.244 (5.791 - 6.197) 0.150 - 0.157 (3.810 - 3.988) JA 110C/W
1
2
3
4
5
6
7
0.053 - 0.069 (1.346 - 1.752) 0.004 - 0.010 (0.101 - 0.254)
0.050 (1.270) TYP
SO14 0392
BA/GP 0492 10K REV 0
(c) LINEAR TECHNOLOGY CORPORATION 1992

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