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LTC490 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 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 for 2.5MB Operation Pin Compatible with the SN75179 Available in 8-Lead PDIP and SO Packages
The LTC(R)490 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. 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.
, LTC and LT are registered trademarks of Linear Technology Corporation.
APPLICATIO S
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Low Power RS485/RS422 Transceiver Level Translator
TYPICAL APPLICATIO
LTC490
5 D 3 DRIVER 120 6 4000 FT BELDEN 9841 120 RECEIVER R
8 R 2 RECEIVER 120 7 4000 FT BELDEN 9841 120 DRIVER D
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LTC490
LTC490 * TA01
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1
LTC490
ABSOLUTE
(Note 1)
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RATI GS
PACKAGE/ORDER I FOR ATIO
TOP VIEW VCC 1 R2 D3
D
Supply Voltage (VCC) ............................................... 12V Driver Input Currents ........................... - 25mA to 25mA Driver Input Voltages ...................... -0.5V to VCC + 0.5V Driver Output Voltages .......................................... 14V Receiver Input Voltages ......................................... 14V Receiver Output Voltages ............... -0.5V to VCC + 0.5V Operating Temperature Range LTC490C ................................................ 0C to 70C LTC490I ............................................. - 40C to 85C Storage Temperature Range ................. - 65C to 150C Lead Temperature (Soldering, 10 sec).................. 300C
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R
A B Z Y
ORDER PART NUMBER LTC490CN8 LTC490CS8 LTC490IN8 LTC490IS8 S8 PART MARKING 490 490I
7 6 5
GND 4
N8 PACKAGE 8-LEAD PDIP S8 PACKAGE 8-LEAD PLASTIC SO
TJMAX = 125C, JA = 100C/ W (N8) TJMAX = 150C, JA = 150C/ W (S8)
Consult LTC Marketing for parts specified with wider operating temperature ranges.
DC ELECTRICAL CHARACTERISTICS
SYMBOL VOD1 VOD2 VOD VOC VOC VIH VIL lIN1 lIN2 VTH VTH VOH VOL IOZR ICC RIN IOSD1 IOSD2 IOSR IOZ 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 (D) Input Low Voltage (D) Input Current (D) 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
The q denotes the specificatiions which apply over the full operating temperature range, otherwise specifications are at TA = 25C. VCC = 5V 5%
CONDITIONS IO = 0 R = 50 (RS422) R = 27 (RS485) (Figure 1) R = 27 or R = 50 (Figure 1) R = 27 or R = 50 (Figure 1) R = 27 or R = 50 (Figure 1)
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 - 0.8 - 0.2 70 3.5 0.4 1 300 12 100 100 7 2 250 250 85 200 500 0.2
VCC = 0V or 5.25V - 7V VCM 12V VCM = 0V
VIN = 12V VIN = - 7V
q q q q q q q q q q q q q
IO = -4mA, VID = 0.2V IO = 4mA, VID = - 0.2V VCC = Max 0.4V VO 2.4V No Load; D = GND or VCC - 7V VO 12V VO = - 7V VO = 12V 0V VO VCC VO = - 7V to 12V
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V A mA mA V mV V V A A k mA mA mA A
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LTC490
SWITCHI G CHARACTERISTICS
SYMBOL tPLH tPHL tSKEW tr, tf tPLH tPHL tSKD PARAMETER Driver Input to Output Driver Input to Output Driver Output to Output Driver Rise or Fall Time Receiver Input to Output Receiver Input to Output tPLH - tPHL Differential Receiver Skew
The q denotes the specificatiions which apply over the full operating temperature range, otherwise specifications are at TA = 25C. VCC = 5V 5%
CONDITIONS RDIFF = 54, CL1 = CL2 = 100pF (Figures 2, 3) RDIFF = 54, CL1 = CL2 = 100pF (Figures 2, 3) RDIFF = 54, CL1 = CL2 = 100pF (Figures 2, 3) RDIFF = 54, CL1 = CL2 = 100pF (Figures 2, 3) RDIFF = 54, CL1 = CL2 = 100pF (Figures 2, 4) RDIFF = 54, CL1 = CL2 = 100pF (Figures 2, 4) RDIFF = 54, CL1 = CL2 = 100pF (Figures 2, 4)
q q q q q q q
Note 1: Absolute maximum ratings are those beyond which the safety of the device cannot be guaranteed. 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.
TYPICAL PERFOR A CE CHARACTERISTICS
Driver Output High Voltage vs Output Current
-96
OUTPUT CURRENT (mA)
TA = 25C
OUTPUT CURRENT (mA)
-72
48
OUTPUT CURRENT (mA)
- 48
-24
0 0 1 2 3 OUTPUT VOLTAGE (V) 4
LTC490 * TPC01
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MIN 10 10 5 40 40
TYP 30 30 5 5 70 70 13
MAX 50 50 25 150 150
UNITS ns ns ns ns ns ns ns
Note 3: All typicals are given for VCC = 5V and Temperature = 25C.
Driver Differential Output Voltage vs Output Current
64 TA = 25C 80
Driver Output Low Voltage vs Output Current
TA = 25C
60
32
40
16
20
0 0 1 2 3 OUTPUT VOLTAGE (V) 4
LTC490 * TPC02
0 0 1 2 3 OUTPUT VOLTAGE (V) 4
LTC490 * TPC03
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LTC490 TYPICAL PERFOR A CE CHARACTERISTICS
TTL Input Threshold vs Temperature
1.63 5
INPUT THRESHOLD VOLTAGE (V)
SUPPLY CURRENT (A)
1.61
TIME (ns)
1.59
1.57
1.55 -50
0 50 TEMPERATURE (C )
Driver Differential Output Voltage vs Temperature
2.3 RO = 54
DIFFERENTIAL VOLTAGE (V)
2.1
OUTPUT VOLTAGE (V)
1.9
TIME (ns)
1.7
1.5 -50
0 50 TEMPERATURE (C )
4
UW
100
LTC490 * TPC04
Driver Skew vs Temperature
350
Supply Current vs Temperature
4
340
3
330
2
320
1 -50
0 50 TEMPERATURE (C )
100
LTC490 * TPC05
310 -50
0 50 TEMPERATURE (C )
100
LTC490 * TPC06
Receiver tPLH-tPHL vs Temperature
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0.8
Receiver Output Low Voltage vs Temperature
I = 8mA
6
0.6
5
0.4
4
0.2
100
LTC490 * TPC07
3 -50
0 50 TEMPERATURE (C )
100
LTC490 * TPC08
0 -50
0 50 TEMPERATURE (C )
100
LTC490 * TPC09
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LTC490
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VCC (Pin 1): Positive Supply; 4.75V VCC 5.25V. R (Pin 2): Receiver Output. If A > B by 200mV, R will be high. If A < B by 200mV, then R will be low. D (Pin 3): Driver Input. A low on D forces the driver outputs Y low and Z high. A high on D will force Y high and Z low. GND (Pin 4): Ground Connection. Y (Pin 5): Driver Output. Z (Pin 6): Driver Output. B (Pin 7): Receiver Input. A (Pin 8): Receiver Input.
TEST CIRCUITS
Y R VOD2 R Z
LTC490 * F01 LTC490 * F02
Figure 1. Driver DC Test Load
SWITCHI G TI E WAVEFOR S
3V D 0V tPLH VO -VO 50% 10% tr Z VO Y 1/2 VO t SKEW 1/2 VO t SKEW
LTC490 * F03
1.5V
f = 1MHz : t r 10ns : t f 10ns
80%
VDIFF = V(Y) - V(Z)
Figure 3. Driver Propagation Delays
VOD2 A-B -VOD2 VOH R VOL
0V tPLH 1.5V
f = 1MHz ; t r 10ns : t f 10ns
Figure 4. Receiver Propagation Delays
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Y D VOC DRIVER Z RDIFF
CL1
A RECEIVER R 15pF
CL2
B
Figure 2. Driver/Receiver Timing Test Circuit
1.5V tPHL 90% tf 50% 20%
INPUT 0V tPHL OUTPUT 1.5V
LTC490 * F04
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LTC490
APPLICATIO S I FOR ATIO
Typical Application
A typical connection of the LTC490 is shown in Figure 5. Two twisted-pair wires connect two driver/receiver pairs for full duplex data transmission. Note that the driver and receiver outputs are always enabled. If the outputs must be disabled, use the LTC491. There are no restrictions on where the chips are connected, and it isn't necessary to have the chips connected at the ends of the wire. However, the wires must be terminated only at the ends with a resistor equal to their characteristic impedance, typically 120. Because only one driver can be connected on the bus, the cable can be terminated only at the receiving end. The optional shields around the twisted pair help reduce unwanted noise, and are connected to GND at one end. The LTC490 can also be used as a line repeater as shown in Figure 6. If the cable length is longer than 4000 feet, the LTC490 is inserted in the middle of the cable with the receiver output connected back to the driver input. Thermal Shutdown The LTC490 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
5V 1 LTC490 8 RX 2 RECEIVER 120 7 SHIELD
+
0.01F DX 3 4 DRIVER
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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 LTC490 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
LTC490 8 RX 2 RECEIVER 120 7 DATA IN 6 DX 3 DRIVER 5 DATA OUT
LTC490 * F06
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Figure 6. Line Repeater
5V LTC490 5 6 DRIVER 3 DX 1
SHIELD 7 120 8 RECEIVER 2 RX
+
0.01F
4
LTC490 * F05
Figure 5. Typical Connection
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LTC490
APPLICATIO S I FOR ATIO
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 overall loss (Figure 7). When using low loss cables, Figure 8 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
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LOSS PER 100 FT (dB)
1.0
0.1 0.1 1.0 10 100
LTC490 * F07
FREQUENCY (MHz)
Figure 7. Attenuation vs Frequency for Belden 9841
10k
CABLE LENGTH (FT)
1k
100
10 10k
100k
1M
2.5M
10M
LTC490 * F09
DATA RATE (bps)
LTC490 * F08
Figure 8. RS485 Cable Length Specification. Applies for 24 Gauge, Polyethylene Dielectric Twisted Pair
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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. Cable Termination The proper termination of the cable is very important. If the cable is not terminated with its 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 9). 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). If the
PROBE HERE Rt DX DRIVER RECEIVER RX Rt = 120 Rt = 47 Rt = 470
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Figure 9. Termination Effects
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LTC490
APPLICATIO S I FOR ATIO
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 LTC490. One way to eliminate the unwanted current is by AC coupling the termination resistors as shown in Figure 10. 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.
120
C
RECEIVER
RX
C = LINE LENGTH (FT) x 16.3pF
LTC490 * F10
Figure 10. AC Coupled Termination
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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). 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(R) from each line side pin to ground (Figure 11). 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 Instruments, GSI 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.
TransZorb is a registered trademark of General Instruments, GSI
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Y DRIVER 120 Z
LTC490 * F11
Figure 11. ESD Protection with TransZorbs
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LTC490
TYPICAL APPLICATIO S
RS232 Receiver
RS232 IN 5.6k
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RS232 IN 5.6k RECEIVER 1/2 LTC490
LTC490 * TA02
RX
RS232 to RS485 Level Transistor with Hysteresis
R = 220k
Y 10k DRIVER 1/2 LTC490 120 Z
VY - VZ 19k HYSTERESIS = 10k * -------- ---- R R
LTC490 * TA03
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LTC490
PACKAGE DESCRIPTIO
.300 - .325 (7.620 - 8.255)
.008 - .015 (0.203 - 0.381)
(
+.035 .325 -.015 8.255 +0.889 -0.381
)
INCHES MILLIMETERS *THESE DIMENSIONS DO NOT INCLUDE MOLD FLASH OR PROTRUSIONS. MOLD FLASH OR PROTRUSIONS SHALL NOT EXCEED .010 INCH (0.254mm)
NOTE: 1. DIMENSIONS ARE
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N8 Package 8-Lead PDIP (Narrow .300 Inch)
(Reference LTC DWG # 05-08-1510)
.400* (10.160) MAX 8 7 6 5 .255 .015* (6.477 0.381) 1 2 3 4 .130 .005 (3.302 0.127) .045 - .065 (1.143 - 1.651) .065 (1.651) TYP .120 (3.048) .020 MIN (0.508) MIN .018 .003 (0.457 0.076)
N8 1002
.100 (2.54) BSC
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LTC490
PACKAGE DESCRIPTIO
.050 BSC 8
.245 MIN
.030 .005 TYP RECOMMENDED SOLDER PAD LAYOUT .010 - .020 x 45 (0.254 - 0.508) .008 - .010 (0.203 - 0.254) 0- 8 TYP
.016 - .050 (0.406 - 1.270) NOTE: 1. DIMENSIONS IN
INCHES (MILLIMETERS) 2. DRAWING NOT TO SCALE 3. THESE DIMENSIONS DO NOT INCLUDE MOLD FLASH OR PROTRUSIONS. MOLD FLASH OR PROTRUSIONS SHALL NOT EXCEED .006" (0.15mm)
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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S8 Package 8-Lead Plastic Small Outline (Narrow .150 Inch)
(Reference LTC DWG # 05-08-1610)
.189 - .197 (4.801 - 5.004) NOTE 3 7 6 5 .045 .005 .160 .005 .228 - .244 (5.791 - 6.197) .150 - .157 (3.810 - 3.988) NOTE 3 1 2 3 4 .053 - .069 (1.346 - 1.752) .004 - .010 (0.101 - 0.254) .014 - .019 (0.355 - 0.483) TYP .050 (1.270) BSC
SO8 0303
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LTC490
RELATED PARTS
PART NUMBER LTC486/LTC487 LTC488/LTC489 LTC1480 LTC1481 LTC1482 LTC1483 LTC1484 LTC1485 LTC1518/LTC1519 LTC1520 LTC1535 LTC1685 LTC1686/LTC1687 LTC1688/LTC1689 LTC1690 LT1785/LTC1785A LT1791/LTC1791A DESCRIPTION Low Power Quad RS485 Drivers Low Power Quad RS485 Receivers 3.3V Supply RS485 Transceiver Low Power RS485 Transceiver with Shutdown RS485 Transceiver with Carrier Detect Low Power, Low EMI RS485 Transceiver RS485 Transceiver with Fail-Safe 10Mbps RS485 Transceiver 52Mbps Quad RS485 Receivers LVDS-Compatible Quad Receiver 2500V Isolated RS485 Transceiver 52Mbps RS485 Transceiver 52Mbps Full-Duplex RS485 Transceiver 100Mbps Quad RS485 Drivers Full-Duplex RS485 Transceiver with Fail-Safe 60V Protected RS485 Transceivers 60V Protected Full-Duplex RS485 Transceivers COMMENTS 110A Supply Current 7mA Supply Current Lower Supply Voltage Lowest Power 15kV ESD, Fail-Safe Slew Rate Limited Driver Outputs, Lowest Power 15kV ESD, MSOP Package High Speed Higher Speed, LTC488/LTC489 Pin-Compatible 100mV Threshold, Low Channel-to-Channel Skew Full-Duplex, Self-Powered Using External Transformer Industry-Standard Pinout, 500ps Propagation Delay Skew LTC490/LTC491 Pin Compatible Highest Speed, LTC486/LTC487 Pin Compatible 15kV ESD, LTC490 Pin Compatible 15kV ESD, Fail-Safe (LT1785A) 15kV ESD, Fail-Safe (LT1791A), LTC491 Pin Compatible
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Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 q FAX: (408) 434-0507
q
LT/TP 0104 1K REV B * PRINTED IN USA
www.linear.com
(c) LINEAR TECHNOLOGY CORPORATION 1993

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