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LTC486 Quad Low Power RS485 Driver FEATURES

DESCRIPTIO
Very Low Power: ICC = 110A Typ Designed for RS485 or RS422 Applications Single 5V Supply - 7V to 12V Bus Common-Mode Range Permits 7V GND Difference Between Devices on the Bus Thermal Shutdown Protection Power-Up/Down Glitch-Free Driver Outputs Permit Live Insertion/Removal of Package Driver Maintains High Impedance in Three-State or with the Power Off 28ns Typical Driver Propagation Delays with 5ns Skew Pin Compatible with the SN75172, DS96172, A96172, and DS96F172
The LTC486 is a low power differential bus/line driver designed for multipoint data transmission standard RS485 applications with extended common-mode range (12V to -7V). It also meets RS422 requirements. The CMOS design offers significant power savings over its bipolar counterpart without sacrificing ruggedness against overload or ESD damage. The driver features 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. Both AC and DC specifications are guaranteed from 0C to 70C (Commercial), - 40C to 85C (Industrial), over the 4.75V to 5.25V supply voltage range.
APPLICATI

S
Low Power RS485/RS422 Drivers Level Translator
TYPICAL APPLICATI
CABLE LENGTH (FT)
EN
EN
4 DI 1 DRIVER 12 1/4 LTC486 EN 120 4000 FT BELDEN 9841 120
2
4 RECEIVER 3 RO
1
1/4 LTC488
LTC486 * TA01
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RS485 Cable Length Specification*
10k 1k 100 10 10k 100k 1M 2.5M 10M DATA RATE (bps)
LTC486 * TA09
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* APPLIES FOR 24 GAUGE, POLYETHYLENE DIELECTRIC TWISTED PAIR
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LTC486 ABSOLUTE
(Note 1)
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RATI GS
PACKAGE/ORDER I FOR ATIO
TOP VIEW DI1 DO1A DO1B EN DO2B DO2A DI2 GND 1 2 3 4 5 6 7 8 16 VCC 15 DI4 14 DO4A 13 DO4B 12 EN 11 DO3B 10 DO3A 9 DI3
Supply Voltage (VCC) ............................................... 12V Control Input Voltages .................... - 0.5V to VCC + 0.5V Driver Input Voltages ...................... - 0.5V to VCC + 0.5V Driver Output Voltages ......................................... 14V Control Input Currents ....................................... 25mA Driver Input Currents ......................................... 25mA Operating Temperature Range LTC486C ................................................ 0C to 70C LTC486I ............................................ - 40C to 85C Storage Temperature Range ................. - 65C to 150C Lead Temperature (Soldering, 10 sec)................. 300C
ORDER PART NUMBER LTC486CN LTC486CSW LTC486IN LTC486ISW
N PACKAGE S PACKAGE 16-LEAD PLASTIC DIP 16-LEAD PLASTIC SOL
TJMAX = 125C, JA = 70C/W (N) TJMAX = 150C, JA = 95C/W (S)
Consult factory for Military grade parts
DC ELECTRICAL CHARACTERISTICS
VCC = 5V 5%, 0C Temperature 70C (Commercial), - 40C Temperature 85C (Industrial) (Note 2, 3)
SYMBOL VOD1 VOD2 VOD VOC VOC VIH VIL IIN1 ICC IOSD1 IOSD2 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 Input Low Voltage Input Current Supply Current Driver Short-Circuit Current, VOUT = High Driver Short-Circuit Current, VOUT = Low High Impedance State Output Current No Load VO = - 7V VO = 12V VO = - 7V to 12V Output Enabled Output Disabled 110 110 100 100 10 DI, EN, EN 2.0 0.8 2 200 200 250 250 200 CONDITIONS IO = 0 R = 50; (RS422) R = 27; (RS485) (Figure 1) R = 27 or R = 50 (Figure 1) 2 1.5 5 0.2 3 0.2 MIN TYP MAX 5 UNITS V V V V V V V V A A A mA mA A
SWITCHI G CHARACTERISTICS
SYMBOL tPLH tPHL tSKEW tr, tf tZH PARAMETER Driver Input to Output Driver Input to Output Driver Output to Output Driver Rise or Fall Time Driver Enable to Output High
VCC = 5V 5%, 0C Temperature 70C (Commercial), - 40C Temperature 85C (Industrial) (Note 2, 3)
CONDITIONS RDIFF = 54, CL1 = CL2 = 100pF (Figures 2, 4) MIN 10 10 5 CL = 100pF (Figures 3, 5) S2 Closed TYP 30 30 5 15 35 MAX 50 50 15 25 70 UNITS ns ns ns ns ns
2
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LTC486
VCC = 5V 5%, 0C Temperature 70C (Commercial), - 40C Temperature 85C (Industrial) (Note 2, 3)
SYMBOL tZL tLZ tHZ PARAMETER Driver Enable to Output Low Driver Disable Time from Low Driver Disable Time from High CONDITIONS CL = 100pF (Figures 3, 5) S1 Closed CL = 15pF (Figures 3, 5) S1 Closed CL = 15pF (Figures 3, 5) S2 Closed MIN TYP 35 35 35 MAX 70 70 70 UNITS ns ns ns
SWITCHI G CHARACTERISTICS
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
SWITCHI G TI E WAVEFOR S
3V DI 0V t PLH B VO A VO -VO 1/2 VO 80% 10% tr
3V EN 0V t ZL 5V A, B VOL VOH A, B 0V tZH tHZ
LTC486 * TA06
1.5V
t SKEW VDIFF = V(A) - V(B)
Figure 4. Driver Propagation Delays
1.5V f = 1MHz : t r 10ns : t f 10ns
2.3V
2.3V
Figure 5. Driver Enable and Disable Times
TYPICAL PERFOR A CE CHARACTERISTICS
Driver Output High Voltage vs Output Current TA = 25C
-96 OUTPUT CURRENT (mA) 64
Driver Differential Output Voltage vs Output Current TA = 25C
80
OUTPUT CURRENT (mA)
OUTPUT CURRENT (mA)
-72
48
- 48
32
-24
16
0 0 1 2 3 4
LTC486 * TPC01
0 0 1 2 3 4
LTC486* TPC02
OUTPUT VOLTAGE (V)
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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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f = 1MHz : t r < 10ns : t f < 10ns
1.5V t PHL
1/2 VO 90%
t SKEW 20% tf
LTC486 * TA05
1.5V t LZ
OUTPUT NORMALLY LOW
0.5V
OUTPUT NORMALLY HIGH
0.5V
Driver Output Low Voltage vs Output Current TA = 25C
60
40
20
0 0 1 2 3 4
LTC486 * TPC03
OUTPUT VOLTAGE (V)
OUTPUT VOLTAGE (V)
3
LTC486
TYPICAL PERFOR A CE CHARACTERISTICS
TTL Input Threshold vs Temperature
1.63 INPUT THRESHOLD VOLTAGE (V) 5
1.61
1.59
TIME (ns)
0 50 100
LTC486 * TPC04
1.57
1.55 -50
Supply Current vs Temperature
130
DIFFERENTIAL VOLTAGE (V)
SUPPLY CURRENT (A)
120
110
100
90 -50
0
FU CTI
INPUT DI H L H L X
TABLE
ENABLES OUTPUTS EN X X L L H OUTA H L H L Z OUTB L H L H Z H: High Level L: Low Level X: Irrelevant Z: High Impedance (Off)
EN H H X X L
4
UW
Driver Skew vs Temperature
4
3
2
1 -50
0
50
100
LTC486 * TPC05
TEMPERATURE (C )
TEMPERATURE (C )
Driver Differential Output Voltage vs Temperature RO = 54
2.3
2.1
1.9
1.7
50
100
LTC486 * TPC06
1.5 -50
0
50
100
LTC486 * TPC07
TEMPERATURE (C )
TEMPERATURE (C )
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LTC486
PI FU CTI
DI1 (Pin 1): Driver 1 Input. If Driver 1 is enabled, then a low on DI1 forces the driver outputs DO1A low and DO1B high. A high on DI1 with the driver outputs enabled will force DO1A high and DO1B low. DO1A (Pin 2): Driver 1 Output. DO1B (Pin 3): Driver 1 Output. EN (Pin 4): Driver Outputs Enabled. See Function Table for details. DO2B (Pin 5): Driver 2 Output. DO2A (Pin 6): Driver 2 Output. DI2 (Pin 7): Driver 2 Input. Refer to DI1.
TEST CIRCUITS
A R VOD R B
Figure 1. Driver DC Test Load
APPLICATI
S I FOR ATIO
Typical Application A typical connection of the LTC486 is shown in Figure 6. A twisted pair of wires connect up to 32 drivers and receivers for half 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 optional shields around the twisted pair help reduce unwanted noise, and are connected to GND at one end. Thermal Shutdown The LTC486 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
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GND (Pin 8): Ground Connection. DI3 (Pin 9): Driver 3 Input. Refer to DI1. DO3A (Pin 10): Driver 3 Output. DO3B (Pin 11): Driver 3 Output. EN (Pin 12): Driver Outputs Disabled. See Function Table for details. DO4B (Pin 13): Driver 4 Output. DO4A (Pin 14): Driver 4 Output. DI4 (Pin 15): Driver 4 Input. Refer to DI1. VCC (Pin 16): Positive Supply; 4.75V < VCC < 5.25V .
EN CI1 A DI DRIVER B CI2
LTC486 * TA03
S1 VCC OUTPUT UNDER TEST 500
RDIFF
VOC
CL
S2
LTC486 * TA04
LTC486 * TA02
EN
Figure 2. Driver Timing Test Circuit
Figure 3. Driver Timing Test Load #2
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 LTC486 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. Cable 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 less flexible, more bulky, and more costly than twisted pairs. Many cable manufacturers offer a broad range of 120 cables designed for RS485 applications.
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LTC486
APPLICATI
DX
S I FOR ATIO
EN 4 3 120 2 12 EN 1/4 LTC486 DX 1 DX EN 4 DX
1
12 EN 1/4 LTC486
Figure 6. Typical Connection
CABLE LENGTH (FT)
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, with relatively low overall loss (Figure 7).
10
LOSS PER 100 FT (dB)
1
0.1 0.1 1 10 100
LTC486 * TA08
FREQUENCY (MHz)
Figure 7. Attenuation vs Frequency for Belden 9841
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 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,
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EN SHIELD SHIELD 2 120 1 12 EN 3 1 4 RX 2 2 12 EN 1/4 LTC488 3 RX EN 1/4 LTC488 4 RX 3 RX
LTC486 * TA07
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10k
1k
100
10 10k
100k
1M
2.5M
10M
DATA RATE (bps)
LTC486 * TA09
Figure 8. Cable Length vs Data Rate
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 (e.g., 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/ft). If the cable is lightly loaded (e.g., 470),
LTC486
APPLICATI
DX
S I FOR ATIO
Rt RECEIVER
PROBE HERE
DRIVER
Rt = 120
Rt = 47
Rt = 470
LTC486 * TA10
Figure 9. Termination Effects
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 ft. 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. When no data is being sent 33mA of DC current flows in the cable . This DC current is about 220 times greater than the supply current of the LTC486. One way to eliminate the unwanted current is by AC coupling the termination resistors as shown in Figure 10.
120
C
RECEIVER
RX
C = LINE LENGTH (FT) x 16.3pF
LTC486 * TA11
Figure 10. AC Coupled Termination
The coupling capacitor allows high frequency energy to flow to the termination, but blocks 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
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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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, 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). 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. All LTC RS485 receivers have 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. If the receiver output must be forced to a known state, the circuits of Figure 11 can be used. 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
5V 110 130 130 110 RECEIVER 5V 1.5k 140 RECEIVER RX RX
1.5k 100k 5V 120 RECEIVER RX C
LTC486 * TA12
Figure 11. Forcing "0" When All Dirvers Are Off
7
LTC486
APPLICATI S I FOR ATIO U
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.
Y DRIVER 120 Z
LTC486 * TA13
solution is to use an AC termination with a pull-up resistor. Simply swap the receiver inputs for data protocols ending in logic 1. 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 greater protection. The best protection method is to connect a bidirectional TransZorb(R) from each line side pin to ground (Figure 12). A TransZorb(R) 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
TYPICAL APPLICATI
10k RS232 IN 5.6k DRIVER 1/4 LTC486
PACKAGE DESCRIPTIO
0.300 - 0.325 (7.620 - 8.255)
N Package 16-Lead Plastic DIP
0.009 - 0.015 (0.229 - 0.381)
(
+0.025 0.325 -0.015 8.255 +0.635 -0.381
)
0.005 (0.127) RAD MIN
0.291 - 0.299 (7.391 - 7.595) (NOTE 2) 0.010 - 0.029 x 45 (0.254 - 0.737)
S Package 16-Lead Plastic SOL
0.009 - 0.013 (0.229 - 0.330) NOTE 1
0.014 - 0.019 0.016 - 0.050 (0.356 - 0.482) (0.406 - 1.270) TYP NOTE: 1. PIN 1 IDENT, NOTCH ON TOP AND CAVITIES ON THE BOTTOM OF PACKAGES ARE THE MANUFACTURING OPTIONS. THE PART MAY BE SUPPLIED WITH OR WITHOUT ANY OF THE OPTIONS. 2. THESE DIMENSIONS DO NOT INCLUDE MOLD FLASH OR PROTRUSIONS. MOLD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.006 INCH (0.15mm).
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Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7487
(408) 432-1900 FAX: (408) 434-0507 TELEX: 499-3977
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Figure 12. ESD Protection
TransZorb(R) is a registrated trademark of General Instruments, GSI
RS232 to RS485 Level Translator with Hysteresis
R = 220k Y 120 Z VY - VZ 19k HYSTERESIS = 10k x -------- ---- R R
LTC486 * TA14
Dimensions in inches (millimeters) unless otherwise noted.
0.130 0.005 (3.302 0.127) 0.045 - 0.065 (1.143 - 1.651) 0.770 (19.558) MAX 16 15 14 13 12 11 10 9
0.015 (0.381) MIN
0.065 (1.651) TYP
0.260 0.010 (6.604 0.254)
1 0.125 (3.175) MIN 0.045 0.015 (1.143 0.381) 0.100 0.010 (2.540 0.254) 0.018 0.003 (0.457 0.076)
2
3
4
5
6
7
8
0.398 - 0.413 (10.109 - 10.490) (NOTE 2) 0.093 - 0.104 (2.362 - 2.642) 0.037 - 0.045 (0.940 - 1.143) 16 15 14 13 12 11 10 9
0 - 8 TYP
0.050 (1.270) TYP
0.004 - 0.012 (0.102 - 0.305)
NOTE 1
0.394 - 0.419 (10.007 - 10.643)
1
2
3
4
5
6
7
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sn486 486fas LT/GP 0294 5K REV A * PRINTED IN USA
(c) LINEAR TECHNOLOGY CORPORATION 1994

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