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LTC1485 Differential Bus Transceiver
FEATURES
s s s
DESCRIPTIO
s s s
s s s
s
s s
ESD Protection over 10kV Low Power: ICC = 1.8mA Typ 28ns Typical Driver Propagation Delays with 4ns Skew 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 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 60mV Typical Input Hysteresis Pin Compatible with the SN75176A, DS75176A, and SN75LBC176
The LTC (R)1485 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 with Schottky 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. I/O pins are protected against multiple ESD strikes of over 10kV. 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 - 40C to 85C and 4.75V to 5.25V supply voltage range.
, LTC and LT are registered trademarks of Linear Technology Corporation.
APPLICATI
s s
S
Low Power RS485/RS422 Transceiver Level Translator
TYPICAL APPLICATI
DE 3
5V 8 LTC1485 6 6 120 7
5V 8 LTC1485
DI
4
DRIVER
120 7
4000 FT 24 GAUGE TWISTED PAIR
RO
1
RECEIVER
2 RE
5
5
U
DE 3 DRIVER 4 DI RECEIVER 1 RO 2 RE
1485 TA01
UO
UO
1
LTC1485 ABSOLUTE
(Note 1)
AXI U
RATI GS
PACKAGE/ORDER I FOR ATIO
TOP VIEW RO 1 RE 2 DE 3 DI 4 D R 8 VCC 7B 6 A
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 LTC1485C............................................... 0C to 70C LTC1485I .......................................... - 40C to 85C Storage Temperature Range ................ - 65C to 150C Lead Temperature (Soldering, 10 sec.)................ 300C
ORDER PART NUMBER LTC1485CN8 LTC1485IN8 LTC1485CS8 LTC1485IS8 S8 PART MARKING 1485 1485I
5 GND
N8 PACKAGE S8 PACKAGE 8-LEAD PLASTIC DIP 8-LEAD PLASTIC SOIC
TJMAX = 125C, JA = 100C/ W (N) TJMAX = 150C, JA = 150C/ W (S)
Consult factory for Military grade parts.
DC ELECTRICAL CHARACTERISTICS
VCC = 5V (Notes 2, 3), unless otherwise noted.
SYMBOL VOD1 VOD2 VOD VOC | VOC | VINH VINL IIN1 IIN2 VTH VTH VOH VOL IOZR ICC 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 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) DI, DE, RE DI, DE, RE DI, DE, RE VCC = 0V or 5.25V, VIN = 12V VCC = 0V or 5.25V, VIN = - 7V - 7V VCM 12V VCM = 0V IO = - 4mA, VID = 0.2V IO = 4mA, VID = - 0.2V VCC = Max 0.4V VO 2.4V No Load; DI = GND or VCC Outputs Enabled Outputs Disabled - 7V VCM 12V VO = - 7V VO = 10 V 0V VO VCC MIN
q q q q q q q q q q q q q q q q q q q q q q
TYP 5
MAX V 5 0.2 3 0.2
UNITS V V V V V V V A mA mA V mV V V A mA mA k mA mA mA
2 1.5
2.0 0.8 2 1.0 - 0.8 0.2 60 3.5 0.4 1 1.8 1.7 12 250 250 85 2.3 2.3
- 0.2
RIN IOSD1 IOSD2 IOSR
Receiver Input Resistance Driver Short-Circuit Current, VOUT = High Driver Short-Circuit Current, VOUT = Low Receiver Short-Circuit Current
7
2
U
W
U
U
WW
W
LTC1485
SWITCHI G CHARACTERISTICS
VCC = 5V (Notes 2, 3), unless otherwise noted.
SYMBOL tPLH tPHL tSKEW t r, t f t ZH t ZL t LZ t HZ t PLH t PHL t SKEW t ZL t ZH t LZ t HZ 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 | t PLH - t PHL | Differential Receiver Skew Receiver Enable to Output Low Receiver Enable to Output High Receiver Disable from Low Receiver Disable from High CONDITIONS RDIFF = 54, CL1 = CL2 = 100pF (Figures 2, 5) RDIFF = 54, CL1 = CL2 = 100pF (Figures 2, 5) RDIFF = 54, CL1 = CL2 = 100pF (Figures 2, 5) RDIFF = 54, CL1 = CL2 = 100pF (Figures 2, 5) 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) RDIFF = 54, CL1 = CL2 = 100pF (Figures 2, 7) 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
q q q q q q q q q q q q q q q
The q denotes specifications which apply over the operating temperature range. Note 1: Absolute Maximum Ratings are those values beyond which the safety of the device cannot be guaranteed.
TYPICAL PERFOR A CE CHARACTERISTICS
Receiver Output Low Voltage vs Output Current
36 32
OUTPUT CURRENT (mA)
TA = 25C
OUTPUT CURRENT (mA)
28 24 20 16 12 8 4 0 0 0.5 1.5 1.0 OUTPUT VOLTAGE (V) 2.0
1485 G01
-12 -10 -8 -6 -4 -2 0 5 4 3 OUTPUT VOLTAGE (V)
1485 G02
OUTPUT VOLTAGE (V)
UW
U
MIN 10 10
TYP 30 30 4
MAX 50 50 10 25 70 70 70 70 50 55 15 45 45 45 45
UNITS ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns
5
15 40 40 40 40 25 30 5 30 30 30 30
15 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 TA = 25C.
Receiver Output High Voltage vs Output Current
-18 -16 -14 TA = 25C 4.8 4.6 4.4 4.2 4.0 3.8 3.6 3.4 3.2 2
Receiver Output High Voltage vs Temperature
I = 8mA
3.0 -50
-25
0 75 50 25 TEMPERATURE (C)
100
125
1485 G03
3
LTC1485
TYPICAL PERFOR A CE CHARACTERISTICS
Receiver Output Low Voltage vs Temperature
0.9 0.8 0.7 I = 8mA
OUTPUT CURRENT (mA)
0.6 0.5 0.4 0.3 0.2 0.1 0 -50
48
DIFFERENTIAL VOLTAGE (V)
OUTPUT VOLTAGE (V)
-25
0 75 50 25 TEMPERATURE (C)
Driver Output Low Voltage vs Output Current
80 TA = 25C
OUTPUT CURRENT (mA)
60
-72
INPUT THRESHOLD VOLTAGE (V)
OUTPUT CURRENT (mA)
40
20
0 0 1 3 2 OUTPUT VOLTAGE (V) 4
1485 G07
Receiver | tPLH - tPHL | vs Temperature
5 5
4
4
SUPPLY CURRENT (mA)
TIME (ns)
3
TIME (ns)
2
1 -50
-25
0 75 25 50 TEMPERATURE (C)
4
UW
100
1485 G04
Driver Differential Output Voltage vs Output Current
64 TA = 25C 2.4
Driver Differential Output Voltage vs Temperature
RL =54
2.2
32
2.0
16
1.8
0
125
0
1
3 2 OUTPUT VOLTAGE (V)
4
1485 G05
1.6 -50
-25
0 75 25 50 TEMPERATURE (C)
100
125
1485 G06
Driver Output High Voltage vs Output Current
-96 TA = 25C
1.63
TTL Input Threshold vs Temperature
1.61
-48
1.59
-24
1.57
0 0 1 3 2 OUTPUT VOLTAGE (V) 4
1485 G08
1.55 -50
-25
0 75 25 50 TEMPERATURE (C)
100
125
1485 G09
Driver Skew vs Temperature
1.8
Supply Current vs Temperature
DRIVER ENABLED
1.7
3
1.6 DRIVER DISABLED 1.5
2
100
125
1 -50
-25
0 75 25 50 TEMPERATURE (C)
100
125
1.4 -50
-25
0 75 25 50 TEMPERATURE (C)
100
125
1485 G10
1485 G11
1485 G12
LTC1485
PI FU CTIO S
RO (Pin 1): Receiver Output. If the receiver output is enabled (RE low), then if A > B by 200mV, RO will be high. If A < B by 200mV, then RO will be low. RE (Pin 2): Receiver Output Enable. A low enables the receiver output, RO. A high input forces the receiver output into a high impedance state. DE (Pin 3): Driver Output Enable. A high on DE enables the driver outputs, A and B. A low input will force the driver outputs into a high impedance state. DI (Pin 4): Driver Input. If the driver outputs are enabled (DE high), then a low on DI forces the driver outputs A low and B high. A high on DI will force A high and B low. GND (Pin 5): Ground Connection. A (Pin 6): Driver Output/Receiver Input. B (Pin 7): Driver Output/Receiver Input. VCC (Pin 8): Positive Supply. 4.75V VCC 5.25V.
TEST CIRCUITS
A R VOD2 R B VOC
Figure 1. Driver DC Test Load
RECEIVER OUTPUT CL 1k S2
Figure 3. Receiver Timing Test Load
U
U
U
A DI DRIVER B RDIFF
CL1
A RECEIVER RO 15pF
CL2
B
1485 F02
1485 F01
Figure 2. Driver/Receiver Timing Test Circuit
S1
1k VCC
OUTPUT UNDER TEST 500 CL
1485 F03
S1 VCC
S2
1485 F04
Figure 4. Driver Timing Test Load
5
LTC1485
SWITCHI G TI E WAVEFOR S
3V DI 0V 1.5V tPLH f = 1MHz; tr 10ns; t f 10ns 1.5V tPHL
VO V A - VB -VO 10% tr B VO A 50%
90%
1/2 VO tSKEW
Figure 5. Driver Propagation Delays
3V DE 0V tZL 5V A,B VOL 2.3V 1.5V
VOH A,B 0V tZH tHZ 2.3V OUTPUT NORMALLY HIGH
Figure 6. Driver Enable and Disable Times
VOD2 VA - VB -VOD2 0V tPLH
VOH RO VOL 1.5V
Figure 7. Receiver Propagation Delays
6
W
W
U
90% 50% 10% tf
1/2 VO tSKEW
1485 F05
f = 1MHz; tr 10ns; t f 10ns
1.5V
tLZ
OUTPUT NORMALLY LOW
0.5V
0.5V
1485 F06
INPUT f = 1MHz; tr 10ns; t f 10ns 0V tPHL OUTPUT 1.5V
1485 F07
LTC1485
SWITCHI G TI E WAVEFOR S
3V RE 0V 1.5V tZL f = 1MHz; tr 10ns; t f 10ns 1.5V tLZ
5V RO VOL 1.5V OUTPUT NORMALLY LOW 0.5V
VOH RO 0V tZH tHZ 1.5V OUTPUT NORMALLY HIGH
Figure 8. Receiver Enable and Disable Times
APPLICATI
S I FOR ATIO
Typical Application A typical connection of the LTC1485 is shown in Figure 9. Two twisted pair wires connect up to 32 driver/receiver pairs 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
LTC1485 1 2 3
RX
RECEIVER
7 DX 4 DRIVER 120 8 LTC1485 1 2 3 7 DRIVER 8 4 DX RX 120 DRIVER 4 DX
Figure 9. Typical Connection
U
W
W
U
W
UO
U
0.5V
1485 F08
ends with a resistor equal to their characteristic impedance, typically 120. The input impedance of a receiver is typically 20k to GND, or 0.6 unit RS485 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.
LTC1485 1 2 3 RX
RECEIVER
1485 F09
RECEIVER
7
LTC1485
APPLICATI
S I FOR ATIO
Thermal Shutdown The LTC1485 has a thermal shutdown feature which protects the part from excessive power dissipation. If the outputs of the driver are accidentally 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 LTC1485 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 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 10). When using low loss cables, Figure 11 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, 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
CABLE LENGTH (FT)
LOSS PER 100 FT (dB)
8
U
10 1 0.1 0.1 1 10 FREQUENCY (MHz) 100
1485 F10
W
U
UO
Figure 10. Attenuation vs Frequency for Belden 9481
10k
1k
100
10 10k
100k 1M DATA RATE (bps)
2.5M
10M
1485 F11
Figure 11. Cable Length vs Data Rate
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 12). 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 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.
LTC1485
APPLICATI
DX
S I FOR ATIO
Rt RECEIVER
PROBE HERE
DRIVER
Rt = 120
Rt = 47
Rt = 470
1485 F12
Figure 12. Termination Effects
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 10 times greater than the supply current of the LTC1485. One way to eliminate the unwanted current is by AC-coupling the termination resistors as shown in Figure 13.
120 C
RECEIVER
1485 F13
C = LINE LENGTH (FT) * 16.3pF
Figure 13. 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
U
RX
W
U
UO
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 400 feet in length. Be aware that the power savings start to decrease once the data rate surpasses 1/(120 * 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. The receiver of the LTC1485 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). If the receiver output must be forced to a known state, the circuits of Figure 14 can be used.
5V
110
130 RECEIVER RX
130
110
5V 1.5k
120
RECEIVER
RX
RX
1.5k
5V 100k
C 120 RECEIVER RX
1485 F14
Figure 14. Forcing "0" When All Drivers Are Off
9
LTC1485
APPLICATI
S I FOR ATIO
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. 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 15). A TransZorb is a silicon transient voltage suppressor that has exceptional surge handling capabilities: fast response
TYPICAL APPLICATI
S
RS232 Receiver
RS232 IN 5.6k RECEIVER RX
RS232 to RS485 Level Translator with Hysteresis
220k A RS232 IN 5.6k 10k DRIVER B 120
1485 TA03
HYSTERESIS = 10k * VA - VB /R 19 (k * VOLT)/R
10
U
A DRIVER B 120
1485 F15
W
UO
U
UO
Figure 15. ESD Protection with TransZorbs
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.
TransZorb is a registered trademark of General Instruments, GSI
1485 TA02
LTC1485
PACKAGE DESCRIPTIO
0.300 - 0.325 (7.620 - 8.255)
0.009 - 0.015 (0.229 - 0.381)
(
+0.025 0.325 -0.015 +0.635 8.255 -0.381
)
*THESE DIMENSIONS DO NOT INCLUDE MOLD FLASH OR PROTRUSIONS. MOLD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.010 INCH (0.254mm).
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 circuits as described herein will not infringe on existing patent rights.
U
Dimensions in inches (millimeters) unless otherwise noted.
N8 Package 8-Lead Plastic DIP
0.400* (10.160) MAX 8 7 6 5
0.255 0.015* (6.477 0.381)
1
2
3
4
0.045 - 0.065 (1.143 - 1.651)
0.130 0.005 (3.302 0.127)
0.065 (1.651) TYP 0.125 (3.175) MIN 0.018 0.003 (0.457 0.076) 0.015 (0.380) MIN
0.045 0.015 (1.143 0.381) 0.100 0.010 (2.540 0.254)
N8 0694
11
LTC1485
PACKAGE DESCRIPTIO U
Dimensions in inches (millimeters) unless otherwise noted.
S8 Package 8-Lead Plastic SOIC
0.189 - 0.197* (4.801 - 5.004) 8 7 6 5
0.228 - 0.244 (5.791 - 6.197)
0.150 - 0.157* (3.810 - 3.988)
1 0.010 - 0.020 x 45 (0.254 - 0.508) 0.008 - 0.010 (0.203 - 0.254) 0- 8 TYP
2
3
4
0.053 - 0.069 (1.346 - 1.752)
0.004 - 0.010 (0.101 - 0.254)
0.016 - 0.050 0.406 - 1.270
0.014 - 0.019 (0.355 - 0.483)
0.050 (1.270) BSC
SO8 0294
*THESE DIMENSIONS DO NOT INCLUDE MOLD FLASH OR PROTRUSIONS. MOLD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.006 INCH (0.15mm).
RELATED PARTS
PART NUMBER LTC486 LTC488 LTC490 LTC1481 DESCRIPTION Quad RS485 Driver Quad RS485 Receiver Full Duplex RS485 Transceiver Ultra-Low Power Half Duplex RS485 Transceiver COMMENTS Fits 75172 Pinout, Only 110A IQ Fits 75173 Pinout, Only 7mA IQ Fits 75179 Pinout, Only 300A IQ Fits 75176 Pinout, 80A IQ
12
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7487
(408) 432-1900 q FAX: (408) 434-0507 q TELEX: 499-3977
LT/GP 0795 2K REV A * PRINTED IN THE USA
(c) LINEAR TECHNOLOGY CORPORATION 1995

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