LINER LTC491 Differential driver and receiver pair Datasheet

LTC491
Differential Driver and
Receiver Pair
U
DESCRIPTIO
FEATURES
■
■
■
■
■
■
■
■
■
■
■
Low Power: ICC = 300µA 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 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.
S
Low Power RS485/RS422 Transceiver
Level Translator
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■
The CMOS design offers significant power savings over its
bipolar counterpart without sacrificing ruggedness against
overload or ESD damage.
Both AC and DC specifications are guaranteed from 0°C to
70°C and 4.75V to 5.25V supply voltage range.
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APPLICATI
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.
TYPICAL APPLICATI
DE
DE
4
9
D
5
DRIVER
120Ω
120Ω
10
RECEIVER
R
4000 FT 24 GAUGE TWISTED PAIR
LTC491
LTC491
12
R
2
RECEIVER
120Ω
120Ω
11
DRIVER
D
4000 FT 24 GAUGE TWISTED PAIR
3
REB
REB
LTC491 • TA01
1
LTC491
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RATI GS
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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.................................................. 0°C to 70°C
LTC491I.............................................. – 40°C to 85°C
Storage Temperature Range ................. – 65°C to 150°C
Lead Temperature (Soldering, 10 sec.)................. 300°C
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(Note 1)
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ABSOLUTE
PACKAGE/ORDER I FOR ATIO
TOP VIEW
NC
1
R
2
REB
3
12 A
DE
4
11 B
D
5
GND
6
9
Y
GND
7
8
NC
ORDER PART
NUMBER
14 VCC
R
13 NC
LTC491CN
LTC491CS
LTC491IN
LTC491IS
10 Z
D
S PACKAGE
N PACKAGE
14-LEAD PLASTIC DIP 14-LEAD PLASTIC SOIC
LTC491 • POI01
Consult factory for Military grade parts.
DC ELECTRICAL CHARACTERISTICS
VCC = 5V ±5%
SYMBOL
PARAMETER
CONDITIONS
VOD1
Differential Driver Output Voltage (Unloaded)
IO = 0
●
MIN
VOD2
Differential Driver Output Voltage (With load)
R = 50Ω; (RS422)
●
2
R = 27Ω; (RS485) (Figure 1)
●
1.5
5
V
∆VOD
Change in Magnitude of Driver Differential Output
Voltage for Complementary Output States
R = 27Ω or R = 50Ω (Figure 1)
●
0.2
V
VOC
Driver Common Mode Output Voltage
●
3
V
∆ VOC
Change in Magnitude of Driver Common Mode
Output Voltage for Complementary Output States
●
0.2
V
VIH
Input High Voltage
VIL
Input Low Voltage
●
0.8
V
lIN1
Input Current
●
±2
µA
lIN2
Input Current (A, B)
VCC = 0V or 5.25V
VIN = 12V
●
1.0
mA
VIN = –7V
●
– 0.8
mA
VTH
Differential Input Threshold Voltage for Receiver
– 7V ≤ VCM ≤ 12V
●
– 0.2
0.2
V
∆VTH
Receiver Input Hysteresis
VCM = 0V
●
70
VOH
Receiver Output High Voltage
IO = –4mA, VID = 0.2V
●
3.5
VOL
Receiver Output Low Voltage
●
0.4
V
IOZR
Three-State Output Current at Receiver
IO = 4mA, VID = –0.2V
VCC = Max 0.4V ≤ VO ≤ 2.4V
●
±1
µA
ICC
Supply Current
No Load; D = GND,
Outputs Enabled
●
300
500
µA
or VCC
Outputs Disabled
●
300
500
µA
D, DE, REB
●
TYP
MAX
5
UNITS
V
V
2.0
V
mV
V
RIN
Receiver Input Resistance
– 7V ≤ VCM ≤ 12V
●
IOSD1
Driver Short Circuit Current, VOUT = High
VO = – 7V
●
100
250
mA
IOSD2
Driver Short Circuit Current, VOUT = Low
VO = 12V
●
100
250
mA
IOSR
Receiver Short Circuit Current
0V ≤ VO ≤ VCC
●
85
mA
IOZ
Driver Three-State Output Current
VO = – 7V to 12V
●
±200
µA
2
12
kΩ
7
±2
LTC491
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SWITCHI G CHARACTERISTICS
VCC = 5V ±5%
SYMBOL
PARAMETER
CONDITIONS
tPLH
Driver Input to Output
tPHL
Driver Input to Output
RDIFF = 54Ω, CL1 = CL2 = 100pF
(Figures 2, 5)
MIN
TYP
MAX
●
10
30
50
tSKEW
tr, tf
Driver Output to Output
Driver Rise or Fall Time
tZH
Driver Enable to Output High
CL = 100pF (Figures 4, 6) S2 Closed
tZL
Driver Enable to Output Low
tLZ
tHZ
UNITS
ns
●
10
30
50
ns
●
●
5
5
15
25
ns
ns
●
40
70
ns
CL = 100pF (Figures 4, 6) S1 Closed
●
40
70
ns
Driver Disable Time From Low
CL = 15pF (Figures 4, 6) S1 Closed
●
40
70
ns
Driver Disable Time From High
CL = 15pF (Figures 4, 6) S2 Closed
●
40
70
ns
tPLH
Receiver Input to Output
●
40
70
150
ns
tPHL
Receiver Input to Output
RDIFF = 54Ω, CL1 = CL2 = 100pF
(Figures 2, 7)
●
40
70
150
tSKD
tZL
 tPLH – tPHL Differential Receiver Skew
Receiver Enable to Output Low
tZH
tLZ
tHZ
ns
●
13
CL = 15pF (Figures 3, 8) S1 Closed
●
20
50
ns
Receiver Enable to Output High
CL = 15pF (Figures 3, 8) S2 Closed
●
20
50
ns
Receiver Disable From Low
CL = 15pF (Figures 3, 8) S1 Closed
●
20
50
ns
Receiver Disable From High
CL = 15pF (Figures 3, 8) S2 Closed
●
20
50
ns
The ● 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.
ns
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 = 25°C.
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PI FU CTI
S
NC (Pin 1): Not Connected.
GND (Pin 6): Ground Connection.
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.
GND (Pin 7): Ground Connection.
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.
Y (Pin 9): Driver output.
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.
NC (Pin 8): Not Connected.
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
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TYPICAL PERFOR A CE CHARACTERISTICS
Driver Output High Voltage vs
Output Current TA = 25°C
Driver Differential Output Voltage vs
Output Current TA = 25°C
–72
– 48
–24
OUTPUT CURRENT (mA)
OUTPUT CURRENT (mA)
OUTPUT CURRENT (mA)
80
64
–96
48
32
16
0
1
2
3
0
4
1
2
3
5.0
1.61
4.0
1.59
1.57
3.0
1.0
–50
100
TEMPERATURE (°C )
0
50
330
320
310
–50
100
6.0
TEMPERATURE (°C )
5.0
3.0
–50
100
LTC491 • TPC06
0.8
0.6
0.4
0.2
0
50
100
0
–50
0
50
100
TEMPERATURE (°C )
TEMPERATURE (°C )
LTC491 • TPC07
50
Receiver Output Low Voltage vs
Temperature at I = 8mA
4.0
100
0
TEMPERATURE (°C )
OUTPUT VOLTAGE (V)
2.1
TIME (ns)
DIFFERENTIAL VOLTAGE (V)
7.0
50
340
Receiver  tPLH tPHL vs
Temperature
2.3
4
Supply Current vs Temperature
LTC491 • TPC05
Driver Differential Output Voltage vs
Temperature RO = 54Ω
0
3
LTC491 • TPC03
TEMPERATURE (°C )
LTC491 • TPC04
1.7
2
350
2.0
1.9
1
OUTPUT VOLTAGE (V)
SUPPLY CURRENT (µA)
1.63
1.5
–50
0
4
Driver Skew vs Temperature
TIME (ns)
INPUT THRESHOLD VOLTAGE (V)
TTL Input Threshold vs Temperature
50
20
LTC491 • TPC02
LTC491 • TPC01
0
40
OUTPUT VOLTAGE (V)
OUTPUT VOLTAGE (V)
1.55
–50
60
0
0
0
4
Driver Output Low Voltage vs
Output Current TA = 25°C
LTC491 • TPC08
LTC491 • TPC09
LTC491
TEST CIRCUITS
Y
R
VOD2
R
VOC
Z
LTC491 • TA02
Figure 1. Driver DC Test Load
CL1
Y
D
A
RDIFF
DRIVER
Z
RECEIVER
CL2
B
R
15pF
LTC491 • TA03
Figure 2. Driver/Receiver Timing Test Circuit
S1
RECEIVER
OUTPUT
S1
1kΩ
VCC
CL
VCC
OUTPUT
UNDER TEST
1kΩ
500Ω
CL
S2
LTC491 • TA04
Figure 3. Receiver Timing Test Load
S2
LTC491 • TA05
Figure 4. Driver Timing Test Load
5
LTC491
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SWITCHI G TI E WAVEFOR S
3V
D
f = 1MHz : tr ≤ 10ns : tf ≤ 10ns
1.5V
1.5V
0V
tPHL
tPLH
VO
80%
50%
10%
–VO
90%
VDIFF = V(Y) – V(Z)
tr
50%
20%
tf
Z
VO
Y
tSKEW
1/2 VO
tSKEW
1/2 VO
LTC491 • TA06
Figure 5. Driver Propagation Delays
3V
DE
f = 1MHz : tr ≤ 10ns : tr ≤ 10ns
1.5V
1.5V
0V
tZL
tLZ
5V
A, B
VOL
2.3V
OUTPUT NORMALLY LOW
2.3V
OUTPUT NORMALLY HIGH
0.5V
VOH
A, B
0.5V
0V
tZH
tHZ
LTC491 • TA07
Figure 6. Driver Enable and Disable Times
INPUT
VOD2
A-B
–VOD2
f = 1MHz ; tr ≤ 10ns : tf ≤ 10ns
0V
tPHL
tPLH
VOH
R
0V
OUTPUT
1.5V
1.5V
VOL
LTC491 • TA08
Figure 7. Receiver Propagation Delays
3V
REB
f = 1MHz : tr ≤ 10ns : tf ≤ 10ns
1.5V
1.5V
0V
tZL
tLZ
5V
R
1.5V
VOL
OUTPUT NORMALLY LOW
0.5V
VOH
R
1.5V
0V
tZH
tHZ
Figure 8. Receiver Enable and Disable Times
6
0.5V
OUTPUT NORMALLY HIGH
LTC491 • TA09
LTC491
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APPLICATI
S I FOR ATIO
Typical Application
typically 20kΩ 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.
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 120Ω. The input impedance of a receiver is
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
RX
2
3
RECEIVER
12
120Ω
120Ω
11
11
RECEIVER
4
DX
3
RX
4
10
5
2
DRIVER
10
120Ω
120Ω
9
9
LTC491
9
10
11
DRIVER
5
DX
LTC491
12
RECEIVER
LTC491
DRIVER
5
4
3 2
LTC491 • TA10
DX
RX
Figure 9. Typical Connection
12
RX
2
3
RECEIVER
120Ω
11
DATA IN
4
10
DX
5
DRIVER
120Ω
9
DATA OUT
LTC491
LTC491 • TA11
Figure 10. Line Repeater
7
LTC491
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APPLICATI
S I FOR ATIO
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
150°C and turns them back on when the temperature
cools to 130°C. 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
less flexible, more bulky, and more costly than twisted
pairs. Many cable manufacturers offer a broad range of
120Ω 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.
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
10k
CABLE LENGTH (ft)
LOSS PER 100 ft (dB)
10
1.0
0.1
0.1
1.0
10
100
FREQUENCY (MHZ)
100
10
10k
100k
1M
2.5M
10M
DATA RATE (bps)
LTC491 • TA12
Figure 11. Attenuation vs Frequency for Belden 9481
8
1k
LTC491 • TA13
Figure 12. Cable Length vs Data Rate
LTC491
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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
If the cable is lightly loaded (470Ω), 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 120Ω 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.
Rt
DX
DRIVER
RECEIVER
RX
120Ω
C
Rt = 120Ω
RECEIVER
RX
C = LINE LENGTH (ft) x 16.3pF
LTC491 • TA15
Rt = 47Ω
Figure 14. AC Coupled Termination
Rt = 470Ω
LTC491 • TA14
Figure 13. Termination Effects
If the cable is loaded excessively (47Ω), 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).
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 120Ω 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/(120Ω × C).
9
LTC491
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APPLICATI
S I FOR ATIO
Receiver Open-Circuit Fail-Safe
Fault Protection
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 120Ω, 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.
All of LTC’s RS485 products are protected against ESD
transients up to 2kV using the human body model (100pF,
1.5kΩ). 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
120Ω
Z
+5V
110Ω
130Ω
130Ω
110Ω
LTC491 • TA17
RECEIVER
RX
+5V
1.5kΩ
140Ω
RECEIVER
RX
RECEIVER
RX
1.5kΩ
100kΩ
+5V
C
120Ω
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.
10
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
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TYPICAL APPLICATI
S
RS232 Receiver
RS232 IN
5.6kΩ
RX
RECEIVER
1/2 LTC491
LTC491 • TA18
RS232 to RS485 Level Transistor with Hysteresis
R = 220kΩ
Y
10kΩ
RS232 IN
DRIVER
5.6kΩ
1/2 LTC491
120Ω
Z
19k
VY - VZ 
HYSTERESIS = 10kΩ • ———— ≈ ————
R
R
LTC491 • TA19
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.
11
LTC491
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PACKAGE DESCRIPTIO
Dimensions in inches (millimeters) unless otherwise noted.
N Package
14-Lead Plastic DIP
0.770
(19.558)
MAX
14
13
11
12
10
9
8
0.260 ± 0.010
(6.604 ± 0.254)
1
0.300 – 0.325
(7.620 – 8.255)
3
2
5
4
θJA
100°C
90°C/W
7
6
0.045 – 0.065
(1.143 – 1.651)
0.015
(0.380)
MIN 0.130 ± 0.005
(3.302 ± 0.127)
TJ MAX
0.065
(1.651)
TYP
0.009 – 0.015
(0.229 – 0.381)
+0.025
0.325 –0.015
(
8.255
+0.635
–0.381
)
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
0.228 – 0.244
(5.791 – 6.197)
2
3
4
5
12
6
7
0.004 – 0.010
(0.101 – 0.254)
0° – 8° TYP
Linear Technology Corporation
110°C/W
0.053 – 0.069
(1.346 – 1.752)
0.008 – 0.010
(0.203 – 0.254)
0.016 – 0.050
0.406 – 1.270
θJA
100°C
0.150 – 0.157
(3.810 – 3.988)
1
0.010 – 0.020
× 45°
(0.254 – 0.508)
TJ MAX
0.014 – 0.019
(0.355 – 0.483)
0.050
(1.270)
TYP
SO14 0392
BA/GP 0492 10K REV 0
1630 McCarthy Blvd., Milpitas, CA 95035-7487
(408) 432-1900 ● FAX: (408) 434-0507 ● TELEX: 499-3977
 LINEAR TECHNOLOGY CORPORATION 1992