LTC491
Differential Driver and
Receiver Pair
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FEATURES
■
■
■
■
■
■
■
■
■
■
■
■
DESCRIPTIO
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 for 2.5MB Operation
Pin Compatible with the SN75180
Available in 14-Lead PDIP and SO Packages
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APPLICATIO S
■
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 0°C to
70°C and 4.75V to 5.25V supply voltage range.
, LTC and LT are registered trademarks of Linear Technology Corporation.
Low Power RS485/RS422 Transceiver
Level Translator
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■
The LTC®491 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 APPLICATIO
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
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LTC491
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RATI GS
W W
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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
ORDER PART
NUMBER
TOP VIEW
NC
1
14 VCC
R
13 NC
R
2
REB
3
12 A
DE
4
11 B
D
5
GND
6
9
Y
GND
7
8
NC
LTC491CN
LTC491CS
LTC491IN
LTC491IS
10 Z
D
N PACKAGE
14-LEAD PDIP
S PACKAGE
14-LEAD PLASTIC SO
TJMAX = 100°C, θJA = 90°C/W (N)
TJMAX = 100°C, θJA = 110°C/W (S)
Consult LTC Marketing for parts specified with wider operating temperature ranges.
DC ELECTRICAL CHARACTERISTICS
The ● denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VCC = 5V ±5%
SYMBOL
PARAMETER
CONDITIONS
MIN
VOD1
Differential Driver Output Voltage (Unloaded)
IO = 0
●
VOD2
Differential Driver Output Voltage (With load)
R = 50Ω; (RS422)
●
2
R = 27Ω; (RS485) (Figure 1)
●
1.5
R = 27Ω or R = 50Ω (Figure 1)
TYP
MAX
5
UNITS
V
V
5
V
●
0.2
V
∆VOD
Change in Magnitude of Driver Differential Output
Voltage for Complementary Output States
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
lIN1
Input Current
lIN2
Input Current (A, B)
D, DE, REB
●
2.0
V
0.8
●
VCC = 0V or 5.25V
V
●
±2
µA
VIN = 12V
●
1.0
mA
VIN = –7V
●
– 0.8
mA
VTH
Differential Input Threshold Voltage for Receiver
– 7V ≤ VCM ≤12V
●
∆VTH
Receiver Input Hysteresis
VCM = 0V
●
VOH
Receiver Output High Voltage
IO = –4mA, VID = 0.2V
●
VOL
Receiver Output Low Voltage
IO = 4mA, VID = –0.2V
●
0.4
V
IOZR
Three-State Output Current at Receiver
VCC = Max 0.4V ≤ VO ≤ 2.4V
●
±1
µA
ICC
Supply Current
No Load; D = GND,
or VCC
Outputs Enabled
●
300
500
µA
Outputs Disabled
●
300
500
RIN
Receiver Input Resistance
– 7V ≤ VCM ≤ 12V
●
IOSD1
Driver Short Circuit Current, VOUT = High
VO = – 7V
●
IOSD2
Driver Short Circuit Current, VOUT = Low
VO = 12V
●
IOSR
Receiver Short Circuit Current
0V ≤ VO ≤ VCC
●
IOZ
Driver Three-State Output Current
VO = – 7V to 12V
●
– 0.2
0.2
70
V
mV
3.5
V
12
µA
kΩ
100
250
mA
100
250
mA
85
mA
±200
µA
7
±2
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LTC491
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SWITCHI G CHARACTERISTICS
The ● denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VCC = 5V ±5%
SYMBOL
PARAMETER
CONDITIONS
tPLH
Driver Input to Output
RDIFF = 54Ω, CL1 = CL2 = 100pF
(Figures 2, 5)
tPHL
Driver Input to Output
tSKEW
tr, tf
Driver Output to Output
Driver Rise or Fall Time
tZH
Driver Enable to Output High
tZL
tLZ
MIN
TYP
MAX
●
10
30
50
ns
●
10
30
50
ns
5
5
15
25
ns
ns
●
●
UNITS
CL = 100pF (Figures 4, 6) S2 Closed
●
40
70
ns
Driver Enable to Output Low
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
tHZ
Driver Disable Time From High
CL = 15pF (Figures 4, 6) S2 Closed
●
40
70
ns
tPLH
Receiver Input to Output
40
70
150
ns
Receiver Input to Output
RDIFF = 54Ω, CL1 = CL2 = 100pF
(Figures 2, 7)
●
tPHL
●
40
70
150
ns
tSKD
tPLH – tPHL Differential Receiver Skew
tZL
Receiver Enable to Output Low
tZH
Receiver Enable to Output High
tLZ
tHZ
●
13
CL = 15pF (Figures 3, 8) S1 Closed
●
20
50
ns
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
Note 1: Absolute Maximum Ratings are those values beyond which the life
of the device may be impaired.
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.
ns
Note 3: All typicals are given for VCC = 5V and temperature = 25°C.
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PI FU CTIO 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.
DE (Pin 4): Driver Output Enable. A high on DE enables the
driver outputs, Y and Z. 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 Y low
and Z high. A high on D will force Y high and Z low.
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.
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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
OUTPUT CURRENT (mA)
OUTPUT CURRENT (mA)
–72
– 48
–24
80
OUTPUT CURRENT (mA)
64
–96
48
32
16
0
0
0
1
2
3
1
2
3
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
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)
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)
1.61
TIME (ns)
INPUT THRESHOLD VOLTAGE (V)
5.0
1.5
–50
0
4
Driver Skew vs Temperature
1.63
50
20
LTC491 • TPC02
TTL Input Threshold vs Temperature
0
40
OUTPUT VOLTAGE (V)
LTC491 • TPC01
1.55
–50
60
0
0
4
OUTPUT VOLTAGE (V)
DIFFERENTIAL VOLTAGE (V)
Driver Output Low Voltage
vs Output Current, TA = 25°C
0
–50
0
50
100
TEMPERATURE (°C )
LTC491 • TPC08
LTC491 • TPC09
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LTC491
TEST CIRCUITS
Y
R
VOD2
R
VOC
Z
LTC491 • F01
Figure 1. Driver DC Test Load
CL1
Y
D
A
RDIFF
DRIVER
RECEIVER
Z
CL2
B
R
15pF
LTC491 • F02
Figure 2. Driver/Receiver Timing Test Circuit
S1
RECEIVER
OUTPUT
1k
VCC
CL
1k
S2
LTC491 • F03
Figure 3. Receiver Timing Test Load
S1
VCC
OUTPUT
UNDER TEST
500Ω
CL
S2
LTC491 • F04
Figure 4. Driver Timing Test Load
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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 • F05
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
LTC491 • F06
tHZ
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 • F07
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
0.5V
OUTPUT NORMALLY HIGH
tZH
tHZ
LTC491 • F08
Figure 8. Receiver Enable and Disable Times
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LTC491
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APPLICATIO S I FOR ATIO
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 120Ω. The input impedance of a receiver is
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.
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.
12
2
RX
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.
3
120Ω
RECEIVER
DATA IN
11
4
10
5
DX
Thermal Shutdown
120Ω
DRIVER
DATA OUT
9
LTC491
LTC491 • F10
The LTC491 has a thermal shutdown feature which protects the part from excessive power dissipation. If the
Figure 10. Line Repeater
12
12
RX
2
3
RECEIVER
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
LTC491
RECEIVER
DRIVER
LTC491 • F09
5
DX
4
3
2
RX
Figure 9. Typical Connection
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LTC491
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APPLICATI
S I FOR ATIO
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
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).
LOSS PER 100 ft (dB)
10
1.0
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.
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).
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.
0.1
0.1
1.0
10
100
PROBE HERE
FREQUENCY (MHZ)
LTC491 • F11
Figure 11. Attenuation vs Frequency for Belden 9481
Rt
DX
DRIVER
RECEIVER
RX
10k
CABLE LENGTH (ft)
Rt = 120Ω
1k
Rt = 47Ω
100
Rt = 470Ω
10
10k
100k
1M
2.5M
10M
DATA RATE (bps)
LTC491 • F13
LTC491 • F12
Figure 12. Cable Length vs Data Rate
Figure 13. Termination Effects
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LTC491
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APPLICATI
S I FOR ATIO
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 (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.
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).
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
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 tend to maintain the last data bit received, but this is not guaranteed.
The termination resistors are used to generate a DC bias
which forces the receiver output to a known state; in the
case of Figure 15, 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.
5V
110Ω
130Ω
130Ω
110Ω
RECEIVER
RX
RECEIVER
RX
RECEIVER
RX
5V
1.5k
140Ω
1.5kΩ
100kΩ
120Ω
C
5V
C
RECEIVER
120Ω
RX
C = LINE LENGTH (ft) x 16.3pF
LTC491 • F14
Figure 14. AC Coupled Termination
LTC491 • F15
Figure 15. Forcing “O” When All Drivers are Off
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LTC491
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APPLICATI
S I FOR ATIO
Fault Protection
Y
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).
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
120Ω
DRIVER
Z
LTC491 • F16
Figure 16. ESD Protection with TransZorbs
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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PACKAGE DESCRIPTIO
N Package
14-Lead PDIP (Narrow .300 Inch)
(Reference LTC DWG # 05-08-1510)
.770*
(19.558)
MAX
14
13
12
11
10
9
8
1
2
3
4
5
6
7
.255 ± .015*
(6.477 ± 0.381)
.130 ± .005
(3.302 ± 0.127)
.300 – .325
(7.620 – 8.255)
.045 – .065
(1.143 – 1.651)
.020
(0.508)
MIN
.065
(1.651)
TYP
.008 – .015
(0.203 – 0.381)
(
+.035
.325 –.015
+0.889
8.255
–0.381
NOTE:
1. DIMENSIONS ARE
)
.120
(3.048)
MIN
.005
(0.125) .100
MIN (2.54)
BSC
INCHES
MILLIMETERS
*THESE DIMENSIONS DO NOT INCLUDE MOLD FLASH OR PROTRUSIONS.
MOLD FLASH OR PROTRUSIONS SHALL NOT EXCEED .010 INCH (0.254mm)
.018 ± .003
(0.457 ± 0.076)
N14 1002
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LTC491
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PACKAGE DESCRIPTIO
S Package
14-Lead Plastic Small Outline (Narrow .150 Inch)
(Reference LTC DWG # 05-08-1610)
.337 – .344
(8.560 – 8.738)
NOTE 3
.045 ±.005
.050 BSC
14
N
12
11
10
9
8
N
.245
MIN
.160 ±.005
.150 – .157
(3.810 – 3.988)
NOTE 3
.228 – .244
(5.791 – 6.197)
1
.030 ±.005
TYP
13
2
3
N/2
N/2
RECOMMENDED SOLDER PAD LAYOUT
1
.010 – .020
× 45°
(0.254 – 0.508)
.008 – .010
(0.203 – 0.254)
2
3
4
5
6
.053 – .069
(1.346 – 1.752)
.004 – .010
(0.101 – 0.254)
0° – 8° TYP
.016 – .050
(0.406 – 1.270)
NOTE:
1. DIMENSIONS IN
.014 – .019
(0.355 – 0.483)
TYP
7
.050
(1.270)
BSC
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)
S14 0502
491fa
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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LTC491
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TYPICAL APPLICATIO S
RS232 Receiver
RS232 to RS485 Level Transistor with Hysteresis
R = 220k
RS232 IN
5.6k
Y
RX
RECEIVER
10k
1/2 LTC491
LTC491 • TA02
RS232 IN
120Ω
DRIVER
5.6k
1/2 LTC491
Z
VY - VZ
19k
HYSTERESIS = 10kΩ • ———— ≈ ————
R
R
LTC491 • TA03
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PART NUMBER
DESCRIPTION
COMMENTS
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LT1791/LTC1791A
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±15kV ESD, Fail-Safe (LT1791A), LTC491 Pin Compatible
491fa
12
Linear Technology Corporation
LT/TP 0104 1K REV A • PRINTED IN USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 ● FAX: (408) 434-0507
●
www.linear.com
LINEAR TECHNOLOGY CORPORATION 1992
工商网监
湘ICP备2023018690号
