LINER LTC487CS

LTC487
Quad Low Power
RS485 Driver
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DESCRIPTIO
FEATURES
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The LTC487® is a low power differential bus/line driver
designed for multipoint data transmission standard RS485
applications with extended common-mode range (– 7V to
12V). It also meets RS422 requirements.
Very Low Power: ICC = 110µA 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 SN75174, DS96174,
µA96174, and DS96F174
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 0°C to
70°C (Commercial), – 40°C to 85°C (Industrial) and over
the 4.75V to 5.25V supply voltage range.
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APPLICATI
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Low Power RS485/RS422 Drivers
Level Translator
and LTC are registered trademarks and LT is a trademark of Linear Technology Corporation.
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TYPICAL APPLICATI
RS485 Cable Length Specification*
10k
EN 12
4
4
2
DI
1
2
120Ω
DRIVER
3
120Ω
4000 FT BELDEN 9841
RECEIVER
1
3
RO
1/4 LTC489
CABLE LENGTH (FT)
EN 12
1k
100
1/4 LTC487
LTC487 • TA01
10
10k
100k
1M
2.5M
10M
DATA RATE (bps)
LTC487 • TA09
* APPLIES FOR 24 GAUGE, POLYETHYLENE
DIELECTRIC TWISTED PAIR
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LTC487
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ABSOLUTE
PACKAGE/ORDER I FOR ATIO
RATI GS
(Note 1)
TOP VIEW
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
Commercial ............................................ 0°C to 70°C
Industrial ........................................... – 40°C to 85°C
Storage Temperature Range ................. – 65°C to 150°C
Lead Temperature (Soldering, 10 sec.)................. 300°C
ORDER PART
NUMBER
DI1
1
16 VCC
DO1A
2
15 DI4
DO1B
3
14 DO4A
EN12
4
13 DO4B
DO2B
5
12 EN34
DO2A
6
11 DO3B
DI2
7
10 DO3A
GND
8
9
LTC487CN
LTC487CS
LTC487IN
LTC487IS
DI3
N PACKAGE
S PACKAGE
16-LEAD PLASTIC DIP 16-LEAD PLASTIC SOL
TJMAX = 125°C, θJA = 70°C/W (N)
TJMAX = 150°C, θJA = 95°C/W (S)
Consult factory for Military grade parts.
DC ELECTRICAL CHARACTERISTICS
VCC = 5V ± 5%, 0°C ≤ TA ≤ 70°C (Commercial), – 40°C ≤ TA ≤ 85°C (Industrial) (Note 2, 3)
SYMBOL
PARAMETER
CONDITIONS
VOD1
Differential Driver Output Voltage (Unloaded)
IO = 0
VOD2
Differential Driver Output Voltage (With Load)
R = 50Ω; (RS422)
R = 27Ω; (RS485) (Figure 3)
MIN
TYP
UNITS
5
V
5
V
0.2
V
3
V
0.2
V
2
V
1.5
VOD
Change in Magnitude of Driver Differential
Output Voltage for Complementary Output States
R = 27Ω or R = 50Ω
(Figure 3)
VOC
Driver Common-Mode Output Voltage
VOC 
Change in Magnitude of Driver Common-Mode
Output Voltage for Complementary Output States
VIH
Input High Voltage
VIL
Input Low Voltage
IIN1
Input Current
ICC
Supply Current
No Load
IOSD1
Driver Short-Circuit Current, VOUT = High
VO = – 7V
IOSD2
Driver Short-Circuit Current, VOUT = Low
VO = 12V
IOZ
High Impedance State Output Current
VO = – 7V to 12V
TYP
DI, EN12, EN34
MAX
2.0
V
0.8
V
±2
µA
Output Enabled
110
200
µA
Output Disabled
110
200
µA
100
250
mA
100
250
mA
±10
± 200
µA
MAX
UNITS
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SWITCHI G CHARACTERISTICS VCC = 5V ± 5%, 0°C ≤ TA ≤ 70°C (Note 2, 3)
SYMBOL
PARAMETER
CONDITIONS
MIN
tPLH
Driver Input to Output
30
50
ns
Driver Input to Output
RDIFF = 54Ω, CL1 = CL2 = 100pF
(Figures 1, 4)
10
tPHL
10
30
50
ns
tSKEW
Driver Output to Output
5
15
ns
tr, tf
Driver Rise or Fall Time
5
20
25
ns
tZH
Driver Enable to Output High
CL = 100pF (Figures 2, 5) S2 Closed
35
70
ns
tZL
Driver Enable to Output Low
CL = 100pF (Figures 2, 5) S1 Closed
35
70
ns
tLZ
Driver Disable Time from Low
CL = 15pF (Figures 2, 5) S1 Closed
35
70
ns
tHZ
Driver Disable Time from High
CL = 15pF (Figures 2, 5) S2 Closed
35
70
ns
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
2
pins are negative. All voltages are referenced to device GND unless
otherwise specified.
Note 3: All typicals are given for VCC = 5V and Temperature = 25°C.
LTC487
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TYPICAL PERFOR A CE CHARACTERISTICS
Driver Output High Voltage
vs Output Current
TA = 25°C
– 48
–24
48
32
16
0
0
1
2
3
OUTPUT VOLTAGE (V)
4.0
TIME (ns)
1.61
1.57
2
3
0
50
TEMPERATURE (°C )
20
0
4
2
3
4
LTC487 • TPC03
Supply Current vs Temperature
130
3.0
1.0
–50
100
1
OUTPUT VOLTAGE (V)
LTC487• TPC02
2.0
0
50
100
TEMPERATURE (°C )
LTC487 • TPC04
LTC487 • TPC05
120
110
100
90
–50
0
50
TEMPERATURE (°C )
100
LTC487 • TPC06
Driver Differential Output
Voltage vs Temperature
RO = 54Ω
2.3
DIFFERENTIAL VOLTAGE (V)
INPUT THRESHOLD VOLTAGE (V)
5.0
1.55
–50
40
Driver Skew vs Temperature
1.63
1.59
1
OUTPUT VOLTAGE (V)
LTC487 • TPC01
TTL Input Threshold vs Temperature
60
0
0
4
SUPPLY CURRENT (µA)
0
TA = 25°C
80
OUTPUT CURRENT (mA)
–72
Driver Output Low Voltage
vs Output Current
TA = 25°C
64
OUTPUT CURRENT (mA)
–96
OUTPUT CURRENT (mA)
Driver Differential Output Voltage
vs Output Current
2.1
1.9
1.7
1.5
–50
0
50
TEMPERATURE (°C )
100
LTC487 • TPC07
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LTC487
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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.
GND (Pin 8): GND connection.
DO1A (Pin 2): Driver 1 output.
DO3B (Pin 11): Driver 3 output.
DO1B (Pin 3): Driver 1 output.
EN34 (Pin 12): Driver 3 and 4 outputs enabled. See
Function Table for details.
EN12 (Pin 4): Driver 1 and 2 outputs enabled. See Function Table for details.
DI3 (Pin 9): Driver 3 input. Refer to DI1.
DO3A (Pin 10): Driver 3 output.
DO4B (Pin 13): Driver 4 output.
DO2B (Pin 5): Driver 2 output.
DO4A (Pin 14): Driver 4 output.
DO2A (Pin 6): Driver 2 output.
DI4 (Pin 15): Driver 4 input. Refer to DI1.
DI2 (Pin 7): Driver 2 input. Refer to DI1.
VCC (Pin 16): Positive supply; 4.75 < VCC < 5.25.
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TABLE
INPUT
ENABLES
OUTPUTS
DI
EN12 or EN34
OUT A
OUT B
H
L
X
H
H
L
H
L
Z
L
H
Z
H: High Level
L: Low Level
X: Irrelevant
Z: High Impedance (Off)
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SWITCHI G TI E WAVEFOR S
3V
DI
f = 1MHz : t r < 10ns : t f < 10ns
1.5V
1.5V
0V
t PLH
t PHL
B
VO
A
VO
–VO
t SKEW
1/2 VO
80%
90%
VDIFF = V(A) – V(B)
10%
tr
20%
tf
Figure 1. Driver Propagation Delays
3V
EN12
t SKEW
1/2 VO
f = 1MHz : t r ≤ 10ns : t f ≤ 10ns
1.5V
LTC487 • TA05
1.5V
0V
t ZL
5V
A, B
VOL
VOH
A, B
t LZ
2.3V
OUTPUT NORMALLY LOW
2.3V
OUTPUT NORMALLY HIGH
0.5V
0.5V
0V
tHZ
tZH
Figure 2. Driver Enable and Disable Times
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LTC487 • TA06
LTC487
TEST CIRCUITS
EN12
CL1
A
A
R
DI
S1
VCC
DRIVER 1
RDIFF
VOD
OUTPUT
UNDER TEST
B
R
VOC
500Ω
CL2
B
CL
S2
LTC487 • TA03
LTC487 • TA02
LTC487 • TA04
Figure 4. Driver Timing Test Circuit
Figure 3. Driver DC Test Load
Figure 5. Driver Timing Test Load #2
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APPLICATI
Typical Application
A typical connection of the LTC487 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 120Ω. The optional shields around
the twisted pair help reduce unwanted noise, and are
connected to GND at one end.
Thermal Shutdown
The LTC487 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 LTC487
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
120Ω cables designed for RS485 applications.
EN12
EN12
DX
1
SHIELD
SHIELD
4
3
4
2
120Ω
DX
120Ω
2
RX
3
RX
1
1/4 LTC487
1/4 LTC489
EN12
EN12
4
DX
1
3
4
1
DX
RX
2
1/4 LTC487
3
RX
2
1/4 LTC489
LTC487 • TA07
Figure 6. Typical Connection
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LTC487
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APPLICATI
S I FOR ATIO
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).
LOSS PER 100 FT (dB)
10
1.0
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).
PROBE HERE
Rt
DX
DRIVER
RECEIVER
RX
0.1
0.1
1.0
10
100
FREQUENCY (MHz)
LTC487 • TA08
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.
Rt = 120Ω
Rt = 47Ω
Rt = 470Ω
10k
LTC487 • TA10
CABLE LENGTH (FT)
Figure 9. Termination Effects
1k
100
10
10k
100k
1M
2.5M
10M
DATA RATE (bps)
LTC487 • TA09
Figure 8. Cable Length vs Data Rate
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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).
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.
LTC487
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APPLICATI
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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 220 times greater than
the supply current of the LTC487. One way to eliminate the
unwanted current is by AC coupling the termination resistors as shown in Figure 10.
has about 70mV of hysteresis, the receiver output will
maintain the last data bit received.
If the receiver output must be forced to a known state, the
circuits of Figure 11 can be used.
5V
110Ω
130Ω
130Ω
110Ω
RECEIVER
RX
RECEIVER
RX
RECEIVER
RX
5V
120Ω
1.5k
C
RECEIVER
RX
140Ω
C = LINE LENGTH (FT) x 16.3pF
1.5k
LTC487 • TA11
Figure 10. 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
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. 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 120Ω, the differential inputs to the receiver are
shorted together, not left floating. Because the receiver
100k
5V
C
120Ω
LTC487 • TA12
Figure 11. Forcing ‘0’ 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.
Fault Protection
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 12).
TransZorb is a registered trademark of General Instruments, GSI
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.
7
LTC487
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APPLICATI
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.
Y
120Ω
DRIVER
Z
LTC487 • TA13
Figure 12. ESD Protection with TransZorbs
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TYPICAL APPLICATI
RS232 to RS485 Level Translator with Hysteresis
R = 220k
Y
10k
RS232 IN
120Ω
DRIVER
5.6k
19k
VY - VZ 
HYSTERESIS = 10kΩ • ———— ≈ ————
R
R
Z
1/4 LTC487
LTC487 • TA14
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PACKAGE DESCRIPTIO
Dimensions in inches (millimeters) unless otherwise noted.
N Package, 16-Lead Plastic DIP
0.300 – 0.325
(7.620 – 8.255)
0.130 ± 0.005
(3.302 ± 0.127)
0.015
(0.381)
MIN
0.009 – 0.015
(0.229 – 0.381)
(
+0.025
0.325 –0.015
8.255
+0.635
–0.381
)
0.770*
(19.558)
MAX
0.045 – 0.065
(1.143 – 1.651)
16
15
14
13
12
11
10
9
1
2
3
4
5
6
7
8
0.260 ± 0.010*
0.065 (6.604 ± 0.254)
(1.651)
TYP
0.125
(3.175)
MIN
0.045 ± 0.015
(1.143 ± 0.381)
0.018 ± 0.003
(0.457 ± 0.076)
N16 0594
0.100 ± 0.010
(2.540 ± 0.254)
*THESE DIMENSIONS DO NOT INCLUDE MOLD FLASH OR PROTRUSIONS.
MOLD FLASH OR PROTURSIONS SHALL NOT EXCEED 0.010 INCH (0.254mm).
S Package, 16-Lead Plastic SOL
0.005
(0.127)
RAD MIN
0.291 – 0.299
(7.391 – 7.595)
(NOTE 2)
0.010 – 0.029 × 45°
(0.254 – 0.737)
16
0.093 – 0.104
(2.362 – 2.642)
NOTE 1
0.016 – 0.050
(0.406 – 1.270)
8
14
13
12
11
10
9
0.394 – 0.419
(10.007 – 10.643)
NOTE 1
0.050
(1.270)
TYP
0.004 – 0.012
(0.102 – 0.305)
0.014 – 0.019
(0.356 – 0.482)
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).
Linear Technology Corporation
15
0.037 – 0.045
(0.940 – 1.143)
0° – 8° TYP
0.009 – 0.013
(0.229 – 0.330)
0.398 – 0.413
(10.109 – 10.490)
(NOTE 2)
1
2
3
4
5
6
7
8
SOL16 0494
LT/GP 0894 0K REV A • PRINTED IN USA
1630 McCarthy Blvd., Milpitas, CA 95035-7487
(408) 432-1900 ● FAX: (408) 434-0507 ● TELEX: 499-3977
 LINEAR TECHNOLOGY CORPORATION 1994