Linear Technology LTC487 User manual

1
LTC487
Quad Low Power
RS485 Driver
D
U
ESCRIPTIO
2
DRIVER
LTC487 • TA01
RECEIVER
DI
EN 12
4
3
1
1/4 LTC487
120Ω120ΩRO
3
EN 12
4
2
11/4 LTC489
4000 FT BELDEN 9841
■
Very Low Power: I
CC
= 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
S
FEATURE
The LTC487
®
is a low power differential bus/line driver
designedformultipoint data transmission standard RS485
applications with extended common-mode range (–7V to
12V). It also meets RS422 requirements.
The CMOS design offers significant power savings over its
bipolarcounterpartwithoutsacrificingruggednessagainst
overload or ESD damage.
The driver features three-state outputs, with the driver
outputs maintaining high impedance over the entire com-
mon-mode range. Excessive power dissipation caused by
bus contention or faults is prevented by a thermal shut-
down 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.
■
Low Power RS485/RS422 Drivers
■
Level Translator
U
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PPLICATI
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PPLICATITYPICAL
DATA RATE (bps)
10k
10
CABLE LENGTH (FT)
100
1k
10k
100k 1M 10M
LTC487 • TA09
2.5M
* APPLIES FOR 24 GAUGE, POLYETHYLENE
DIELECTRIC TWISTED PAIR
and LTC are registered trademarks and LT is a trademark of Linear Technology Corporation.
RS485 Cable Length Specification*

LTC487
2
ELECTRICAL C CHARA TERISTICSCD
A
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W
A
W
U
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ARBSOLUTEXI T
IS
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PACKAGE/ORDER I FOR ATIO
(Note 1)
Supply Voltage (V
CC
) ............................................... 12V
Control Input Voltages .................... –0.5V to V
CC
+ 0.5V
Driver Input Voltages ...................... –0.5V to V
CC
+ 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
1
2
3
4
5
6
7
8
16
15
14
13
12
11
10
9
DI1
DO1A
DO1B
EN12
DO2B
DO2A
DI2
GND DI3
DO3A
DO3B
EN34
DO4B
DO4A
DI4
V
N PACKAGE
16-LEAD PLASTIC DIP
TOP VIEW
CC
S PACKAGE
16-LEAD PLASTIC SOL
T
JMAX
= 125°C, θ
JA
= 70°C/W (N)
T
JMAX
= 150°C, θ
JA
= 95°C/W (S)
LTC487CN
LTC487CSW
LTC487IN
LTC487ISW
pins are negative. All voltages are referenced to device GND unless
otherwise specified.
Note 3: All typicals are given for V
CC
= 5V and Temperature = 25°C.
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
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
t
PLH
Driver Input to Output R
DIFF
= 54Ω, C
L1
= C
L2
= 100pF 10 30 50 ns
t
PHL
Driver Input to Output (Figures 1, 4) 10 30 50 ns
t
SKEW
Driver Output to Output 515ns
t
r,
t
f
Driver Rise or Fall Time 5 20 25 ns
t
ZH
Driver Enable to Output High C
L
= 100pF (Figures 2, 5) S2 Closed 35 70 ns
t
ZL
Driver Enable to Output Low C
L
= 100pF (Figures 2, 5) S1 Closed 35 70 ns
t
LZ
Driver Disable Time from Low C
L
= 15pF (Figures 2, 5) S1 Closed 35 70 ns
t
HZ
Driver Disable Time from High C
L
= 15pF (Figures 2, 5) S2 Closed 35 70 ns
VCC = 5V ±5%, 0°C ≤TA≤70°C (Note 2, 3)
S
U
GC CHARA TERISTICS
WI TCHI
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
V
OD1
Differential Driver Output Voltage (Unloaded) I
O
= 0 5 V
V
OD2
Differential Driver Output Voltage (With Load) R = 50Ω; (RS422) 2 V
R = 27Ω; (RS485) (Figure 3) 1.5 5 V
V
OD
Change in Magnitude of Driver Differential R = 27Ωor R = 50Ω0.2 V
Output Voltage for Complementary Output States (Figure 3)
V
OC
Driver Common-Mode Output Voltage 3V
⏐V
OC
⏐Change in Magnitude of Driver Common-Mode 0.2 V
Output Voltage for Complementary Output States
V
IH
Input High Voltage DI, EN12, EN34 2.0 V
V
IL
Input Low Voltage 0.8 V
I
IN1
Input Current ±2µA
I
CC
Supply Current No Load Output Enabled 110 200 µA
Output Disabled 110 200 µA
I
OSD1
Driver Short-Circuit Current, V
OUT
= High V
O
= –7V 100 250 mA
I
OSD2
Driver Short-Circuit Current, V
OUT
= Low V
O
= 12V 100 250 mA
I
OZ
High Impedance State Output Current V
O
= –7V to 12V ±10 ±200 µA
VCC = 5V ±5%, 0°C ≤TA≤70°C (Commercial), – 40°C ≤TA≤85°C (Industrial) (Note 2, 3)
Consult factory for Military grade parts.

3
LTC487
CCHARA TERISTICS
UW
ATYPICALPER FORCE
Driver Output High Voltage Driver Differential Output Voltage Driver Output Low Voltage
vs Output Current vs Output Current vs Output Current
TTL Input Threshold vs Temperature Driver Skew vs Temperature Supply Current vs Temperature
TEMPERATURE (°C )
–50
SUPPLY CURRENT (µA)
90
100
110
120
130
0 50 100
LTC487 • TPC06
TEMPERATURE (°C )
–50
TIME (ns)
1.0
2.0
3.0
4.0
5.0
0 50 100
LTC487 • TPC05
TEMPERATURE (°C )
–50
INPUT THRESHOLD VOLTAGE (V)
1.55
1.57
1.59
1.61
1.63
0 50 100
LTC487 • TPC04
Driver Differential Output
Voltage vs Temperature
TEMPERATURE (°C )
–50
DIFFERENTIAL VOLTAGE (V)
1.5
1.7
1.9
2.1
2.3
0 50 100
LTC487 • TPC07
R
O
= 54Ω
OUTPUT VOLTAGE (V)
0
OUTPUT CURRENT (mA)
0
–24
– 4 8
–72
–96
1234
LTC487 • TPC01
T
A
= 25°C
OUTPUT VOLTAGE (V)
0
OUTPUT CURRENT (mA)
0
16
32
48
64
1234
LTC487• TPC02
T
A
= 25°C
OUTPUT VOLTAGE (V)
0
OUTPUT CURRENT (mA)
0
20
40
60
80
1234
LTC487 • TPC03
T
A
= 25°C

LTC487
4
GND (Pin 8): GND connection.
DI3 (Pin 9): Driver 3 input. Refer to DI1.
DO3A (Pin 10): Driver 3 output.
DO3B (Pin 11): Driver 3 output.
EN34 (Pin 12): Driver 3 and 4 outputs enabled. 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.
V
CC
(Pin 16): Positive supply; 4.75 < V
CC
< 5.25.
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.
EN12 (Pin 4): Driver 1 and 2 outputs enabled. See Func-
tion Table for details.
DO2B (Pin 5): Driver 2 output.
DO2A (Pin 6): Driver 2 output.
DI2 (Pin 7): Driver 2 input. Refer to DI1.
PI
U
FU
U
C
U
S
O
TI
INPUT ENABLES OUTPUTS
DI EN12 or EN34 OUT A OUT B
HH HL
LH LH
XL ZZ
FU
U
C
UO
TI TABLE
Figure 2. Driver Enable and Disable Times
H: High Level
L: Low Level
X: Irrelevant
Z: High Impedance (Off)
TI
W
EWAVEFORS
U
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WITCHI
W
S
Figure 1. Driver Propagation Delays
LTC487 • TA06
A, B
EN12
3V
0V
f = 1MHz : t 10ns : t 10ns
V
OL
V
OH
1.5V 1.5V
5V
OUTPUT NORMALLY LOW
t
ZL
2.3V
t
LZ
0.5V
≤≤
A, B
0V
t
ZH
2.3V OUTPUT NORMALLY HIGH
t
HZ
0.5V
rf
–V
O
LTC487 • TA05
B
A
DI
V
O
1/2 V
O
3V
0V
tSKEW
1.5V
tPLH
1.5V
tPHL
1/2 V
O
V = V(A) – V(B)
V
O80%
20%
tf
90%
DIFF
10%
tSKEW
tr
f = 1MHz : t 10ns : t 10ns
<<
rf

5
LTC487
TEST CIRCUITS
LTC487 • TA04
OUTPUT
UNDER TEST
CL
S1
500
CC
V
Ω
S2
Figure 5. Driver Timing Test Load #2
DRIVER 1
LTC487 • TA03
DI
A
B
EN12
R
DIFF
C
L1
C
L2
Figure 4. Driver Timing Test Circuit
LT
C
4
8
7 • TA
0
2
A
B
R
R
OD
V
OC
V
Figure 3. Driver DC Test Load
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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 pro-
tects 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 con-
tention 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
120Ωcables designed for RS485 applications.
Figure 6. Typical Connection
EN12
4
LTC487 • TA07
120Ω
DX
1
2
3SHIELD
120ΩRX RX
SHIELD
3
DX
EN12
42
EN12
4
1
2
3
RX RX
3
DX
EN12
41
DX
1/4 LTC4891/4 LTC487
1/4 LTC4891/4 LTC487
2
1

LTC487
6
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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 dielec-
tric losses are of the same order of magnitude, leading to
relatively low overall loss (Figure 7).
FREQUENCY (MHz)
0.1
0.1
LOSS PER 100 FT (dB)
1.0
10
1.0 10 100
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.
DATA RATE (bps)
10k
10
CABLE LENGTH (FT)
100
1k
10k
100k 1M 10M
LTC487 • TA09
2.5M
Figure 8. Cable Length vs Data Rate
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).
Rt
DRIVERDX RECEIVER RX
Rt = 120Ω
Rt = 47Ω
Rt = 470Ω
LTC487 • TA10
PROBE HERE
Figure 9. 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 ampli-
tude 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.

7
LTC487
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AC Cable Termination
Cable termination resistors are necessary to prevent un-
wanted reflections, but they consume power. The typical
differential output voltage of the driver is 2V when the
cable is terminated with two 120Ωresistors, 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 resis-
tors as shown in Figure 10.
LTC487 • TA11
C = LINE LENGTH (FT) x 16.3pF
120Ω
RECEIVER RX
C
Figure 10. AC Coupled Termination
The coupling capacitor must allow high-frequency energy
to flow to the termination, but block DC and low frequen-
cies. 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 120Ωcables. 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
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.
LTC487 • TA12
140ΩRECEIVER RX
5V
1.5k
RECEIVER RX
5V
110Ω
130Ω110Ω130Ω
120Ω
RECEIVER RX
C
5V
100k
1.5k
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).
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 represen-
tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
TransZorb is a registered trademark of General Instruments, GSI

LTC487
8
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7487
[phone]
●
FAX : [phone]
●
TELEX : 499-3977
LTC487 • TA13
120Ω
DRIVER
Z
Y
Figure 12. ESD Protection with TransZorbs
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PACKAGE DESCRIPTIO
Dimensions in inches (millimeters) unless otherwise noted.
N Package, 16-Lead Plastic DIP
S Package, 16-Lead Plastic SOL
N16 0594
0.260 ± 0.010*
(6.604 ± 0.254)
0.770*
(19.558)
MAX
16
12345678
910
11
12
13
14
15
0.015
(0.381)
MIN
0.125
(3.175)
MIN
0.130 ± 0.005
(3.302 ± 0.127)
0.065
(1.651)
TYP
0.045 – 0.065
(1.143 – 1.651)
0.018 ± 0.003
(0.457 ± 0.076)
0.045 ± 0.015
(1.143 ± 0.381)
0.100 ± 0.010
(2.540 ± 0.254)
0.009 – 0.015
(0.229 – 0.381)
0.300 – 0.325
(7.620 – 8.255)
0.325 +0.025
–0.015
+0.635
–0.381
8.255
()
*THESE DIMENSIONS DO NOT INCLUDE MOLD FLASH OR PROTRUSIONS.
MOLD FLASH OR PROTURSIONS SHALL NOT EXCEED 0.010 INCH (0.254mm).
SOL16 0494
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.
NOTE 1
0.398 – 0.413
(10.109 – 10.490)
(NOTE 2)
16 15 14 13 12 11 10 9
12345678
0.394 – 0.419
(10.007 – 10.643)
0.037 – 0.045
(0.940 – 1.143)
0.004 – 0.012
(0.102 – 0.305)
0.093 – 0.104
(2.362 – 2.642)
0.050
(1.270)
TYP 0.014 – 0.019
(0.356 – 0.482)
TYP
0° – 8° TYP
NOTE 1
0.005
(0.127)
RAD MIN
0.009 – 0.013
(0.229 – 0.330)
0.016 – 0.050
(0.406 – 1.270)
0.291 – 0.299
(7.391 – 7.595)
(NOTE 2)
×45°
0.010 – 0.029
(0.254 – 0.737)
2. THESE DIMENSIONS DO NOT INCLUDE MOLD FLASH OR PROTRUSIONS.
MOLD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.006 INCH (0.15mm).
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 break-
down 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.
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PPLICATITYPICAL
RS232 to RS485 Level Translator with Hysteresis
LTC487 • TA14
HYSTERESIS = 10kΩ• ≈
⎜VY - VZ ⎜
————
R
19k
————
R
120Ω
DRIVER
Y
Z
R = 220k
10k
RS232 IN
5.6k 1/4 LTC487
© LINEAR TECHNOLOGY CORPORATION 1994
sn487 487fas LT/GP 0894 0K REV A • PRINTED IN USA