1
LTC490
490fb
Differential Driver and
Receiver Pair
■
Low Power: I
CC
= 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 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 SN75179
■
Available in 8-Lead PDIP and SO Packages
The LTC
®
490 is a low power differential bus/line trans-
ceiver 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 design offers significant power savings over its
bipolar counterpart without sacrificing ruggedness against
overload or ESD damage.
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.
■
Low Power RS485/RS422 Transceiver
■
Level Translator
FEATURES
DESCRIPTIO
U
APPLICATIO S
U
TYPICAL APPLICATIO
U
, LTC and LT are registered trademarks of Linear Technology Corporation.
LTC490 • TA01
120Ω120Ω
120Ω120Ω
4000 FT BELDEN 9841
4000 FT BELDEN 9841
RECEIVER
LTC490
DRIVER RECEIVER
LTC490
DRIVER
R
D
R
D3
2
7
8
6
5
LTC490
2
490fb
(Note 1)
Supply Voltage (V
CC
) ............................................... 12V
Driver Input Currents ........................... –25mA to 25mA
Driver Input Voltages ...................... –0.5V to V
CC
+ 0.5V
Driver Output Voltages .......................................... ±14V
Receiver Input Voltages ......................................... ±14V
Receiver Output Voltages ............... –0.5V to V
CC
+ 0.5V
Operating Temperature Range
LTC490C ................................................ 0°C to 70°C
LTC490I............................................. –40°C to 85°C
Storage Temperature Range ................. –65°C to 150°C
Lead Temperature (Soldering, 10 sec).................. 300°C
T
JMAX
= 125°C, θ
JA
= 100°C/W (N8)
T
JMAX
= 150°C, θ
JA
= 150°C/W (S8)
N8 PACKAGE
8-LEAD PDIP
S8 PACKAGE
8-LEAD PLASTIC SO
1
2
3
4
8
7
6
5
TOP VIEW
V
CC
R
D
GND
A
B
Z
Y
R
D
ORDER PART
NUMBER
S8 PART MARKING
LTC490CN8
LTC490CS8
LTC490IN8
LTC490IS8
490
490I
The ● denotes the specificatiions which apply over the full
operating temperature range, otherwise specifications are at TA = 25°C. VCC = 5V ±5%
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
V
OD1
Differential Driver Output Voltage (Unloaded) I
O
= 0 ●5V
V
OD2
Differential Driver Output Voltage (with Load) R = 50Ω (RS422) ●2V
R = 27Ω (RS485) (Figure 1) ●1.5 5 V
∆V
OD
Change in Magnitude of Driver Differential Output R = 27Ω or R = 50Ω (Figure 1) ●0.2 V
Voltage for Complementary Output States
V
OC
Driver Common Mode Output Voltage R = 27Ω or R = 50Ω (Figure 1) ●3V
∆ V
OC
Change in Magnitude of Driver Common Mode R = 27Ω or R = 50Ω (Figure 1) ●0.2 V
Output Voltage for Complementary Output States
V
IH
Input High Voltage (D) ●2.0 V
V
IL
Input Low Voltage (D) ●0.8 V
l
IN1
Input Current (D) ●±2µA
l
IN2
Input Current (A, B) V
CC
= 0V or 5.25V V
IN
= 12V ●1mA
V
IN
= –7V ●–0.8 mA
V
TH
Differential Input Threshold Voltage for Receiver –7V ≤ V
CM
≤ 12V ●–0.2 0.2 V
∆V
TH
Receiver Input Hysteresis V
CM
= 0V ●70 mV
V
OH
Receiver Output High Voltage I
O
= –4mA, V
ID
= 0.2V ●3.5 V
V
OL
Receiver Output Low Voltage I
O
= 4mA, V
ID
= – 0.2V ●0.4 V
I
OZR
Three-State Output Current at Receiver V
CC
= Max 0.4V ≤ V
O
≤ 2.4V ●±1µA
I
CC
Supply Current No Load; D = GND or V
CC
●300 500 µA
R
IN
Receiver Input Resistance –7V ≤ V
O
≤ 12V ●12 kΩ
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
OSR
Receiver Short-Circuit Current 0V ≤ V
O
≤ V
CC
●785mA
I
OZ
Driver Three-State Output Current V
O
= –7V to 12V ●±2±200 µA
ABSOLUTE AXI U RATI GS
WWWU
Consult LTC Marketing for parts specified with wider operating temperature ranges.
PACKAGE/ORDER I FOR ATIO
UU
W
DC ELECTRICAL CHARACTERISTICS
3
LTC490
490fb
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
pins are negative. All voltages are referenced to device ground unless
otherwise specified.
Note 3: All typicals are given for V
CC
= 5V and Temperature = 25°C.
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
t
PLH
Driver Input to Output R
DIFF
= 54Ω, C
L1
= C
L2
= 100pF (Figures 2, 3) ●10 30 50 ns
t
PHL
Driver Input to Output R
DIFF
= 54Ω, C
L1
= C
L2
= 100pF (Figures 2, 3) ●10 30 50 ns
t
SKEW
Driver Output to Output R
DIFF
= 54Ω, C
L1
= C
L2
= 100pF (Figures 2, 3) ●5ns
t
r
, t
f
Driver Rise or Fall Time R
DIFF
= 54Ω, C
L1
= C
L2
= 100pF (Figures 2, 3) ●5525 ns
t
PLH
Receiver Input to Output R
DIFF
= 54Ω, C
L1
= C
L2
= 100pF (Figures 2, 4) ●40 70 150 ns
t
PHL
Receiver Input to Output R
DIFF
= 54Ω, C
L1
= C
L2
= 100pF (Figures 2, 4) ●40 70 150 ns
t
SKD
t
PLH
– t
PHL
Differential Receiver Skew R
DIFF
= 54Ω, C
L1
= C
L2
= 100pF (Figures 2, 4) ●13 ns
SWITCHI G CHARACTERISTICS
U
Driver Output High Voltage Driver Differential Output Voltage Driver Output Low Voltage
vs Output Current vs Output Current vs Output Current
TYPICAL PERFOR A CE CHARACTERISTICS
UW
OUTPUT VOLTAGE (V)
0
OUTPUT CURRENT (mA)
0
–24
– 4 8
–72
–96
1234
LTC490 • TPC01
T
A
= 25°C
OUTPUT VOLTAGE (V)
0
OUTPUT CURRENT (mA)
0
16
32
48
64
1234
LTC490 • TPC02
TA = 25°C
OUTPUT VOLTAGE (V)
0
OUTPUT CURRENT (mA)
0
20
40
60
80
1234
LTC490 • TPC03
T
A
= 25°C
The ● denotes the specificatiions which apply over the full
operating temperature range, otherwise specifications are at TA = 25°C. VCC = 5V ±5%
LTC490
4
490fb
TTL Input Threshold
vs Temperature Driver Skew vs Temperature Supply Current vs Temperature
TYPICAL PERFOR A CE CHARACTERISTICS
UW
TEMPERATURE (°C )
–50
INPUT THRESHOLD VOLTAGE (V)
1.55
1.57
1.59
1.61
1.63
0 50 100
LTC490 • TPC04
TEMPERATURE (°C )
–50
TIME (ns)
5
4
3
2
10 50 100
LTC490 • TPC05
TEMPERATURE (°C )
–50
SUPPLY CURRENT (µA)
310
320
330
340
350
0 50 100
LTC490 • TPC06
Driver Differential Output Voltage Receiver tPLH-tPHLReceiver Output Low Voltage
vs Temperature vs Temperature vs Temperature
TEMPERATURE (°C )
–50
DIFFERENTIAL VOLTAGE (V)
1.5
1.7
1.9
2.1
2.3
0 50 100
LTC490 • TPC07
R
O
= 54Ω
TEMPERATURE (°C )
–50
TIME (ns)
7
6
5
4
30 50 100
LTC490 • TPC08
TEMPERATURE (°C )
–50
OUTPUT VOLTAGE (V)
0
0.2
0.4
0.6
0.8
0 50 100
LTC490 • TPC09
I = 8mA
5
LTC490
490fb
–V
O
LTC490 • F03
D3V
0V
1.5V
t
PLH
1.5V
V
DIFF
= V(Y) – V(Z)
V
O
80%
20%
50%
10%
Z
Yt
SKEW
t
r
f = 1MHz : t
r
≤ 10ns : t
f
≤ 10ns
90%
50%
t
PHL
t
f
V
O
t
SKEW
1/2 V
O
1/2 V
O
UU
U
PI FU CTIO S
V
CC
(Pin 1): Positive Supply; 4.75V ≤ V
CC
≤ 5.25V.
R (Pin 2): Receiver Output. If A > B by 200mV, R will be
high. If A < B by 200mV, then R will be low.
D (Pin 3): Driver Input. A low on D forces the driver outputs
Y low and Z high. A high on D will force Y high and Z low.
GND (Pin 4): Ground Connection.
Y (Pin 5): Driver Output.
Z (Pin 6): Driver Output.
B (Pin 7): Receiver Input.
A (Pin 8): Receiver Input.
Figure 4. Receiver Propagation Delays
Figure 3. Driver Propagation Delays
Figure 1. Driver DC Test Load Figure 2. Driver/Receiver Timing Test Circuit
TEST CIRCUITS
LTC490 • F01
Y
Z
R
R
VOD2
VOC
LTC490 • F02
DRIVER
D
Y
Z
RECEIVER
R
DIFF
A
B 15pF
C
L1
C
L2
R
SWITCHI G TI E WAVEFOR S
UWW
V
OL LTC490 • F04
A-B
V
OD2
0V
t
PLH
0V
OUTPUT
V
OH
1.5V
f = 1MHz ; t
r
≤ 10ns : t
f
≤ 10ns
t
PHL
–V
OD2
1.5V
INPUT
R
LTC490
6
490fb
Typical Application
A typical connection of the LTC490 is shown in Figure 5.
Two twisted-pair wires connect two driver/receiver pairs
for full duplex data transmission. Note that the driver and
receiver outputs are always enabled. If the outputs must
be disabled, use the LTC491.
There are no restrictions on where the chips are con-
nected, and it isn’t necessary to have the chips connected
at the ends of the wire. However, the wires must be
terminated only at the ends with a resistor equal to their
characteristic impedance, typically 120Ω. Because only
one driver can be connected on the bus, the cable can be
terminated only at the receiving end. The optional shields
around the twisted pair help reduce unwanted noise, and
are connected to GND at one end.
The LTC490 can also be used as a line repeater as shown
in Figure 6. If the cable length is longer than 4000 feet, the
LTC490 is inserted in the middle of the cable with the
receiver output connected back to the driver input.
Thermal Shutdown
The LTC490 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 LTC490
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.
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
Figure 5. Typical Connection
Figure 6. Line Repeater
APPLICATIO S I FOR ATIO
WUUU
LTC490 • F05
120Ω
120Ω
SHIELD
LTC490
DRIVER RECEIVER
LTC490
DRIVER DX
RX
DX
RX 2
3
5
6
7
8
RECEIVER
0.01µF
1
4
5V
SHIELD
8
7
6
5
0.01µF
5V
4
2
3
1
+
+
LTC490 • F06
120Ω
LTC490
DRIVERDX
RX 2
3
5
6
7
8
RECEIVER DATA IN
DATA OUT
7
LTC490
490fb
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 overall loss (Figure 7).
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
APPLICATIO S I FOR ATIO
WUUU
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 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). 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 re-
flected 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
Figure 7. Attenuation vs Frequency for Belden 9841
Figure 8. RS485 Cable Length Specification. Applies for 24
Gauge, Polyethylene Dielectric Twisted Pair Figure 9. Termination Effects
FREQUENCY (MHz)
0.1
0.1
LOSS PER 100 FT (dB)
1.0
10
1.0 10 100
LTC490 • F07
DATA RATE (bps)
10k
10
CABLE LENGTH (FT)
100
1k
10k
100k 1M 10M
LTC490 • F08
2.5M
Rt
DRIVERDX RECEIVER RX
Rt = 120Ω
Rt = 47Ω
Rt = 470Ω
LTC490 • F09
PROBE HERE
LTC490
8
490fb
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 un-
wanted 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 LTC490. One way to eliminate the
unwanted current is by AC coupling the termination resis-
tors as shown in Figure 10.
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 120Ω cables.
APPLICATIO S I FOR ATIO
WUUU
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).
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 11). A TransZorb is a silicon transient voltage
suppressor that has exceptional surge handling capabili-
ties, fast response time, and low series resistance. They
are available from General Instruments, GSI 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
Figure 10. AC Coupled Termination Figure 11. ESD Protection with TransZorbs
LTC490 • F10
120Ω
RECEIVER RX
C
C = LINE LENGTH (FT) × 16.3pF
LTC490 • F11
120Ω
DRIVER
Z
Y
9
LTC490
490fb
TYPICAL APPLICATIO S
U
RS232 Receiver
RS232 to RS485 Level Transistor with Hysteresis
LTC490 • TA02
5.6k RECEIVER
RS232 IN
1/2 LTC490
RX
120Ω
DRIVER
Y
Z
R = 220k
10k
RS232 IN
5.6k
LTC490 • TA03
HYSTERESIS = 10k • ≈
1/2 LTC490
VY – VZ
————
R
19k
——
R
LTC490
10
490fb
N8 Package
8-Lead PDIP (Narrow .300 Inch)
(Reference LTC DWG # 05-08-1510)
U
PACKAGE DESCRIPTIO
N8 1002
.065
(1.651)
TYP
.045 – .065
(1.143 – 1.651)
.130 ± .005
(3.302 ± 0.127)
.020
(0.508)
MIN
.018 ± .003
(0.457 ± 0.076)
.120
(3.048)
MIN
12 34
8765
.255 ± .015*
(6.477 ± 0.381)
.400*
(10.160)
MAX
.008 – .015
(0.203 – 0.381)
.300 – .325
(7.620 – 8.255)
.325 +.035
–.015
+0.889
–0.381
8.255
()
NOTE:
1. DIMENSIONS ARE INCHES
MILLIMETERS
*THESE DIMENSIONS DO NOT INCLUDE MOLD FLASH OR PROTRUSIONS.
MOLD FLASH OR PROTRUSIONS SHALL NOT EXCEED .010 INCH (0.254mm)
.100
(2.54)
BSC
11
LTC490
490fb
U
PACKAGE DESCRIPTIO
S8 Package
8-Lead Plastic Small Outline (Narrow .150 Inch)
(Reference LTC DWG # 05-08-1610)
.016 – .050
(0.406 – 1.270)
.010 – .020
(0.254 – 0.508)× 45°
0°– 8° TYP
.008 – .010
(0.203 – 0.254)
SO8 0303
.053 – .069
(1.346 – 1.752)
.014 – .019
(0.355 – 0.483)
TYP
.004 – .010
(0.101 – 0.254)
.050
(1.270)
BSC
1234
.150 – .157
(3.810 – 3.988)
NOTE 3
8765
.189 – .197
(4.801 – 5.004)
NOTE 3
.228 – .244
(5.791 – 6.197)
.245
MIN .160 ±.005
RECOMMENDED SOLDER PAD LAYOUT
.045 ±.005
.050 BSC
.030 ±.005
TYP
INCHES
(MILLIMETERS)
NOTE:
1. DIMENSIONS IN
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)
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.
LTC490
12
490fb
LINEAR TECHNOLOGY CORPORATION 1993
LT/TP 0104 1K REV B • PRINTED IN USA
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
FAX: (408) 434-0507
●
www.linear.com
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