LTC490CN8 [Linear]
Differential Driver and Receiver Pair; 差分驱动器和接收器对
| 型号： | LTC490CN8 |
| 厂家： | 
Linear |
| 描述： | Differential Driver and Receiver Pair |
| 文件： | 总8页 (文件大小：205K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
LTC490
Differential Driver and
Receiver Pair
U
DESCRIPTIO
EATURE
Low Power: ICC = 300µA Typical
Designed for RS485 or RS422 Applications
Single 5V Supply
S
F
■
■
■
■
TheLTC490isalowpowerdifferentialbus/linetransceiver
designedformultipointdatatransmissionstandardRS485
applications with extended common-mode range (12V to
–7V). It also meets the requirements of RS422.
–7V to 12V Bus Common-Mode Range
Permits ±7V Ground Difference Between Devices
on the Bus
TheCMOSdesignofferssignificantpowersavingsoverits
bipolarcounterpartwithoutsacrificingruggednessagainst
overload or ESD damage.
■
■
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
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.
■
■
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 SN75179
■
■
Both AC and DC specifications are guaranteed from 0°C to
70°C and 4.75V to 5.25V supply voltage range.
■
O U
PPLICATI
S
A
■
Low Power RS485/RS422 Transceiver
■
Level Translator
U
O
TYPICAL APPLICATI
LTC490
LTC490
5
6
3
2
120Ω
120Ω
120Ω
D
R
DRIVER
RECEIVER
R
D
4000 FT BELDEN 9841
4000 FT BELDEN 9841
8
7
120Ω
DRIVER
RECEIVER
LTC490 • TA01
1
LTC490
W W W
U
ABSOLUTE AXI U RATI GS
/O
PACKAGE RDER I FOR ATIO
(Note 1)
Supply Voltage (VCC) ............................................... 12V
Driver Input Currents ........................... –25mA to 25mA
Driver Input Voltages ....................... –0.5V to VCC +0.5V
Driver Output Voltages .......................................... ±14V
Receiver Input Voltages ......................................... ±14V
Receiver Output Voltages ................ –0.5V to VCC +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
ORDER PART
TOP VIEW
NUMBER
V
1
2
3
4
8
7
6
5
A
B
Z
Y
CC
R
R
LTC490CN8
LTC490CS8
LTC490IN8
LTC490IS8
D
D
GND
N8 PACKAGE
8-LEAD PLASTIC DIP
S8 PACKAGE
8-LEAD PLASTIC SOIC
S8 PART MARKING
490
490I
TJMAX = 125°C, θJA = 100°C/W (N8)
TJMAX = 150°C, θJA = 150°C/W (S8)
Consult factory for Military grade parts.
DC ELECTRICAL CHARACTERISTICS
VCC = 5V ±5%
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
●
●
●
●
V
V
Differential Driver Output Voltage (Unloaded)
Differential Driver Output Voltage (with Load)
I = 0
5
V
V
V
V
OD1
OD2
O
R = 50Ω (RS422)
2
R = 27Ω (RS485) (Figure 1)
R = 27Ω or R = 50Ω (Figure 1)
1.5
5
∆V
Change in Magnitude of Driver Differential Output
Voltage for Complementary Output States
0.2
OD
●
●
V
Driver Common-Mode Output Voltage
R = 27Ω or R = 50Ω (Figure 1)
R = 27Ω or R = 50Ω (Figure 1)
3
V
V
OC
∆ V
Change in Magnitude of Driver Common Mode
Output Voltage for Complementary Output States
0.2
OC
●
●
●
●
●
●
●
●
●
●
●
●
●
●
●
●
V
V
Input High Voltage (D)
Input Low Voltage (D)
Input Current (D)
2.0
V
V
IH
0.8
±2
IL
l
µA
mA
mA
V
IN1
l
Input Current (A, B)
V
= 0V or 5.25V
V
IN
= 12V
= –7V
1
IN2
CC
V
IN
–0.8
0.2
V
TH
Differential Input Threshold Voltage for Receiver
Receiver Input Hysteresis
–7V ≤ V ≤ 12V
–0.2
3.5
CM
∆V
V
CM
= 0V
70
mV
V
TH
V
V
Receiver Output High Voltage
Receiver Output Low Voltage
Three-State Output Current at Receiver
Supply Current
I = –4mA, V = 0.2V
O ID
OH
I = 4mA, V = –0.2V
O
0.4
±1
V
OL
OZR
CC
ID
I
I
V
CC
= Max 0.4V ≤ V ≤ 2.4V
µA
µA
kΩ
mA
mA
mA
µA
O
No Load; D = GND or V
300
500
CC
R
Receiver Input Resistance
–7V ≤ V ≤ 12V
12
7
IN
O
I
I
I
I
Driver Short-Circuit Current, V
Driver Short-Circuit Current, V
Receiver Short-Circuit Current
= High
= Low
V = – 7V
O
100
100
250
250
85
OSD1
OSD2
OSR
OZ
OUT
V = 12V
O
OUT
0V ≤ V ≤ V
O
CC
Driver Three-State Output Current
V = –7V to 12V
O
±2
±200
2
LTC490
U
SWI I
TCH G CHARACTERISTICS
VCC = 5V ±5%
SYMBOL
PARAMETER
CONDITIONS
MIN
10
TYP
30
30
5
MAX
UNITS
ns
●
●
●
●
●
●
●
t
t
t
Driver Input to Output
Driver Input to Output
Driver Output to Output
Driver Rise or Fall Time
Receiver Input to Output
Receiver Input to Output
R
DIFF
R
DIFF
R
DIFF
R
DIFF
R
DIFF
R
DIFF
R
DIFF
= 54Ω, C = C = 100pF (Figures 2, 3)
50
50
PLH
L1
L2
= 54Ω, C = C = 100pF (Figures 2, 3)
10
ns
PHL
L1
L2
= 54Ω, C = C = 100pF (Figures 2, 3)
ns
SKEW
L1
L2
t , t
= 54Ω, C = C = 100pF (Figures 2, 3)
5
5
25
ns
r
f
PLH
PHL
SKD
L1
L2
t
t
t
= 54Ω, C = C = 100pF (Figures 2, 4)
40
40
70
70
13
150
150
ns
L1
L2
= 54Ω, C = C = 100pF (Figures 2, 4)
ns
L1
L2
t
– t
Differential Receiver Skew
= 54Ω, C = C = 100pF (Figures 2, 4)
ns
PLH
PHL
L1
L2
●
The denotes specifications which apply over the full operating
temperature range.
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 1: Absolute maximum ratings are those beyond which the safety of
the device cannot be guaranteed.
Note 3: All typicals are given for V = 5V and Temperature = 25°C.
CC
U W
TYPICAL PERFOR A CE CHARACTERISTICS
Driver Output High Voltage vs
Output Current
Driver Differential Output Voltage
vs Output Current
Driver Output Low Voltage vs
Output Current
T
= 25°C
T = 25°C
A
T
A
= 25°C
A
64
48
80
60
–96
–72
32
16
0
40
20
0
– 4 8
–24
0
0
0
1
2
3
4
1
2
3
4
0
1
2
3
4
OUTPUT VOLTAGE (V)
OUTPUT VOLTAGE (V)
OUTPUT VOLTAGE (V)
LTC490 • TPC01
LTC490 • TPC02
LTC490 • TPC03
TTL Input Threshold vs
Temperature
Driver Skew vs Temperature
Supply Current vs Temperature
350
340
330
320
310
1.63
1.61
1.59
1.57
1.55
5
4
3
2
1
–50
0
50
100
–50
0
50
100
–50
0
50
100
TEMPERATURE (°C )
TEMPERATURE (°C )
TEMPERATURE (°C )
LTC490 • TPC04
LTC490 • TPC06
LTC490 • TPC05
3
LTC490
TYPICAL PERFOR A CE CHARACTERISTICS
U W
Driver Differential Output Voltage
vs Temperature
Receiver tPLH-tPHL vs
Temperature
Receiver Output Low Voltage vs
Temperature
R
= 54Ω
I = 8mA
O
0.8
0.6
0.4
0.2
0
2.3
2.1
1.9
1.7
1.5
7
6
5
4
3
–50
0
50
100
–50
0
50
100
–50
0
50
100
TEMPERATURE (°C )
TEMPERATURE (°C )
TEMPERATURE (°C )
LTC490 • TPC09
LTC490 • TPC07
LTC490 • TPC08
U
O
U
U
PI
FU CTI
S
VCC (Pin 1): Positive Supply; 4.75V ≤ VCC ≤ 5.25V.
Y (Pin 5): Driver Output.
Z (Pin 6): Driver Output.
B (Pin 7): Receiver Input.
A (Pin 8): Receiver Input.
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(Pin3):DriverInput.AlowonDforcesthedriveroutputs
A low and B high. A high on D will force A high and B low.
GND (Pin 4): Ground Connection.
TEST CIRCUITS
Y
R
C
C
A
B
L1
Y
Z
V
OD2
R
DRIVER
RECEIVER
R
D
DIFF
R
V
OC
Z
L2
15pF
LTC490 • TA02
LTC490 • TA03
Figure 1. Driver DC Test Load
Figure 2. Driver/Receiver Timing Test Circuit
4
LTC490
U
W
W
SWITCHI G TI E WAVEFOR S
3V
f = 1MHz : t ≤ 10ns : t ≤ 10ns
D
1.5V
1.5V
PHL
r
f
0V
t
t
PLH
V
O
O
80%
90%
50%
10%
V
= V(Y) – V(Z)
50%
20%
DIFF
–V
t
t
f
r
Z
V
O
Y
t
t
SKEW
1/2 V
1/2 V
O
SKEW
O
LTC490 • TA04
Figure 3. Driver Propagation Delays
INPUT
V
OD2
f = 1MHz ; t ≤ 10ns : t ≤ 10ns
A-B
0V
0V
r
f
–V
OD2
t
t
PHL
PLH
V
OH
OUTPUT
R
1.5V
1.5V
V
OL
LTC490 • TA05
Figure 4. Receiver Propagation Delays
O U
W
U
PPLICATI
A
S I FOR ATIO
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
receiveroutputsarealwaysenabled.Iftheoutputsmustbe
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
5V
5V
1
1
LTC490
LTC490
SHIELD
8
7
5
2
3
120Ω
RX
DRIVER
DX
RX
RECEIVER
6
SHIELD
+
+
6
5
7
8
0.01µF
0.01µF
2
4
3
4
120Ω
RECEIVER
DX
DRIVER
LTC490 • TA06
Figure 5. Typical Connection
5
LTC490
PPLICATI
O U
W
U
A
S I FOR ATIO
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.
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).
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.
10
LTC490
8
1.0
2
3
120Ω
RX
DX
RECEIVER
DATA IN
7
6
5
0.1
DATA OUT
DRIVER
0.1
1.0
10
100
FREQUENCY (MHz)
LTC490 • TA08
LTC490 • TA07
Figure 7. Attenuation vs Frequency for Belden 9841
Figure 6. Line Repeater
When using low loss cables, Figure 8 can be used as a
guidelineforchoosingthemaximumlinelengthforagiven
datarate. WithlowerqualityPVCcables, thedielectricloss
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, theyareacceptableandmuchmoreeconomical.
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.
10k
1k
100
10
Cables and Data Rate
10k
100k
1M 2.5M
10M
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.
DATA RATE (bps)
LTC490 • TA09
Figure 8. RS485 Cable Length Specification. Applies for 24
Gauge, Polyethylene Dielectric Twisted Pair.
6
LTC490
O U
W
U
PPLICATI
Cable Termination
S I FOR ATIO
A
AC 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.
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
thesupplycurrentoftheLTC490. Onewaytoeliminatethe
unwanted current is by AC coupling the termination resis-
tors as shown in Figure 10.
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
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.
120Ω
RECEIVER
RX
C
C = LINE LENGTH (FT) × 16.3pF
LTC490 • TA11
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 120Ω cables.
PROBE HERE
Rt
DX
DRIVER
RECEIVER
RX
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).
Rt = 120Ω
Rt = 47Ω
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
Rt = 470Ω
LTC490 • TA10
Figure 9. Termination Effects
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 represen-
tationthattheinterconnectionofitscircuitsasdescribedhereinwillnotinfringeonexistingpatentrights.
7
LTC490
PPLICATI
O U
W
U
A
S I FOR ATIO
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
avarietyofbreakdownvoltagesandprices. Besuretopick
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
LTC490 • TA12
Figure 11. ESD Protection with TransZorbs®
U
O
TYPICAL APPLICATI S
RS232 Receiver
RS232 to RS485 Level Transistor with Hysteresis
R = 220k
Y
RS232 IN
10k
120Ω
RX
RECEIVER
1/2 LTC490
5.6k
RS232 IN
DRIVER
5.6k
Z
1/2 LTC490
VY - VZ
R
19k
LTC490 • TA13
———— ——
HYSTERESIS = 10k •
≈
R
LTC490 • TA14
U
PACKAGE DESCRIPTIO
Dimensions in inches (millimeters) unless otherwise noted.
N8 Package
8-Lead Plastic DIP
0.400
(10.160)
MAX
0.130 ± 0.005
(3.302 ± 0.127)
0.300 – 0.320
0.045 – 0.065
(1.143 – 1.651)
(7.620 – 8.128)
8
1
7
6
5
4
0.065
(1.651)
TYP
0.250 ± 0.010
(6.350 ± 0.254)
0.009 – 0.015
(0.229 – 0.381)
0.125
(3.175)
MIN
0.020
(0.508)
MIN
+0.025
–0.015
0.045 ± 0.015
(1.143 ± 0.381)
0.325
2
3
+0.635
8.255
(
)
–0.381
0.100 ± 0.010
(2.540 ± 0.254)
0.018 ± 0.003
(0.457 ± 0.076)
S8 Package
8-Lead Plastic SOIC
0.189 – 0.197
(4.801 – 5.004)
0.010 – 0.020
(0.254 – 0.508)
7
5
8
6
× 45°
0.053 – 0.069
(1.346 – 1.752)
0.004 – 0.010
(0.101 – 0.254)
0.008 – 0.010
(0.203 – 0.254)
0°– 8° TYP
0.228 – 0.244
0.150 – 0.157
(5.791 – 6.197)
(3.810 – 3.988)
0.016 – 0.050
0.406 – 1.270
0.050
(1.270)
BSC
0.014 – 0.019
(0.355 – 0.483)
1
2
3
4
BA/LT/GP 0893 5K REV A • PRINTED IN USA
LINEAR TECHNOLOGY CORPORATION 1993
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7487
8
●
●
(408) 432-1900 FAX: (408) 434-0507 TELEX: 499-3977
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