Datasheet LTC490 Datasheet (Linear Technology)

LTC490

Differential Driver and

Receiver Pair

EATU

F

■

Low Power: ICC = 300µA Typical

■

Designed for RS485 or RS422 Applications

■

Single 5V Supply

■

–7V to 12V Bus Common-Mode Range

RE

S

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

■

Pin Compatible with the SN75179

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DUESCRIPTIO

The LTC490 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.

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

O

A

PPLICATITYPICAL

LTC490

3

D

R

DRIVER

2

RECEIVER

U

5

120Ω 120Ω

6

4000 FT BELDEN 9841

8

120Ω 120Ω

7

4000 FT BELDEN 9841

LTC490

RECEIVER

DRIVER

R

D

LTC490 • TA01

1

LTC490

WU

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PACKAGE

/

O

RDER I FOR ATIO

N8 PACKAGE

8-LEAD PLASTIC DIP

S8 PACKAGE

8-LEAD PLASTIC SOIC

1
2
3
4

8
7
6
5



TOP VIEW

V

CC


R
D

GND

A
B
Z
Y

R

D

A

S

(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

VCC = 5V ±5%

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

V

OD1

V

OD2

∆V

OD

V

OC

∆ VOC Change in Magnitude of Driver Common Mode R = 27Ω or R = 50Ω (Figure 1)

V

IH

V

IL

l

IN1

l

IN2

V

TH

∆V

TH

V

OH

V

OL

I

OZR

I

CC

R

IN

I

OSD1

I

OSD2

I

OSR

IOZDriver Three-State Output Current VO = –7V to 12V

2

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O

LUTEXI T

LECTRICAL C CHARA TERIST

E

Differential Driver Output Voltage (Unloaded) IO = 0
Differential Driver Output Voltage (with Load) R = 50Ω (RS422)

Change in Magnitude of Driver Differential Output R = 27Ω or R = 50Ω (Figure 1)
Voltage for Complementary Output States

Driver Common-Mode Output Voltage R = 27Ω or R = 50Ω (Figure 1)

Output Voltage for Complementary Output States
Input High Voltage (D)
Input Low Voltage (D)
Input Current (D)
Input Current (A, B) VCC = 0V or 5.25V VIN = 12V

Differential Input Threshold Voltage for Receiver –7V ≤ VCM ≤ 12V
Receiver Input Hysteresis VCM = 0V
Receiver Output High Voltage IO = –4mA, V
Receiver Output Low Voltage IO = 4mA, V
Three-State Output Current at Receiver VCC = Max 0.4V ≤ VO ≤ 2.4V
Supply Current No Load; D = GND or V
Receiver Input Resistance – 7V ≤ VO ≤ 12V
Driver Short-Circuit Current, V
Driver Short-Circuit Current, V
Receiver Short-Circuit Current 0V ≤ VO ≤ V

A

WUW

ARB

= High VO = – 7V

OUT

= Low VO = 12V

OUT

U
G

I

S

T

JMAX

T

JMAX

Consult factory for Military grade parts.

ICSCD

R = 27Ω (RS485) (Figure 1)

VIN = –7V

= 0.2V

ID

= –0.2V

ID

CC

= 125°C, θJA = 100°C/W (N8)
= 150°C, θJA = 150°C/W (S8)

●

●

●

●

●

●

●

●

●

●

●

●

●

●

●

●

CC

●

●

●

●

●

●

ORDER PART

NUMBER

LTC490CN8
LTC490CS8
LTC490IN8
LTC490IS8

S8 PART MARKING

490
490I

5V

2V

1.5 5 V

0.2 V

3V

0.2 V

2.0 V

0.8 V
±2 µA

1mA

–0.8 mA

–0.2 0.2 V

70 mV

3.5 V

0.4 V
±1 µA

300 500 µA

12 kΩ

100 250 mA
100 250 mA

785mA

±2 ±200 µA

LTC490

OUTPUT VOLTAGE (V)

0

OUTPUT CURRENT (mA)

0

20

40

60

80

1234

LTC490 • TPC03

TA = 25°C

TEMPERATURE (°C )

–50

SUPPLY CURRENT (µA)

310

320

330

340

350

0 50 100

LTC490 • TPC06

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VCC = 5V ±5%

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

t

PLH

t

PHL

t

SKEW

t

, t

r

f

t

PLH

t

PHL

t

SKD

I

TCH

GC CHARA TERIST

Driver Input to Output R
Driver Input to Output R
Driver Output to Output R
Driver Rise or Fall Time R
Receiver Input to Output R
Receiver Input to Output R
 t

– t

PLH

 Differential Receiver Skew R

PHL

ICS

= 54Ω, CL1 = CL2 = 100pF (Figures 2, 3)

DIFF

= 54Ω, CL1 = CL2 = 100pF (Figures 2, 3)

DIFF

= 54Ω, CL1 = CL2 = 100pF (Figures 2, 3)

DIFF

= 54Ω, CL1 = CL2 = 100pF (Figures 2, 3)

DIFF

= 54Ω, CL1 = CL2 = 100pF (Figures 2, 4)

DIFF

= 54Ω, CL1 = CL2 = 100pF (Figures 2, 4)

DIFF

= 54Ω, CL1 = CL2 = 100pF (Figures 2, 4)

DIFF

●

10 30 50 ns

●

10 30 50 ns

●

●

5525 ns

●

40 70 150 ns

●

40 70 150 ns

●

5ns

13 ns

The ● denotes specifications which apply over the full operating
temperature range.

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

= 5V and Temperature = 25°C.

CC

UW

Y

PICA

–96

–72

– 4 8

OUTPUT CURRENT (mA)

–24

0

LPER

F

O

R

AT

CCHARA TERIST

E

C

ICS

Driver Output High Voltage vs Driver Differential Output Voltage Driver Output Low Voltage vs
Output Current vs Output Current Output Current

TA = 25°C

64

48

32

OUTPUT CURRENT (mA)

16

0

0

1234

OUTPUT VOLTAGE (V)

LTC490 • TPC02

0

1234

OUTPUT VOLTAGE (V)

TA = 25°C

LTC490 • TPC01

1.63

1.61

1.59

1.57

INPUT THRESHOLD VOLTAGE (V)

1.55
–50

TTL Input Threshold vs
Temperature Driver Skew vs Temperature Supply Current vs Temperature

5

4

3

TIME (ns)

2

0 50 100
TEMPERATURE (°C )

LTC490 • TPC04

1

–50

0 50 100
TEMPERATURE (°C )

LTC490 • TPC05

3

LTC490

TEMPERATURE (°C )

–50

OUTPUT VOLTAGE (V)

0

0.2

0.4

0.6

0.8

0 50 100

LTC490 • TPC09

I = 8mA

Y

PICA

LPER

F

O

R

AT

UW

CCHARA TERIST

E

C

ICS

Driver Differential Output Voltage Receiver t
vs Temperature Temperature Temperature

O

U
S

RO = 54Ω

LTC490 • TPC07

7

6

5

TIME (ns)

4

3

–50

2.3

2.1

1.9

1.7

DIFFERENTIAL VOLTAGE (V)

1.5

–50

0 50 100

TEMPERATURE (°C )

U

PI

V

FUUC

(Pin 1): Positive Supply; 4.75V ≤ VCC ≤ 5.25V.

CC

TI

PLH-tPHL

0 50 100
TEMPERATURE (°C )

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
A low and B high. A high on D will force A high and B low.

 vs Receiver Output Low Voltage vs

LTC490 • TPC08

Y (Pin 5): Driver Output.
Z (Pin 6): Driver Output.
B (Pin 7): Receiver Input.
A (Pin 8): Receiver Input.

GND (Pin 4): Ground Connection.

TEST CIRCUITS

4

Y

R

V

OD2

R

Z

LTC490 • TA02

Figure 1. Driver DC Test Load

A

C

Y

D

V

OC

DRIVER

Figure 2. Driver/Receiver Timing Test Circuit

R

DIFF

Z

L1

RECEIVER

C

L2

B 15pF

R

LTC490 • TA03

LTC490

WITCHI

3V

D

0V

V

O

–V

O

Z

V

O

Y

1/2 V

V

OD2

A-B

–V

OD2

V

OH

R

V

OL

O

U

G

10%

t

50%

t

TI

1.5V

PLH

0V

PLH

W

WAVEFORS

E

80%

t

r

t

SKEW

f = 1MHz ; t

1.5V

Figure 4. Receiver Propagation Delays

W

S

V

DIFF

≤ 10ns : t

r

= V(Y) – V(Z)

f = 1MHz : t

Figure 3. Driver Propagation Delays

INPUT

≤ 10ns : t

r

OUTPUT

≤ 10ns

f

≤ 10ns

f

1/2 V

1.5V

t

PHL

90%

O

50%

20%

t

f

t

SKEW

LTC490 • TA04

0V

t

PHL

1.5V

LTC490 • TA05

PPLICATI

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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.

5V

1

2

RX

+

0.01µF

DX

3

4

LTC490

RECEIVER

DRIVER

8

120Ω

7

6

5

SHIELD

SHIELD

There are no restrictions on where the chips are con­nected, 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

1

3

DX

+

2

RX

4

0.01µF

120Ω

LTC490

5

6

7

8

DRIVER

RECEIVER

Figure 5. Typical Connection

LTC490 • TA06

5

LTC490

PPLICATI

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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.

LTC490

8

RX

2

RECEIVER DATA IN

3

DRIVERDX

Figure 6. Line Repeater

120Ω

7

6

5

DATA OUT

LTC490 • TA07

Thermal Shutdown

The LTC490 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 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 con­tention faults when two drivers are active on the bus at the
same time.

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).

10

1.0

LOSS PER 100 FT (dB)

0.1

0.1

Figure 7. Attenuation vs Frequency for Belden 9841

1.0 10 100

FREQUENCY (MHz)

LTC490 • TA08

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.

10k

1k

100

CABLE LENGTH (FT)

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.

6

10

10k

Figure 8. RS485 Cable Length Specification. Applies for 24
Gauge, Polyethylene Dielectric Twisted Pair.

100k 1M 10M

DATA RATE (bps)

2.5M

LTC490 • TA09

LTC490

PPLICATI

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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 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.

PROBE HERE

DRIVERDX RECEIVER RX

Rt

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 LTC490. One way to eliminate the
unwanted current is by AC coupling the termination resis­tors as shown in Figure 10.

120Ω

C

C = LINE LENGTH (FT) × 16.3pF

Figure 10. AC Coupled Termination

RECEIVER

RX

LTC490 • TA11

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.

Rt = 120Ω

Rt = 47Ω

Rt = 470Ω

Figure 9. Termination Effects

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.

LTC490 • TA10

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

TransZorb® is a registered trademark of General Instruments, GSI

7

LTC490

LTC490 • TA12

120Ω

DRIVER

Z

Y

PPLICATI

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suppressor that has exceptional surge handling capabili­ties, 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.

U

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1/2 LTC490

LTC490 • TA13

RX

RS232 IN

RS232 IN

PPLICATITYPICAL

RS232 Receiver

5.6k

RECEIVER

Figure 11. ESD Protection with TransZorbs

RS232 to RS485 Level Transistor with Hysteresis

R = 220k

Y

5.6k

10k

DRIVER

1/2 LTC490

HYSTERESIS = 10k • ≈

120Ω

VY - VZ
————

Z

19k

——

R

R

LTC490 • TA14

®

PACKAGEDESCRIPTI

0.300 – 0.320

(7.620 – 8.128)

0.009 – 0.015

(0.229 – 0.381)

+0.025

0.325

–0.015

+0.635

8.255

()

–0.381

8

0.010 – 0.020

(0.254 – 0.508)

0.008 – 0.010

(0.203 – 0.254)

Linear Technology Corporation

1630 McCarthy Blvd., Milpitas, CA 95035-7487

[phone]

× 45°

0.016 – 0.050

0.406 – 1.270

●

FAX

: [phone]

O

0.045 ± 0.015

(1.143 ± 0.381)

0°– 8° TYP

●

TELEX

U

Dimensions in inches (millimeters) unless otherwise noted.

N8 Package

8-Lead Plastic DIP

0.130 ± 0.005

(3.302 ± 0.127)

0.125

(3.175)

MIN

0.018 ± 0.003

(0.457 ± 0.076)

0.004 – 0.010

(0.101 – 0.254)

0.050

(1.270)

BSC



0.020

(0.508)

MIN

0.228 – 0.244

(5.791 – 6.197)

876

1234

8

1

0.065

(1.651)

TYP

0.100 ± 0.010

(2.540 ± 0.254)

0.053 – 0.069

(1.346 – 1.752)

0.014 – 0.019

(0.355 – 0.483)

: 499-3977

0.045 – 0.065

(1.143 – 1.651)

S8 Package

8-Lead Plastic SOIC

0.400

(10.160)

MAX

5

0.250 ± 0.010

(6.350 ± 0.254)

0.189 – 0.197

(4.801 – 5.004)

7

2

BA/LT/GP 0893 5K REV A • PRINTED IN USA

 LINEAR TECHNOLOGY CORPORATION 1993

5

6

0.150 – 0.157

(3.810 – 3.988)

3

4



