LTC490_04_技术文档

LTC490

Differential Driver and

Receiver Pair

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FEATURES

DESCRIPTIO

The LTC^®490 is a low power differential bus/line trans-

ceiverdesignedformultipointdatatransmissionstandard

RS485 applications with extended common mode range

(12V to –7V). It also meets the requirements of RS422.

■

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

TheCMOSdesignofferssignificantpowersavingsoverits

bipolarcounterpartwithoutsacrificingruggednessagainst

overload or ESD damage.

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

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.

, LTC and LT are registered trademarks of Linear Technology Corporation.

■

^{Available in 8-Le}U^{ad PDIP and SO Packages}

APPLICATIO S

■

Low Power RS485/RS422 Transceiver

■

Level Translator

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TYPICAL APPLICATIO

LTC490

5

3

120Ω

RECEIVER

D

R

DRIVER

R

D

6

4000 FT BELDEN 9841

8

7

2

120Ω

DRIVER

RECEIVER

LTC490 • TA01

490fb

1

LTC490

W W

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ABSOLUTE AXI U RATI GS

PACKAGE/ORDER I FOR ATIO

(Note 1)

ORDER PART

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

TOP VIEW

NUMBER

V

1

2

3

4

8

7

6

5

A

B

Z

Y

CC

R

LTC490CN8

LTC490CS8

LTC490IN8

LTC490IS8

D

GND

N8 PACKAGE

8-LEAD PDIP

S8 PACKAGE

S8 PART MARKING

8-LEAD PLASTIC SO

490

490I

T_JMAX= 125°C, θ_JA= 100°C/W (N8)

T_JMAX= 150°C, θ_JA= 150°C/W (S8)

Consult LTC Marketing for parts specified with wider operating temperature ranges.

DC ELECTRICAL CHARACTERISTICS

The ● denotes the specificatiions which apply over the full

operating temperature range, otherwise specifications are at T_A= 25°C. V_CC= 5V ±5%

SYMBOL

PARAMETER

CONDITIONS

MIN

TYP

MAX

UNITS

●

V

Differential Driver Output Voltage (Unloaded)

Differential Driver Output Voltage (with Load)

I = 0

5

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)

3

V

OC

∆ V

Change in Magnitude of Driver Common Mode

Output Voltage for Complementary Output States

0.2

OC

●

V

Input High Voltage (D)

Input Low Voltage (D)

Input Current (D)

2.0

V

IH

IL

0.8

±2

l

µA

mA

V

IN1

IN2

Input Current (A, B)

V

= 0V or 5.25V

V

= 12V

= –7V

1

CC

IN

V

–0.8

0.2

IN

V

Differential Input Threshold Voltage for Receiver

Receiver Input Hysteresis

–7V ≤V

≤12V

CM

–0.2

3.5

TH

∆V

V

= 0V

CM

70

mV

V

TH

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

V

= Max 0.4V ≤V ≤2.4V

µA

kΩ

mA

µA

CC

O

No Load; D = GND or V

300

500

CC

R

Receiver Input Resistance

–7V ≤V ≤12V

12

7

IN

O

I

Driver Short-Circuit Current, V

Receiver Short-Circuit Current

= High

= Low

V = – 7V

100

250

85

OSD1

OSD2

OSR

OZ

OUT

O

V = 12V

OUT

O

0V ≤V ≤V

O

CC

Driver Three-State Output Current

V = –7V to 12V

±2

±200

O

490fb

2

LTC490

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The ● denotes the specificatiions which apply over the full

operating temperature range, otherwise specifications are at T_A= 25°C. V_CC= 5V ±5%

SWITCHI G CHARACTERISTICS

SYMBOL

PARAMETER

CONDITIONS

MIN

10

TYP

30

5

MAX

UNITS

ns

●

t

Driver Input to Output

Driver Output to Output

Driver Rise or Fall Time

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

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

25

ns

r

f

PLH

PHL

SKD

L1

L2

t

= 54Ω, C = C = 100pF (Figures 2, 4)

40

70

13

150

ns

L1

L2

= 54Ω, C = C = 100pF (Figures 2, 4)

ns

L1

L2

t

– t

PHL

Differential Receiver Skew

= 54Ω, C = C = 100pF (Figures 2, 4)

ns

PLH

L1

L2

Note 1: Absolute maximum ratings are those beyond which the safety of

Note 3: All typicals are given for V = 5V and Temperature = 25°C.

CC

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.

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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

A

= 25°C

T

= 25°C

T

A

= 25°C

A

–96

–72

64

48

80

60

– 4 8

–24

0

32

16

0

40

20

0

1

2

3

4

0

1

2

3

4

1

2

3

4

OUTPUT VOLTAGE (V)

LTC490 • TPC01

LTC490 • TPC02

LTC490 • TPC03

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3

LTC490

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TYPICAL PERFOR A CE CHARACTERISTICS

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 )

LTC490 • TPC06

LTC490 • TPC04

LTC490 • TPC05

Driver Differential Output Voltage

vs Temperature

Receiver t_PLH-t_PHL

vs Temperature

Receiver Output Low Voltage

vs Temperature

R

O

= 54Ω

I = 8mA

2.3

2.1

1.9

1.7

1.5

0.8

0.6

0.4

0.2

0

7

6

5

4

3

–50

0

50

100

–50

0

50

100

–50

0

50

100

TEMPERATURE (°C )

LTC490 • TPC07

LTC490 • TPC08

LTC490 • TPC09

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4

LTC490

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PI FU CTIO S

V_CC(Pin 1): Positive Supply; 4.75V ≤ V_CC≤ 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

Y low and Z high. A high on D will force Y high and Z low.

GND (Pin 4): Ground Connection.

TEST CIRCUITS

Y

R

C

A

B

L1

L2

Y

V

OD2

R

DRIVER

RECEIVER

R

D

DIFF

R

V

OC

Z

15pF

LTC490 • F01

LTC490 • F02

Figure 1. Driver DC Test Load

Figure 2. Driver/Receiver Timing Test Circuit

U

W

SWITCHI G TI E WAVEFOR S

3V

f = 1MHz : t ≤ 10ns : t ≤ 10ns

D

1.5V

PLH

1.5V

r

f

0V

t

PHL

V

O

80%

90%

50%

10%

V

= V(Y) – V(Z)

DIFF

20%

–V

t

f

r

Z

V

O

Y

LTC490 • F03

t

SKEW

1/2 V

O

SKEW

O

Figure 3. Driver Propagation Delays

INPUT

V

OD2

f = 1MHz ; t ≤ 10ns : t ≤ 10ns

A-B

0V

r

f

–V

OD2

t

PHL

PLH

V

OH

OUTPUT

R

1.5V

V

OL

LTC490 • F04

Figure 4. Receiver Propagation Delays

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5

LTC490

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APPLICATIO S I FOR ATIO

Typical Application

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.

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.

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

LTC490

8

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.

2

3

120Ω

RX

DX

RECEIVER

DATA IN

7

6

5

Thermal Shutdown

DATA OUT

DRIVER

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

LTC490 • F06

Figure 6. Line Repeater

5V

1

LTC490

SHIELD

8

7

5

6

2

3

120Ω

RX

DRIVER

DX

RX

RECEIVER

+

SHIELD

+

6

5

7

8

0.01µF

2

4

3

4

120Ω

RECEIVER

DX

DRIVER

LTC490 • F05

Figure 5. Typical Connection

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6

LTC490

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APPLICATIO S I FOR ATIO

less flexible, more bulky, and more costly than twisted

pairs. Many cable manufacturers offer a broad range of

120Ω cables designed for RS485 applications.

U

terrible losses at high data rates (>100kbs), and greatly

reduce the maximum cable length. At low data rates

however, theyareacceptableandmuchmoreeconomical.

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).

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

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

10

1.0

0.1

1.0

10

100

PROBE HERE

FREQUENCY (MHz)

LTC490 • F07

Rt

DX

DRIVER

RECEIVER

RX

Figure 7. Attenuation vs Frequency for Belden 9841

10k

Rt = 120Ω

Rt = 47Ω

1k

100

10

Rt = 470Ω

10k

100k

1M 2.5M

10M

DATA RATE (bps)

LTC490 • F09

LTC490 • F08

Figure 8. RS485 Cable Length Specification. Applies for 24

Gauge, Polyethylene Dielectric Twisted Pair

Figure 9. Termination Effects

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LTC490

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APPLICATIO S I FOR ATIO

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.

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).

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

thesupplycurrentoftheLTC490. Onewaytoeliminatethe

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.

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

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.

TransZorb is a registered trademark of General Instruments, GSI

Y

120Ω

DRIVER

120Ω

Z

C

RECEIVER

RX

LTC490 • F11

C = LINE LENGTH (FT) × 16.3pF

LTC490 • F10

Figure 10. AC Coupled Termination

Figure 11. ESD Protection with TransZorbs

490fb

8

LTC490

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TYPICAL APPLICATIO S

RS232 Receiver

RS232 IN

RX

RECEIVER

1/2 LTC490

5.6k

LTC490 • TA02

RS232 to RS485 Level Transistor with Hysteresis

R = 220k

Y

10k

120Ω

RS232 IN

DRIVER

5.6k

Z

1/2 LTC490

VY – VZ

R

19k

———— ——

HYSTERESIS = 10k •

≈

R

LTC490 • TA03

490fb

9

LTC490

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PACKAGE DESCRIPTIO

N8 Package

8-Lead PDIP (Narrow .300 Inch)

(Reference LTC DWG # 05-08-1510)

.400*

(10.160)

MAX

8

7

6

5

4

.255 ± .015*

(6.477 ± 0.381)

1

2

3

.130 ± .005

.300 – .325

.045 – .065

(3.302 ± 0.127)

(1.143 – 1.651)

(7.620 – 8.255)

.065

(1.651)

TYP

.008 – .015

(0.203 – 0.381)

.120

.020

(0.508)

MIN

(3.048)

MIN

+.035

.325

–.015

.018 ± .003

(0.457 ± 0.076)

.100

(2.54)

BSC

+0.889

8.255

(

)

N8 1002

–0.381

NOTE:

INCHES

MILLIMETERS

1. DIMENSIONS ARE

*THESE DIMENSIONS DO NOT INCLUDE MOLD FLASH OR PROTRUSIONS.

MOLD FLASH OR PROTRUSIONS SHALL NOT EXCEED .010 INCH (0.254mm)

490fb

10

LTC490

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PACKAGE DESCRIPTIO

S8 Package

8-Lead Plastic Small Outline (Narrow .150 Inch)

(Reference LTC DWG # 05-08-1610)

.189 – .197

(4.801 – 5.004)

.045 ±.005

.160 ±.005

NOTE 3

.050 BSC

7

5

8

6

.245

MIN

.150 – .157

(3.810 – 3.988)

NOTE 3

.228 – .244

(5.791 – 6.197)

.030 ±.005

TYP

1

3

4

2

RECOMMENDED SOLDER PAD LAYOUT

.010 – .020

(0.254 – 0.508)

× 45°

.053 – .069

(1.346 – 1.752)

.004 – .010

(0.101 – 0.254)

.008 – .010

(0.203 – 0.254)

0°– 8° TYP

.016 – .050

(0.406 – 1.270)

.050

(1.270)

BSC

.014 – .019

(0.355 – 0.483)

TYP

NOTE:

INCHES

1. DIMENSIONS IN

(MILLIMETERS)

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)

SO8 0303

490fb

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.

11

LTC490

RELATED PARTS

PART NUMBER

LTC486/LTC487

LTC488/LTC489

LTC1480

DESCRIPTION

COMMENTS

Low Power Quad RS485 Drivers

110µA Supply Current

Low Power Quad RS485 Receivers

3.3V Supply RS485 Transceiver

7mA Supply Current

Lower Supply Voltage

LTC1481

Low Power RS485 Transceiver with Shutdown

RS485 Transceiver with Carrier Detect

Low Power, Low EMI RS485 Transceiver

RS485 Transceiver with Fail-Safe

10Mbps RS485 Transceiver

Lowest Power

LTC1482

±15kV ESD, Fail-Safe

LTC1483

Slew Rate Limited Driver Outputs, Lowest Power

±15kV ESD, MSOP Package

LTC1484

LTC1485

High Speed

LTC1518/LTC1519

LTC1520

52Mbps Quad RS485 Receivers

Higher Speed, LTC488/LTC489 Pin-Compatible

100mV Threshold, Low Channel-to-Channel Skew

Full-Duplex, Self-Powered Using External Transformer

Industry-Standard Pinout, 500ps Propagation Delay Skew

LTC490/LTC491 Pin Compatible

Highest Speed, LTC486/LTC487 Pin Compatible

±15kV ESD, LTC490 Pin Compatible

±15kV ESD, Fail-Safe (LT1785A)

±15kV ESD, Fail-Safe (LT1791A), LTC491 Pin Compatible

LVDS-Compatible Quad Receiver

2500V Isolated RS485 Transceiver

52Mbps RS485 Transceiver

LTC1535

LTC1685

LTC1686/LTC1687

LTC1688/LTC1689

LTC1690

52Mbps Full-Duplex RS485 Transceiver

100Mbps Quad RS485 Drivers

Full-Duplex RS485 Transceiver with Fail-Safe

±60V Protected RS485 Transceivers

±60V Protected Full-Duplex RS485 Transceivers

LT1785/LTC1785A

LT1791/LTC1791A

490fb

LT/TP 0104 1K REV B • PRINTED IN USA

LinearTechnology Corporation

1630 McCarthy Blvd., Milpitas, CA 95035-7417

12

●

LINEAR TECHNOLOGY CORPORATION 1993

(408) 432-1900 FAX: (408) 434-0507 www.linear.com


型号：	LTC490_04
厂家：	Linear
描述：	Differential Driver and Receiver Pair 差分驱动器和接收器对驱动器
文件：	总12页 (文件大小：165K)
中文：	中文翻译
下载：	下载PDF数据表文档文件

LTC490_04 [Linear]

相关型号：

LTC491

LTC491C

LTC491CN

LTC491CN#PBF

LTC491CS

LTC491CS#TR

LTC491CS#TRPBF

LTC491I

LTC491IN

LTC491IS

LTC491IS#TR

LTC491IS#TRPBF