LTC491CN_技术文档

LTC491

Differential Driver and

Receiver Pair

U

DESCRIPTIO

EATURE

S

F

■

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

TheLTC491isalowpowerdifferentialbus/linetransceiver

designedformultipointdatatransmissionstandardRS485

applications with extended common mode range (+12V to

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

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 in Three-State or

with the Power Off

The driver and receiver feature three-state outputs, with

the driver outputs maintaining high impedance over the

entire common mode range. 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.

■

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 SN75180

■

Thereceiverhasafailsafefeaturewhichguaranteesahigh

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.

O U

PPLICATI

A

S

■

Low Power RS485/RS422 Transceiver

Level Translator

■

U

O

TYPICAL APPLICATI

DE

4

9

5

2

120Ω

RECEIVER

D

R

DRIVER

R

D

10

4000 FT 24 GAUGE TWISTED PAIR

LTC491

DRIVER

12

11

120Ω

RECEIVER

3

REB

LTC491 • TA01

1

LTC491

W W W

U

/O

ABSOLUTE AXI U RATI GS

PACKAGE RDER I FOR ATIO

(Note 1)

TOP VIEW

Supply Voltage (V_CC) ............................................... 12V

Control Input Voltages ..................... –0.5V to V_CC+0.5V

Control Input Currents .......................... –50mA to 50mA

Driver Input Voltages ....................... –0.5V to V_CC+0.5V

Driver Input Currents ............................ –25mA to 25mA

Driver Output Voltages .......................................... ±14V

Receiver Input Voltages ......................................... ±14V

Receiver Output Voltages ................ –0.5V to V_CC+0.5V

Operating Temperature Range

LTC491C.................................................. 0°C to 70°C

LTC491I.............................................. –40°C to 85°C

Storage Temperature Range ................. –65°C to 150°C

Lead Temperature (Soldering, 10 sec.)................. 300°C

ORDER PART

NC

R

1

2

3

4

5

6

7

14

V

CC

NUMBER

R

13 NC

12

11

10

9

REB

DE

A

B

LTC491CN

LTC491CS

LTC491IN

LTC491IS

D

Z

D

Y

GND

8

NC

N PACKAGE

S PACKAGE

14-LEAD PLASTIC DIP 14-LEAD PLASTIC SOIC

LTC491 • POI01

Consult factory for Military grade parts.

DC ELECTRICAL CHARACTERISTICS

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

●

3

V

OC

∆ V

Change in Magnitude of Driver Common Mode

Output Voltage for Complementary Output States

0.2

OC

V

Input High Voltage

Input Low Voltage

Input Current

D, DE, REB

●

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

1.0

CC

IN

V

–0.8

0.2

IN

V

Differential Input Threshold Voltage for Receiver

Receiver Input Hysteresis

–7V ≤ V ≤ 12V

–0.2

70

TH

CM

∆V

V

= 0V

CM

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

3.5

OH

ID

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, Outputs Enabled

or V Outputs Disabled

300

500

CC

R

Receiver Input Resistance

–7V ≤ V ≤ 12V

12

7

IN

CM

I

Driver Short Circuit Current, V

Receiver Short Circuit Current

= High

= Low

V = –7V

O

100

250

85

OSD1

OSD2

OSR

OZ

OUT

V = 12V

O

0V ≤ V ≤ V

O

CC

Driver Three-State Output Current

V = –7V to 12V

O

±2

±200

2

LTC491

U

SWI I

TCH G CHARACTERISTICS

V_CC= 5V ±5%

SYMBOL PARAMETER

CONDITIONS

= 54Ω, C = C = 100pF

(Figures 2, 5)

MIN

TYP

MAX

UNITS

t

Driver Input to Output

R

DIFF

●

10

30

50

ns

PLH

L1

L2

t

10

30

ns

PHL

Driver Output to Output

Driver Rise or Fall Time

●

5

15

ns

SKEW

t , t

5

25

70

r

f

t

Driver Enable to Output High

Driver Enable to Output Low

Driver Disable Time From Low

Driver Disable Time From High

Receiver Input to Output

C = 100pF (Figures 4, 6) S2 Closed

L

●

40

70

13

20

ns

ZH

ZL

LZ

HZ

C = 100pF (Figures 4, 6) S1 Closed

L

70

C = 15pF (Figures 4, 6) S1 Closed

L

70

C = 15pF (Figures 4, 6) S2 Closed

L

70

R

DIFF

= 54Ω, C = C = 100pF

40

150

PLH

PHL

SKD

ZL

L1

L2

(Figures 2, 7)

Receiver Input to Output

t

– t

Differential Receiver Skew

PHL

PLH

Receiver Enable to Output Low

Receiver Enable to Output High

Receiver Disable From Low

Receiver Disable From High

C = 15pF (Figures 3, 8) S1 Closed

L

50

C = 15pF (Figures 3, 8) S2 Closed

L

ZH

C = 15pF (Figures 3, 8) S1 Closed

L

LZ

C = 15pF (Figures 3, 8) S2 Closed

L

HZ

The

●

denotes specifications which apply over the full operating

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.

temperature range.

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

O

U

PI

FU CTI

S

NC (Pin 1): Not Connected.

GND (Pin 6): Ground Connection.

GND (Pin 7): Ground Connection.

NC (Pin 8): Not Connected.

Y (Pin 9): Driver output.

R(Pin2):Receiveroutput.Ifthereceiveroutputisenabled

(REB low), then if A > B by 200mV, R will be high. If A < B

by 200mV, then R will be low.

REB (Pin 3): Receiver output enable. A low enables the

receiver output, R. A high input forces the receiver output

into a high impedance state.

Z (Pin 10): Driver output.

B (Pin 11): Receiver input.

A (Pin 12): Receiver input.

NC (Pin 13): Not Connected.

DE (Pin 4): Driver output enable. A high on DE enables the

driver outputs, A and B. A low input forces the driver

outputs into a high impedance state.

D (Pin 5): Driver input. If the driver outputs are enabled

(DE high), then A low on D forces the driver outputs A low

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

V_CC(Pin 14): Positive supply; 4.75V ≤ V_CC≤ 5.25V.

3

LTC491

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TYPICAL PERFOR A CE CHARACTERISTICS

Driver Output High Voltage vs

Driver Differential Output Voltage vs

Driver Output Low Voltage vs

Output Current T_A= 25°C

80

60

–96

–72

64

48

40

20

0

– 4 8

–24

0

32

16

0

1

2

3

4

1

2

3

4

1

2

3

4

OUTPUT VOLTAGE (V)

LTC491 • TPC02

LTC491 • TPC03

LTC491 • TPC01

TTL Input Threshold vs Temperature

Driver Skew vs Temperature

Supply Current vs Temperature

1.63

1.61

1.59

1.57

1.55

5.0

4.0

3.0

2.0

1.0

350

340

330

320

310

–50

0

50

100

–50

0

50

100

–50

0

50

100

TEMPERATURE (°C )

LTC491 • TPC04

LTC491 • TPC05

LTC491 • TPC06

Driver Differential Output Voltage vs

Temperature R_O= 54Ω

Receiver t_{PLH PHL}

t

vs

Receiver Output Low Voltage vs

Temperature at I = 8mA

Temperature

2.3

2.1

1.9

1.7

1.5

7.0

6.0

5.0

4.0

3.0

0.8

0.6

0.4

0.2

0

–50

0

50

100

–50

0

50

100

–50

0

50

100

TEMPERATURE (°C )

LTC491 • TPC07

LTC491 • TPC08

LTC491 • TPC09

4

LTC491

TEST CIRCUITS

Y

Z

R

V

OD2

V

OC

LTC491 • TA02

Figure 1. Driver DC Test Load

C

A

B

L1

Y

Z

R

DRIVER

RECEIVER

R

D

DIFF

L2

15pF

LTC491 • TA03

Figure 2. Driver/Receiver Timing Test Circuit

S1

S2

1kΩ

RECEIVER

OUTPUT

V

CC

500Ω

OUTPUT

UNDER TEST

1kΩ

C

L

C

L

S2

LTC491 • TA04

LTC491 • TA05

Figure 3. Receiver Timing Test Load

Figure 4. Driver Timing Test Load

5

LTC491

U

W

SWITCHI G TI E WAVEFOR S

3V

f = 1MHz : t ≤ 10ns : t ≤ 10ns

D

1.5V

PLH

1.5V

PHL

r

f

0V

t

V

O

80%

90%

50%

10%

V

= V(Y) – V(Z)

50%

20%

DIFF

–V

O

t

f

r

Z

V

O

Y

t

SKEW

t

SKEW

1/2 V

O

LTC491 • TA06

Figure 5. Driver Propagation Delays

3V

f = 1MHz : t ≤ 10ns : t ≤ 10ns

DE

1.5V

r

1.5V

LZ

0V

5V

t

ZL

A, B

OUTPUT NORMALLY LOW

OUTPUT NORMALLY HIGH

2.3V

0.5V

V

OL

OH

0V

V

0.5V

t

ZH

HZ

LTC491 • TA07

Figure 6. Driver Enable and Disable Times

INPUT

V

OD2

f = 1MHz ; t ≤ 10ns : t ≤ 10ns

A-B

R

0V

r

f

–V

t

PHL

PLH

V

OH

OUTPUT

1.5V

V

OL

LTC491 • TA08

Figure 7. Receiver Propagation Delays

3V

f = 1MHz : t ≤ 10ns : t ≤ 10ns

REB

R

1.5V

r

f

1.5V

0V

5V

t

LZ

ZL

OUTPUT NORMALLY LOW

OUTPUT NORMALLY HIGH

1.5V

0.5V

V

OL

OH

0V

V

0.5V

R

1.5V

t

ZH

HZ

LTC491 • TA09

Figure 8. Receiver Enable and Disable Times

6

LTC491

O U

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PPLICATI

A

S I FOR ATIO

Typical Application

typically 20kΩ to GND, or 0.6 unit RS-485 load, so in

practice 50 to 60 transceivers can be connected to the

same wires. The optional shields around the twisted pair

help reduce unwanted noise, and are connected to GND at

one end.

A typical connection of the LTC491 is shown in Figure 9.

Two twisted pair wires connect up to 32 driver/receiver

pairs for full 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 imped-

ance, typically 120Ω. The input impedance of a receiver is

The LTC491 can also be used as a line repeater as shown

inFigure10.Ifthecablelengthislongerthan4000feet,the

LTC491 is inserted in the middle of the cable with the

receiver output connected back to the driver input.

12

2

3

2

3

120Ω

RX

DX

RX

DX

RECEIVER

11

4

10

9

10

9

5

DRIVER

LTC491

9

10

11

12

RECEIVER

LTC491

DRIVER

5

4

3

2

LTC491 • TA10

DX

RX

Figure 9. Typical Connection

12

2

3

120Ω

RX

RECEIVER

11

DATA IN

4

10

5

120Ω

DATA OUT

DX

DRIVER

9

LTC491

LTC491 • TA11

Figure 10. Line Repeater

7

LTC491

PPLICATI

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A

S I FOR ATIO

Thermal Shutdown

less flexible, more bulky, and more costly than twisted

pairs. Many cable manufacturers offer a broad range of

120Ω cables designed for RS485 applications.

The LTC491 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 LTC491

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.

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 over all loss (Figure 11).

When using low loss cables, Figure 12 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.

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

10

10k

1k

1.0

100

10

0.1

1.0

10

100

10k

100k

1M 2.5M

10M

FREQUENCY (MH )

DATA RATE (bps)

Z

LTC491 • TA12

LTC491 • TA13

Figure 11. Attenuation vs Frequency for Belden 9481

Figure 12. Cable Length vs Data Rate

8

LTC491

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PPLICATI

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Cable Termination

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.

The proper termination of the cable is very important.

If the cable is not terminated with it’s 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 13).

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

thesupplycurrentoftheLTC491. Onewaytoeliminatethe

unwanted current is by AC coupling the termination resis-

tors as shown in Figure 14.

PROBE HERE

Rt

DX

DRIVER

RECEIVER

RX

120Ω

RECEIVER

RX

C

Rt = 120Ω

Rt = 47Ω

C = LINE LENGTH (ft) x 16.3pF

LTC491 • TA15

Figure 14. 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

perfootofcablelengthfor120Ω cables. Withthecoupling

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 = 470Ω

LTC491 • TA14

Figure 13. 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

reflectedbackoutofphasebecauseofthemistermination.

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).

9

LTC491

PPLICATI

O U

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A

S I FOR ATIO

Receiver Open-Circuit Fail-Safe

Fault Protection

Some data encoding schemes require that the output of

the receiver maintains a known state (usually a logic 1)

whenthedataisfinishedtransmittingandalldriversonthe

line are forced into three-state. The receiver of the LTC491

has 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.

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 16).

Y

120Ω

DRIVER

Z

+5V

110Ω

130Ω

LTC491 • TA17

Figure 16. ESD Protection with TransZorbs

RECEIVER

RX

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

GeneralSemiconductorIndustriesandcomeinavarietyof

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.

+5V

1.5kΩ

140Ω

RECEIVER

RX

1.5kΩ

120Ω

100kΩ

+5V

C

RECEIVER

RX

LTC491 • TA16

Figure 15. Forcing “O” 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

andthesecondabout8mW.Thelowestpowersolutionisto

use an AC termination with a pull-up resistor. Simply swap

the receiver inputs for data protocols ending in logic 1.

10

LTC491

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TYPICAL APPLICATI S

RS232 Receiver

RS232 IN

5.6kΩ

RX

RECEIVER

1/2 LTC491

LTC491 • TA18

RS232 to RS485 Level Transistor with Hysteresis

R = 220kΩ

Y

10kΩ

120Ω

RS232 IN

DRIVER

5.6kΩ

Z

1/2 LTC491

VY - VZ

R

19k

———— ————

HYSTERESIS = 10kΩ •

≈

R

LTC491 • TA19

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

LTC491

U

Dimensions in inches (millimeters) unless otherwise noted.

PACKAGE DESCRIPTIO

N Package

14-Lead Plastic DIP

0.770

(19.558)

MAX

14

13

12

11

10

9

8

7

T_{J MAX}

θ_JA

0.260 ± 0.010

(6.604 ± 0.254)

100°C

90°C/W

1

2

3

5

6

4

0.065

(1.651)

TYP

0.300 – 0.325

(7.620 – 8.255)

0.045 – 0.065

(1.143 – 1.651)

0.015

(0.380)

MIN

0.130 ± 0.005

(3.302 ± 0.127)

0.009 – 0.015

(0.229 – 0.381)

+0.025

–0.015

0.325

0.075 ± 0.015

(1.905 ± 0.381)

0.018 ± 0.003

(0.457 ± 0.076)

0.125

(3.175)

MIN

+0.635

8.255

(

)

–0.381

0.100 ± 0.010

(2.540 ± 0.254)

N14 0392

S Package

14-Lead Plastic SOIC

0.337 – 0.344

(8.560 – 8.738)

13

12

11

10

8

14

9

T_{J MAX}

θ_JA

100°C

110°C/W

0.228 – 0.244

0.150 – 0.157

(5.791 – 6.197)

(3.810 – 3.988)

1

2

3

4

5

6

7

0.010 – 0.020

(0.254 – 0.508)

× 45°

0.053 – 0.069

(1.346 – 1.752)

0.008 – 0.010

(0.203 – 0.254)

0.004 – 0.010

(0.101 – 0.254)

0° – 8° TYP

0.050

(1.270)

TYP

0.016 – 0.050

0.406 – 1.270

0.014 – 0.019

(0.355 – 0.483)

SO14 0392

BA/GP 0492 10K REV 0

Linear Technology Corporation

1630 McCarthy Blvd., Milpitas, CA 95035-7487

12

●

(408) 432-1900 FAX: (408) 434-0507 TELEX: 499-3977

LINEAR TECHNOLOGY CORPORATION 1992


型号：	LTC491CN
厂家：	Linear
描述：	Differential Driver and Receiver Pair 差分驱动器和接收器对驱动器接口集成电路光电二极管
文件：	总12页 (文件大小：238K)
中文：	中文翻译
下载：	下载PDF数据表文档文件

LTC491CN [Linear]

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