LTC486CN_技术文档

LTC486

Quad Low Power

RS485 Driver

U

EATURE

Very Low Power: I_CC= 110µA Typ

Designed for RS485 or RS422 Applications

Single 5V Supply

–7V to 12V Bus Common-Mode Range Permits

±7V GND Difference Between Devices on the Bus

Thermal Shutdown Protection

Power-Up/Down Glitch-Free Driver Outputs Permit

Live Insertion/Removal of Package

Driver Maintains High Impedance in Three-State or

with the Power Off

S

F

■

DESCRIPTIO

The LTC486 is a low power differential bus/line driver

designedformultipointdatatransmissionstandardRS485

applications with extended common-mode range (12V to

–7V). It also meets RS422 requirements.

TheCMOSdesignofferssignificantpowersavingsoverits

bipolarcounterpartwithoutsacrificingruggednessagainst

overload or ESD damage.

■

The driver features three-state outputs, with the driver

outputs maintaining high impedance over the entire

common-moderange.Excessivepowerdissipationcaused

by bus contention or faults is prevented by a thermal

shutdown circuit which forces the driver outputs into a

high impedance state.

28ns Typical Driver Propagation Delays

with 5ns Skew

Pin Compatible with the SN75172, DS96172,

µA96172, and DS96F172

Both AC and DC specifications are guaranteed from 0°C to

70°C (Commercial), –40°C to 85°C (Industrial), over the

4.75V to 5.25V supply voltage range.

O U

PPLICATI

A

S

■

Low Power RS485/RS422 Drivers

Level Translator

■

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TYPICAL APPLICATI

RS485 Cable Length Specification*

10k

EN

4

EN

1k

4

RECEIVER

1/4 LTC488

2

1

120Ω

3

120Ω

DI

DRIVER

12

RO

100

10

4000 FT BELDEN 9841

1/4 LTC486

EN

LTC486 • TA01

10k

100k

1M 2.5M

10M

DATA RATE (bps)

LTC486 • TA09

* APPLIES FOR 24 GAUGE, POLYETHYLENE

DIELECTRIC TWISTED PAIR

1

LTC486

W W W

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ABSOLUTE AXI U RATI GS

PACKAGE RDER I FOR ATIO

(Note 1)

TOP VIEW

ORDER PART

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

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

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

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

Control Input Currents ....................................... ±25mA

Driver Input Currents ......................................... ±25mA

Operating Temperature Range

LTC486C................................................ 0°C to 70°C

LTC486I ............................................ –40°C to 85°C

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

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

DI1

DO1A

DO1B

EN

1

2

3

4

5

6

7

8

16 V

CC

NUMBER

15 DI4

14 DO4A

13 DO4B

LTC486CN

LTC486CS

LTC486IN

LTC486IS

DO2B

DO2A

DI2

12

EN

11 DO3B

10 DO3A

GND

9

DI3

N PACKAGE

S PACKAGE

16-LEAD PLASTIC DIP 16-LEAD PLASTIC SOL

T_JMAX= 125°C, θ_JA= 70°C/W (N)

_JMAX= 150°C, θ_JA= 95°C/W (S)

T

Consult factory for Military grade parts

DC ELECTRICAL CHARACTERISTICS

V_CC= 5V ±5%, 0°C ≤ Temperature ≤ 70°C (Commercial), –40°C ≤ Temperature ≤ 85°C (Industrial) (Note 2, 3)

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)

1.5

5

V

Change in Magnitude of Driver Differential

Output Voltage for Complementary Output States

R = 27Ω or R = 50Ω

(Figure 1)

0.2

OD

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

DI, EN, EN

2.0

V

IH

0.8

±2

IL

I

µA

mA

µA

IN1

CC

Supply Current

No Load

Output Enabled

Output Disabled

110

100

±10

200

250

±200

I

Driver Short-Circuit Current, V

= High

= Low

V = –7V

O

OSD1

OSD2

OZ

OUT

V = 12V

O

OUT

High Impedance State Output Current

V = –7V to 12V

O

U

SWI I

TCH G CHARACTERISTICS

V_CC= 5V ±5%, 0°C ≤ Temperature ≤ 70°C (Commercial), –40°C ≤ Temperature ≤ 85°C (Industrial) (Note 2, 3)

SYMBOL

PARAMETER

CONDITIONS

R = 54Ω, C = C = 100pF

DIFF

(Figures 2, 4)

MIN

10

TYP

30

5

MAX

50

UNITS

ns

t

Driver Input to Output

Driver Output to Output

Driver Rise or Fall Time

Driver Enable to Output High

PLH

L1

L2

10

50

ns

PHL

15

ns

SKEW

t t

r, f

5

15

35

25

ns

t

C = 100pF (Figures 3, 5) S2 Closed

L

70

ns

ZH

2

LTC486

U

SWI I

TCH G CHARACTERISTICS

V_CC= 5V ±5%, 0°C ≤ Temperature ≤ 70°C (Commercial), –40°C ≤ Temperature ≤ 85°C (Industrial) (Note 2, 3)

SYMBOL

PARAMETER

CONDITIONS

C = 100pF (Figures 3, 5) S1 Closed

MIN

TYP

35

MAX

UNITS

ns

t

Driver Enable to Output Low

Driver Disable Time from Low

Driver Disable Time from High

70

ZL

LZ

HZ

L

C = 15pF (Figures 3, 5) S1 Closed

L

35

ns

C = 15pF (Figures 3, 5) S2 Closed

L

35

ns

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

the device cannot be guaranteed.

pins are negative. All voltages are referenced to device ground unless

otherwise specified.

Note 2: All currents into device pins are positive; all currents out of device

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

CC

U

W

SWITCHI G TI E WAVEFOR S

3V

f = 1MHz : t 10ns : t 10ns

DI

<

f

1.5V

r

0V

B

t

PHL

PLH

V

O

A

t

SKEW

1/2 V

SKEW

O

V

O

90%

20%

80%

V

= V(A) – V(B)

DIFF

–V

10%

O

t

f

r

Figure 4. Driver Propagation Delays

LTC486 • TA05

3V

0V

5V

f = 1MHz : t 10ns : t 10ns

≤

f

EN

r

1.5V

LZ

t

ZL

A, B

V

OUTPUT NORMALLY LOW

OUTPUT NORMALLY HIGH

2.3V

0.5V

OL

V

OH

0.5V

A, B

0V

t

HZ

ZH

LTC486 • TA06

Figure 5. Driver Enable and Disable Times

U W

TYPICAL PERFOR A CE CHARACTERISTICS

Driver Output High Voltage

vs Output Current T_A= 25°C

Driver Differential Output Voltage

vs Output Current T_A= 25°C

Driver Output Low Voltage

vs Output Current T_A= 25°C

–96

–72

64

48

80

60

– 4 8

–24

0

32

16

0

40

20

0

1

2

3

4

1

2

3

4

0

1

2

3

4

OUTPUT VOLTAGE (V)

LTC486• TPC02

LTC486 • TPC01

OUTPUT VOLTAGE (V)

LTC486 • TPC03

3

LTC486

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TYPICAL PERFOR A CE CHARACTERISTICS

TTL Input Threshold vs Temperature

Driver Skew vs Temperature

1.63

5

4

3

2

1

1.61

1.59

1.57

1.55

–50

0

50

100

–50

0

50

100

TEMPERATURE (°C )

LTC486 • TPC04

TEMPERATURE (°C )

LTC486 • TPC05

Driver Differential Output Voltage

vs Temperature R_O= 54Ω

Supply Current vs Temperature

130

120

110

100

90

2.3

2.1

1.9

1.7

1.5

–50

0

50

100

–50

0

50

100

TEMPERATURE (°C )

LTC486 • TPC06

TEMPERATURE (°C )

LTC486 • TPC07

O

U

FU CTI

TABLE

INPUT

ENABLES

OUTPUTS

OUTB

DI

EN

OUTA

H

L

H

L

H

X

L

X

L

H

L

H

L

Z

L

H

L

H

Z

H: High Level

L: Low Level

X: Irrelevant

X

Z: High Impedance (Off)

4

LTC486

U

O

U

PI

FU CTI

S

GND (Pin 8): Ground Connection.

DI3 (Pin 9): Driver 3 Input. Refer to DI1.

DO3A (Pin 10): Driver 3 Output.

DO3B (Pin 11): Driver 3 Output.

DI1(Pin1):Driver1Input.IfDriver1isenabled,thenalow

onDI1forcesthedriveroutputsDO1AlowandDO1Bhigh.

A high on DI1 with the driver outputs enabled will force

DO1A high and DO1B low.

DO1A (Pin 2): Driver 1 Output.

DO1B (Pin 3): Driver 1 Output.

EN (Pin 12): Driver Outputs Disabled. See Function Table

for details.

EN(Pin4): DriverOutputsEnabled. SeeFunctionTablefor

details.

DO4B (Pin 13): Driver 4 Output.

DO4A (Pin 14): Driver 4 Output.

DI4 (Pin 15): Driver 4 Input. Refer to DI1.

DO2B (Pin 5): Driver 2 Output.

DO2A (Pin 6): Driver 2 Output.

V

_CC(Pin 16): Positive Supply; 4.75V < V_CC< 5.25V .

DI2 (Pin 7): Driver 2 Input. Refer to DI1.

TEST CIRCUITS

EN

S1

A

V

CC

CI1

500Ω

R

OUTPUT

UNDER TEST

A

V

OD

DI

DRIVER

R

DIFF

C

L

R

S2

V

OC

B

CI2

LTC486 • TA04

LTC486 • TA03

EN

LTC486 • TA02

Figure 1. Driver DC Test Load

Figure 2. Driver Timing Test Circuit

Figure 3. Driver Timing Test Load #2

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PPLICATI

A

Typical Application

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°Candturnsthembackonwhenthetemperaturecools

to 130°C. If the outputs of two or more LTC486 drivers are

shorted directly, the driver outputs can not supply enough

current to activate the thermal shutdown. Thus, the ther-

mal shutdown circuit will not prevent contention faults

when two drivers are active on the bus at the same time.

A typical connection of the LTC486 is shown in Figure 6.

A twisted pair of wires connect up to 32 drivers and

receivers for half 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

impedance, typically 120Ω. The optional shields around

the twisted pair help reduce unwanted noise, and are

connected to GND at one end.

Cable and Data Rate

The transmission line of choice for RS485 applications is

atwistedpair.Therearecoaxialcables(twinaxial)madefor

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.

Thermal Shutdown

The LTC486 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

5

LTC486

W

I FOR ATIO

S

EN

U

O U

PPLICATI

A

EN

SHIELD

4

3

4

2

3

1

120Ω

DX

RX

DX

RX

2

1

12

EN

1/4 LTC486

EN

1/4 LTC488

4

3

1

3

1

DX

RX

2

12

EN

1/4 LTC486

12

EN

1/4 LTC488

LTC486 • TA07

Figure 6. Typical Connection

10k

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 dielec-

tric losses are of the same order of magnitude, with

relatively low overall loss (Figure 7).

1k

100

10

10k

100k

1M 2.5M

10M

DATA RATE (bps)

1

LTC486 • TA09

Figure 8. Cable Length vs Data Rate

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).

0.1

1

10

100

FREQUENCY (MHz)

LTC486 • TA08

Figure 7. Attenuation vs Frequency for Belden 9841

When using low loss cables, Figure 8 can be used as a

guidelineforchoosingthemaximumlinelengthforagiven

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.

If the cable is loaded excessively (e.g., 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

(about1.5ns/ft). If the cable is lightly loaded (e.g., 470Ω),

Cable Termination

Theproperterminationofthecableisveryimportant.Ifthe

cable is not terminated with its characteristic impedance,

6

LTC486

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PPLICATI

A

PROBE HERE

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, 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

DX

DRIVER

RECEIVER

RX

Rt = 120Ω

Rt = 47Ω

Receiver Open-Circuit Fail-Safe

Some data encoding schemes require that the output of

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

when the data is finished transmitting and all drivers on

the line are forced into three-state. All LTC RS485

receivers have a fail-safe feature which guarantees the

output to be in a logic 1 state when the receiver inputs

areleftfloating(open-circuit). However, whenthecable

is terminated with 120Ω, the differential inputs to the

receiver are shorted together, not left floating.

Rt = 470Ω

LTC486 • TA10

Figure 9. Termination Effects

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 ft. of cable.

AC Cable Termination

If the receiver output must be forced to a known state,

the circuits of Figure 11 can be used.

Cable termination resistors are necessary to prevent un-

wanted reflections, but they consume power. The typical

differentialoutputvoltageofthedriveris2Vwhenthecable

is terminated with two 120Ω resistors. When no data is

being sent 33mA of DC current flows in the cable . This DC

current is about 220 times greater than the supply current

of the LTC486. One way to eliminate the unwanted current

is by AC coupling the termination resistors as shown in

Figure 10.

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 and the second about 8mW. The lowest power

5V

110Ω

130Ω

RECEIVER

RX

5V

120Ω

1.5k

RECEIVER

RX

C

140Ω

1.5k

C = LINE LENGTH (FT) × 16.3pF

LTC486 • TA11

C

100k

Figure 10. AC Coupled Termination

5V

120Ω

The coupling capacitor allows high frequency energy to

flow to the termination, but blocks 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

RECEIVER

RX

LTC486 • TA12

Figure 11. Forcing “0” When All Dirvers Are Off

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

LTC486

PPLICATI

solution is to use an AC termination with a pull-up resistor.

Simply swap the receiver inputs for data protocols ending

in logic 1.

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S

A

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.

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,someapplicationsneedgreater

protection. The best protection method is to connect a

bidirectionalTransZorb^®fromeachlinesidepintoground

(Figure 12).

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

Y

120Ω

DRIVER

Z

LTC486 • TA13

Figure 12. ESD Protection

TransZorb^®is a registrated trademark of General Instruments, GSI

U

O

TYPICAL APPLICATI

RS232 to RS485 Level Translator with Hysteresis

R = 220k

Y

10k

120Ω

RS232 IN

5.6k

DRIVER

VY - VZ

R

19k

Z

1/4 LTC486

———— ——

HYSTERESIS = 10k ×

≈

R

LTC486 • TA14

U

PACKAGE DESCRIPTIO Dimensions in inches (millimeters) unless otherwise noted.

0.770

(19.558)

MAX

0.300 – 0.325

0.130 ± 0.005

0.045 – 0.065

(7.620 – 8.255)

(3.302 ± 0.127)

(1.143 – 1.651)

14

12

10

9

8

15

13

11

16

0.015

(0.381)

MIN

0.260 ± 0.010

(6.604 ± 0.254)

N Package

16-Lead Plastic DIP

0.065

(1.651)

TYP

0.009 – 0.015

(0.229 – 0.381)

+0.025

–0.015

2

1

3

4

6

5

7

0.325

0.125

(3.175)

MIN

0.045 ± 0.015

(1.143 ± 0.381)

0.018 ± 0.003

(0.457 ± 0.076)

+0.635

8.255

(

)

–0.381

0.100 ± 0.010

(2.540 ± 0.254)

0.398 – 0.413

(10.109 – 10.490)

(NOTE 2)

0.291 – 0.299

(7.391 – 7.595)

(NOTE 2)

0.037 – 0.045

(0.940 – 1.143)

0.093 – 0.104

(2.362 – 2.642)

15 14

12

10

9

16

13

11

0.005

(0.127)

RAD MIN

0.010 – 0.029

× 45°

(0.254 – 0.737)

S Package

16-Lead Plastic SOL

0° – 8° TYP

0.050

(1.270)

TYP

0.394 – 0.419

(10.007 – 10.643)

NOTE 1

0.004 – 0.012

(0.102 – 0.305)

0.009 – 0.013

(0.229 – 0.330)

NOTE 1

0.014 – 0.019

0.016 – 0.050

(0.356 – 0.482)

TYP

(0.406 – 1.270)

NOTE:

1. PIN 1 IDENT, NOTCH ON TOP AND CAVITIES ON THE BOTTOM OF PACKAGES ARE THE MANUFACTURING OPTIONS.

THE PART MAY BE SUPPLIED WITH OR WITHOUT ANY OF THE OPTIONS.

2. THESE DIMENSIONS DO NOT INCLUDE MOLD FLASH OR PROTRUSIONS.

MOLD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.006 INCH (0.15mm).

2

3

5

7

8

1

4

6

LT/GP 0294 5K REV A • PRINTED IN USA

LINEAR TECHNOLOGY CORPORATION 1994

Linear Technology Corporation

1630 McCarthy Blvd., Milpitas, CA 95035-7487

8

●

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


型号：	LTC486CN
厂家：	Linear
描述：	Quad Low Power RS485 Driver 四通道，低功耗RS485驱动器线路驱动器或接收器驱动程序和接口接口集成电路光电二极管
文件：	总8页 (文件大小：229K)
中文：	中文翻译
下载：	下载PDF数据表文档文件

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