LTC2855HDE#TRPBF_技术文档

LTC2854/LTC2855

3.3V 20Mbps RS485/RS422

Transceivers with Integrated

Switchable Termination

DESCRIPTION

FEATURES

The LTC^®2854 and LTC2855 are low power, 20Mbps

RS485/RS422 transceivers operating on 3.3V supplies.

The receiver includes a logic-selectable 120Ω termina-

tion, one-eighth unit load supporting up to 256 nodes per

bus (C, I-Grade), and a failsafe feature that guarantees a

high output state under conditions of ﬂoating or shorted

inputs.

■

Integrated, Logic-Selectable 120Ω Termination

Resistor

■

3.3V Supply Voltage

■

20Mbps Maximum Data Rate

■

No Damage or Latchup Up to 2ꢀ5V ꢁHM

■

ꢁigh Input Impedance Supports 2ꢀ6 Nodes

(C, I-Grade)

Operation Up to 12ꢀ°C (ꢁ-Grade)

■

The driver maintains a high output impedance over the

entire common mode range when disabled or when the

supplyisremoved.Excessivepowerdissipationcausedby

bus contention or a fault is prevented by current limiting

all outputs and by a thermal shutdown.

■

Guaranteed Failsafe Receiver Operation Over the

Entire Common Mode Range

■

Current Limited Drivers and Thermal Shutdown

■

Delayed Micropower Shutdown: 5μA Maximum

(C, I-Grade)

■

EnhancedESDprotectionallowstheLTC2854towithstand

25ꢀV(humanbodymodel)andtheLTC2855towithstand

15ꢀV on the transceiver interface pins without latchup

or damage.

Power Up/Down Glitch-Free Driver Outputs

■

Low Operating Current: 370μA Typical in

Receive Mode

■

Compatible with TIA/EIA-485-A Speciﬁcations

■

Available in 10-Pin 3mm × 3mm DFN, 12-Pin

4mm × 3mm DFN and 16-Pin SSOP Pacꢀages

PRODUCT SELECTION GUIDE

APPLICATIONS

PART NUMHER

DUPLEX

PACKAGE

LTC2854

HALF

DFN-10

■

Low Power RS485/RS422 Transceiver

Level Translator

LTC2855

FULL

SSOP-16, DFN-12

■

Bacꢀplane Transceiver

, LT, LTC and LTM are registered trademarꢀs of Linear Technology Corporation.

All other trademarꢀs are the property of their respective owners.

TYPICAL APPLICATION

LTC28ꢀ4 at 20Mbps into ꢀ4

Ω

LTC2854

RO

RE

TE

R

RO

RE

TE

DI

120Ω

DE

120Ω

A

B

D

DI

2V/DIV

A-B

285455 TA01

LTC2854

120Ω

285455 TA01b

R

20ns/DIV

D

RO RE TE DE DI

285455fb

1

LTC2854/LTC2855

ABSOLUTE MAXIMUM RATINGS ^{(Note 1)}

Supply Voltage (V ) ................................... –0.3V to 7V

Operating Temperature (Note 4)

CC

Logic Input Voltages (RE, DE, DI, TE)............ –0.3V to 7V

LTC2854C, LTC2855C .............................. 0°C to 70°C

LTC2854I, LTC2855I............................. –40°C to 85°C

LTC2854H, LTC2855H........................ –40°C to 125°C

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

Lead Temperature (Soldering, 10 sec)

Interface I/O:

A, B, Y, Z...................................... (V –15V) to +15V

CC

(A-B) or (B-A) with Terminator Enabled..................6V

Receiver Output Voltage (RO)........ –0.3V to (V +0.3V)

CC

GN Pacꢀage ...................................................... 300°C

PIN CONFIGURATION

TOP VIEW

1

2

3

4

5

6

7

8

16

15

14

13

12

11

10

9

RO

RE

V

A

B

Z

CC

RO

RE

1

2

3

4

5

6

12

11

10

9

V

A

B

Z

Y

CC

RO

RE

DE

DI

1

2

3

4

5

10

9

V

B

A

CC

DE

13

11

DI

8

DI

TE

7

NC

Y

TE

8

TE

6

GND

NC

GND

7

NC

DD PACKAGE

10-LEAD (3mm × 3mm) PLASTIC DFN

DE PACKAGE

12-LEAD (4mm × 3mm) PLASTIC DFN

EXPOSED PAD (PIN 11) PCB GND CONNECTION

GN PACKAGE

16-LEAD (NARROW 0.150) PLASTIC SSOP

T

JMAX

= 150°C, θ = 43°C/W

EXPOSED PAD (PIN 13) PCB GND CONNECTION

JA

θ

= 2.96°C/W

T

JMAX

= 150°C, θ = 44°C/W

JC

JA

T

= 150°C, θ = 110°C/W

JMAX

JA

θ

= 4.3°C/W

JC

θ

= 40°C/W

JC

ORDER INFORMATION

LEAD FREE FINISꢁ

LTC2854CDD#PBF

LTC2854IDD#PBF

LTC2854HDD#PBF

LTC2855CDE#PBF

LTC2855IDE#PBF

LTC2855HDE#PBF

LTC2855CGN#PBF

LTC2855IGN#PBF

LTC2855HGN#PBF

LEAD HASED FINISꢁ

LTC2854CDD

TAPE AND REEL

PART MARKING*

LCQG

2855

2855I

2855H

PART MARKING*

LCQG

2855

PACKAGE DESCRIPTION

TEMPERATURE RANGE

0°C to 70°C

–40°C to 85°C

–40°C to 125°C

0°C to 70°C

–40°C to 85°C

–40°C to 125°C

0°C to 70°C

–40°C to 85°C

–40°C to 125°C

TEMPERATURE RANGE

0°C to 70°C

–40°C to 85°C

–40°C to 125°C

0°C to 70°C

–40°C to 85°C

–40°C to 125°C

0°C to 70°C

LTC2854CDD#TRPBF

LTC2854IDD#TRPBF

LTC2854HDD#TRPBF

LTC2855CDE#TRPBF

LTC2855IDE#TRPBF

LTC2855HDE#TRPBF

LTC2855CGN#TRPBF

LTC2855IGN#TRPBF

LTC2855HGN#TRPBF

TAPE AND REEL

10-Lead (3mm × 3mm) Plastic DFN

12-Lead (4mm × 3mm) Plastic DFN

16-Lead (Narrow 0.150) Plastic SSOP

PACKAGE DESCRIPTION

10-Lead (3mm × 3mm) Plastic DFN

12-Lead (4mm × 3mm) Plastic DFN

16-Lead (Narrow 0.150) Plastic SSOP

LTC2854CDD#TR

LTC2854IDD#TR

LTC2854IDD

LTC2854HDD

LTC2855CDE

LTC2854HDD#TR

LTC2855CDE#TR

LTC2855IDE

LTC2855IDE#TR

LTC2855HDE

LTC2855CGN

LTC2855IGN

LTC2855HDE#TR

LTC2855CGN#TR

LTC2855IGN#TR

2855I

2855H

–40°C to 85°C

–40°C to 125°C

LTC2855HGN

LTC2855HGN#TR

Consult LTC Marꢀeting for parts speciﬁed with wider operating temperature ranges. *The temperature grade is identiﬁed by a label on the shipping container.

For more information on lead free part marꢀing, go to: http://www.linear.com/leadfree/

For more information on tape and reel speciﬁcations, go to: http://www.linear.com/tapeandreel/

285455fb

2

LTC2854/LTC2855

ELECTRICAL CHARACTERISTICS ^The● denotes the speciﬁcations which apply over the full operating

temperature range, otherwise speciﬁcations are at T_A= 2ꢀ°C, V_CC= 3.3V unless otherwise noted (Note 2).

SYMHOL

Driver

PARAMETER

CONDITIONS

MIN

TYP

MAX

UNITS

●

|V

|

OD

Differential Driver Output Voltage

R = ∞, V = 3V (Figure 1)

V

CC

R = 27Ω, V = 3V (Figure 1)

1.5

2

CC

R = 50Ω, V = 3.13V (Figure 1)

CC

●

Δ|V

|

OD

Change in Magnitude of Driver

Differential Output Voltage for

Complementary Output States

R = 27Ω or R = 50Ω (Figure 1)

0.2

V

●

V

Driver Common Mode Output Voltage

R = 27Ω or R = 50Ω (Figure 1)

3

V

OC

Δ|V

|

Change in Magnitude of Driver

Common Mode Output Voltage for

Complementary Output States

0.2

OC

●

I

Driver Three-State (High Impedance)

Output Current on Y and Z

DE = OV, (Y or Z) = –7V, 12V (LTC2855)

–7V ≤ (Y or Z) ≤ 12V (Figure 2)

10

μA

OZD

Maximum Driver Short-Circuit Current

180

250

300

mA

OSD

–250

–100

Receiver

●

I

Receiver Input Current (A, B)

DE = TE = 0V, V = 0V or 3.3V, V = 12V

125

250

μA

IN

CC

IN

(Figure 3) (C, I-Grade)

DE = TE = 0V, V = 0V or 3.3V, V = –7V,

CC

IN

(Figure 3) (C, I-Grade)

●

DE = TE = 0V, V = 0V or 3.3V, V = 12V

μA

CC

IN

(Figure 3) (H-Grade)

DE = TE = 0V, V = 0V or 3.3V, V = –7V,

–145

96

CC

IN

(Figure 3) (H-Grade)

●

R

Receiver Input Resistance

RE = V or 0V, DE = TE = 0V,

125

ꢀΩ

V

IN

CC

V

= –7V, –3V, 3V, 7V, 12V (Figure 3)

IN

(C, I-Grade)

RE = V or 0V, DE = TE = 0V,

48

CC

V

= –7V, –3V, 3V, 7V, 12V (Figure 3)

IN

(H-Grade)

V

Receiver Differential Input Threshold

Voltage

–7V ≤ B ≤ 12V

0.2

TH

ΔV

Receiver Input Hysteresis

B = 0V

25

mV

V

TH

●

V

OH

V

OL

Receiver Output HIGH Voltage

Receiver Output LOW Voltage

I(RO) = –4mA, A-B = 200mV, V = 3V

2.4

CC

I(RO) = 4mA, A-B = –200mV, V = 3V

0.4

1

V

CC

I

Receiver Three-State (High Impedance) RE = V , 0V ≤ RO ≤ V

Output Current on RO

μA

OZR

CC

●

I

Receiver Short-Circuit Current

0V ≤ RO ≤ V

85

mA

Ω

OSR

CC

R

Receiver Input Terminating Resistor

TE = V , V = 2V, V = –7V, 0V, 10V

108

2

120

156

TERM

CC AB

B

(Figure 8)

Logic

●

V

Logic Input High Voltage

Logic Input Low Voltage

Logic Input Current

V

= 3.6V

V

IH

IL

CC

= 3V

0.8

10

CC

I

0

μA

INL

Supplies

I

Supply Current in Shutdown Mode

Supply Current in Receive Mode

DE = 0V, RE = V , TE = 0V

CCS

CCR

CC

●

(C, I-Grade)

(H-Grade)

0

5

μA

15

●

I

DE = 0V, RE = 0V, TE = 0V

370

900

μA

285455fb

3

LTC2854/LTC2855

ELECTRICAL CHARACTERISTICS

The ● denotes the speciﬁcations which apply over the full operating

temperature range, otherwise speciﬁcations are at T_A= 2ꢀ°C, V_CC= 3.3V, T_E= 0V unless otherwise noted (Note 2).

SYMHOL

PARAMETER

CONDITIONS

No Load, DE = V , RE = V , TE = 0V

MIN

TYP

450

MAX

1000

UNITS

μA

●

I

Supply Current in Transmit Mode

CCT

CC

Supply Current with Both Driver and

Receiver Enabled

No Load, DE = V , RE = 0V, TE = 0V

μA

CCTR

CC

●

I

Supply Current in Termination Mode

DE = 0V, RE = V , TE = V

CC

110

450

180

950

μA

CCTERM

CC

Supply Current in Receive and

Termination Mode

DE = 0V, RE = 0V, TE = V

CC

CCTERMR

●

I

Supply Current in Transmit and

Termination Mode

DE = V , RE = V , TE = V

CC

470

1000

μA

CCTERMT

CC

Supply Current with Driver, Receiver

and Termination Enabled

DE = V , RE = 0V, TE = V

CC CC

CCTERMTR

ESD Protection

ESD Protection for RS485/RS422 Pins

A, B on LTC2854, Human Body Model

25

15

ꢀV

Y, Z, A, B on LTC2855, Human Body Model

SWITCHING CHARACTERISTICS ^The● denotes the speciﬁcations which apply over the full operating

temperature range, otherwise speciﬁcations are at T_A= 2ꢀ°C, V_CC= 3.3V, T_E= 0V unless otherwise noted (Note 2).

SYMHOL

Driver

PARAMETER

CONDITIONS

MIN

TYP

MAX

UNITS

●

f

t

Maximum Data Rate

Note 3

20

Mbps

ns

MAX

, t

Driver Input to Output

R

DIFF

R

DIFF

= 54Ω, C = 100pF (Figure 4)

10

1

50

6

PLHD PHLD

L

Δt

Driver Input to Output Difference

= 54Ω, C = 100pF (Figure 4)

ns

PD

L

|t

-t

|

PLHD PHLD

●

t

Driver Output Y to Output Z

Driver Rise or Fall Time

Driver Enable or Disable Time

Driver Enable from Shutdown

Time to Shutdown

R

= 54Ω, C = 100pF (Figure 4)

1

4

6

12.5

70

ns

μs

ns

SKEWD

DIFF

L

, t

RD FD

= 54Ω, C = 100pF (Figure 4)

L

DIFF

, t , t , t

ZLD ZHD LZD HZD

R = 500Ω, C = 50pF, RE = 0 (Figure 5)

L L

, t

R = 500Ω, C = 50pF, RE = V (Figure 5)

L

8

ZHSD ZLSD

L

CC

R = 500Ω, C = 50pF

100

SHDN

L

(DE = ↓, RE = V ) or (DE = 0, RE ↑)

CC

(Figure 5)

Receiver

●

t

, t

Receiver Input to Output

Differential Receiver Sꢀew

C = 15pF, V = 1.5V, |V | = 1.5V, t and t

F

50

1

70

6

ns

PLHR PHLR

L

CM

AB

R

< 4ns (Figure 6)

t

C = 15pF (Figure 6)

L

SKEWR

|t

-t

|

PLHR PHLR

●

t

, t

Receiver Output Rise or Fall Time

Receiver Enable/Disable

C = 15pF (Figure 6)

3

12.5

50

ns

μs

RR FR

L

, t , t , t

ZLR ZHR LZR HZR

R = 1ꢀ, C =15pF, DE = V (Figure 7)

L L CC

, t

Receiver Enable from Shutdown

Termination Enable or Disable Time

R = 1ꢀ, C = 15pF, DE = 0V (Figure 7)

8

ZHSR ZLSR

L

, t

V = 0V, V = 2V, RE = V , DE = 0V

100

RTEN RTZ

B

AB

CC

(Figure 8)

Note 1: Stresses beyond those listed under Absolute Maximum Ratings

may cause permanent damage to the device. Exposure to any Absolute

Maximum Rating condition for extended periods may affect device

reliability and lifetime. High temperatures degrade operating lifetimes.

Operating lifetime is derated at temperatures greater than 105°C

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: Maximum data rate is guaranteed by other measured parameters

and is not tested directly.

Note 4: This IC includes overtemperature protection that is intended

to protect the device during momentary overload conditions.

Overtemperature protection activates at a junction temperature exceeding

150°C. Continuous operation above the speciﬁed maximum operating

junction temperature may result in device degradation or failure.

285455fb

4

LTC2854/LTC2855

TEST CIRCUITS

Y

Z

Y

R

I

GND

OR

CC

OSD

+

OD

–

GND

OR

DI

V

DI

DRIVER

V

CC

Z

+

–7V to +12V

V

OC

–

285455 F01-2

Figure 1. Driver DC Characteristics

Figure 2. Driver Output Short-Circuit Current

I

IN

A OR B

B OR A

RECEIVER

+

V

IN

–

285455 F03

V

I

IN

R

=

IN

Figure 3. Receiver Input Current and Input Resistance

V

CC

t

PHLD

DI

Y, Z

Y-Z

PLHD

Y

Z

0V

C

t

L

SKEWD

DI

DRIVER

R

DIFF

V

1/2 V

O

285455 F04a

90%

0

10%

285455 F04b

t

FD

RD

Figure 4. Driver Timing Measurement

V

CC

DE

Y OR Z

Z OR Y

1/2 V

CC

GND

OR

CC

R

L

Y

0V

t

,

ZLD

t

V

LZD

ZLSD

C

L

V

CC

OL

V

OR

GND

CC

DI

V

1/2 V

O

DRIVER

DE

CC

0.5V

V

CC

L

OR

GND

OH

0V

Z

CC

C

285455 F05b

t

,

t

,

HZD

ZHD

285455 F05a

t

SHDN

ZHSD

Figure ꢀ. Driver Enable and Disable Timing Measurements

285455fb

5

LTC2854/LTC2855

TEST CIRCUITS

t

R

F

V

AB

90%

0

90%

10%

A

A-B

V

/2

AB

10%

–V

RO

t

PHLR

V

RECEIVER

PLHR

CM

B

C

V

L

CC

0

90%

10%

V

90%

AB

1/2 V

CC

RO

V

O

CC

285455 F06a

10%

t

FR

RR

t

= t

– t

PLHR PHLR

SKEWR

285455 F06b

Figure 6. Receiver Propagation Delay Measurements

V

CC

RE

RO

1/2 V

CC

0V

t

,

ZLR

A

B

t

LZR

ZLSR

0V OR V

CC

V

R

V

OR

GND

CC

OL

L

CC

RO

V

O

1/2 V

RECEIVER

RE

CC

0.5V

V

C

V

OR 0V

L

CC

V

OH

1/2 V

CC

0V

285455 F07b

DI = 0V OR V

285455 F07a

CC

t

,

ZHR

HZR

t

ZHSR

Figure 7. Receiver Enable and Disable Timing Measurements

V

AB

R

=

TERM

I

A

I

V

A

CC

A

B

TE

1/2 V

CC

RO

+

–

RECEIVER

V

0V

AB

t

RTEN

t

RTZ

90%

I

A

TE

10%

+

–

B

285455 F08

Figure 8. Termination Resistance and Timing Measurements

285455fb

6

LTC2854/LTC2855

TYPICAL PERFORMANCE CHARACTERISTICS

T_A= 2ꢀ°C, V_CC= 3.3V, unless otherwise noted.

Receiver S5ew

Driver Propagation Delay

vs Temperature

Driver S5ew vs Temperature

vs Temperature

2

18

V

C

= 1.5V

R

C

= 54Ω

DIFF

R

C

= 54Ω

AB

L

DIFF

= 15pF

= 100pF

L

= 100pF

L

16

14

1

0

1

12

10

9

0

–1

6

–1

4

0

20 40 60

120

80 100

–40 –20

0

20 40 60 80 100 120

TEMPERATURE (˚C)

285455 G02

–40 –20

0

20 40 60

120

80 100

–40 –20

TEMPERATURE (˚C)

285455 G01

285455 G03

Driver Output Low/ꢁigh Voltage

vs Output Current

Driver Differential Output Voltage

vs Temperature

R_TERMvs Temperature

135

130

R

DIFF

=

∞

3

V

OH

R

= 100Ω

= 54Ω

DIFF

125

120

115

110

105

2

1

2

1

DIFF

V

OL

100

0

20 40 60

120

80 100

–40 –20

20

30

40

50

70

0

10

60

0

20 40 60

TEMPERATURE (˚C)

120

80 100

–40 –20

TEMPERATURE (˚C)

OUTPUT CURRENT (mA)

285455 G04

285455 G05

285455 G06

Receiver Output Voltage vs

Receiver Propagation Delay

vs Temperature

Output Current (Source and Sin5)

Supply Current vs Data Rate

70

65

60

50

40

30

20

10

0

V

C

= 1.5V

C

= 100pF

AB

L

SOURCE

= 15pF

3

60

R

= 54Ω

DIFF

55

50

45

40

2

1

= 100Ω

DIFF

SINK

R

DIFF

=

∞

35

0

20 40 60

120

80 100

2

3

4

5

6

–40 –20

0

1

0.1

1

10

100

TEMPERATURE (˚C)

DATA RATE (Mbps)

OUTPUT CURRENT (mA)

285455 G08

285455 G07

285455 G09

285455fb

7

LTC2854/LTC2855

PIN FUNCTIONS

(DD/DE/GN)

RO(Pin1):ReceiverOutput.Ifthereceiveroutputisenabled

(RE low) and A > B by 200mV, then RO will be high. If A

< B by 200mV, then RO will be low. If the receiver inputs

are open, shorted, or terminated without a signal, RO will

be high.

TE (Pin ꢀ): Internal Termination Resistance Enable. A high

input will connect a termination resistor (120Ω typical)

between pins A and B.

GND (Pins 6,11/6,13/6): Ground. Pins 11 and 13 are

bacꢀside thermal pad, connected to Ground.

RE (Pin 2): Receiver Enable. A low enables the receiver.

A high input forces the receiver output into a high imped-

ance state.

Y (Pins NA/8/12): Positive Driver Output for LTC2855.

Z (Pins NA/9/13): Negative Driver Output for LTC2855.

DE (Pin 3): Driver Enable. A high on DE enables the driver.

A low input will force the driver outputs into a high imped-

ance. If RE is high with DE and TE low, the part will enter

a low power shutdown state.

H (Pins 9/10/14): Negative Receiver Input (and Negative

Driver Output for LTC2854).

A (Pins 8/11/1ꢀ): Positive Receiver Input (and Positive

Driver Output for LTC2854).

DI (Pin 4): Driver Input. If the driver outputs are enabled

(DE high), then a low on DI forces the driver positive out-

put low and negative output high. A high on DI, with the

driver outputs enabled, forces the driver positive output

high and negative output low.

V

(Pins 10/12/16): Positive Supply. V = 3.0V < V

<

CC

3.6V. Bypass with 0.1μF ceramic capacitor.

285455fb

8

LTC2854/LTC2855

FUNCTION TABLES

LTC28ꢀ4

LOGIC INPUTS

DE

0

RE

0

TE

0

MODE

RECEIVE

A, H

RO

TERMINATOR

R

DRIVEN

HIGH-Z

DRIVEN

OFF

ON

IN

0

1

RECEIVE WITH TERM

SHUTDOWN

R

TERM

0

1

0

R

OFF

ON

IN

0

1

TERM ONLY

R

TERM

1

0

TRANSMIT WITH RECEIVE

DRIVEN

OFF

ON

1

0

1

TRANSMIT WITH RECEIVE

AND TERM

DRIVEN

1

0

1

TRANSMIT

DRIVEN

HIGH-Z

OFF

ON

TRANSMIT WITH TERM

LTC28ꢀꢀ

LOGIC INPUTS

DE

0

RE

0

TE

0

MODE

RECEIVE

A, H

Y, Z

RO

TERMINATOR

R

HIGH-Z

DRIVEN

HIGH-Z

DRIVEN

OFF

ON

IN

0

1

RECEIVE WITH TERM

SHUTDOWN

R

TERM

0

1

0

R

OFF

ON

IN

0

1

TERM ONLY

R

TERM

1

0

TRANSMIT WITH RECEIVE

R

OFF

ON

IN

1

0

1

TRANSMIT WITH RECEIVE

AND TERM

R

TERM

1

0

1

TRANSMIT

R

DRIVEN

HIGH-Z

OFF

ON

IN

TRANSMIT WITH TERM

R

TERM

BLOCK DIAGRAMS

LTC28ꢀ4

LTC28ꢀꢀ

V

CC

V

CC

A

25ꢀV

A

(15ꢀV)

RE

SLEEP/SHUTDOWN

LOGIC AND DELAY

SLEEP/SHUTDOWN

LOGIC AND DELAY

120Ω

TERM

120Ω

TERM

DE

R

DE

R

TE

125ꢀ

IN

125ꢀ

IN

R

RO

RECEIVER

B

25ꢀV

B

(15ꢀV)

125ꢀ

IN

R

IN

R

Z

(15ꢀV)

DI

DRIVER

Y

(15ꢀV)

GND

285455 BD

285455fb

9

LTC2854/LTC2855

APPLICATIONS INFORMATION

Driver

the inputs are either shorted, left open, or terminated

(externally or internally), but not driven. This failsafe fea-

ture is guaranteed to worꢀ for inputs spanning the entire

common mode range of –7V to +12V.

ThedriverprovidesfullRS485/RS422compatibility.When

enabled, if DI is high, Y-Z is positive for the full-duplex

device (LTC2855) and A-B is positive for the half-duplex

device (LTC2854).

The receiver output is internally driven high (to V ) or

CC

low (to ground) with no external pull-up needed. When

the receiver is disabled the RO pin becomes high-Z with

leaꢀage of less than 1μA for voltages within the supply

range.

When the driver is disabled, both outputs are high-

impedance. For the full-duplex LTC2855, the leaꢀage on

the driver output pins is guaranteed to be less than 10μA

over the entire common mode range of –7V to +12V. On

the half-duplex LTC2854, the impedance is dominated by

Receiver Input Resistance

the receiver input resistance, R .

IN

ThereceiverinputresistancefromAorBtogroundisguar-

anteed to be greater than 96ꢀ (C, I-Grade) when the termi-

nation is disabled. This is 8X higher than the requirements

fortheRS485standardandthusthisreceiverrepresentsa

one-eighth unit load. This, in turn, means that 8X the

standard number of receivers, or 256 total, can be con-

nected to a line without loading it beyond what is called

out in the RS485 standard. The receiver input resistance

from A or B to ground on high temperature H-Grade parts

is greater than 48ꢀ providing a one-quarter unit load. The

input resistance of the receivers is unaffected by enabling/

disabling the receiver and by powering/unpowering the

part. The equivalent input resistance looꢀing into A and B

is shown in Figure 9. The termination resistor cannot be

enabled by TE if the device is unpowered or in thermal

shutdown mode.

Driver Overvoltage and Overcurrent Protection

The driver outputs are protected from short-circuits to

any voltage within the Absolute Maximum range of (V

CC

–15V) to +15V. The typical peaꢀ current in this condition

does not exceed 180mA.

If a high driver output is shorted to a voltage just above

V , a reverse current will ﬂow into the supply. When this

CC

voltageexceedsV byabout1.4V,thereversecurrentturns

CC

off. Preventing the driver from turning off with outputs

shorted to output voltages just above V ꢀeeps the driver

CC

activeevenforreceiverloadsthathaveapositivecommon

mode with respect to the driver—a valid condition.

Theworst-casepeaꢀreverseshort-circuitcurrentcanbeas

high as 300mA in extreme cold conditions. If this current

cannot be absorbed by the supply, a 3.6V Zener diode can

be added in parallel with the supply to sinꢀ this current.

>96ꢀΩ

A

All devices also feature thermal shutdown protection that

disablesthedriverandreceiverincaseofexcessivepower

dissipation (see Note 4).

60Ω

TE

Receiver and Failsafe

60Ω

>96ꢀΩ

With the receiver enabled, when the absolute value of the

differentialvoltagebetweentheAandBpinsisgreaterthan

200mV, the state of RO will reﬂect the polarity of (A-B).

B

285455 F09

Figure 9. Equivalent Input Resistance into A and H

(on the LTC28ꢀ4, Valid if Driver is Disabled)

The LTC2854/LTC2855 have a failsafe feature that guaran-

tees the receiver output to be in a logic-high state when

285455fb

10

LTC2854/LTC2855

APPLICATIONS INFORMATION

Switchable Termination

WhentheTEpinishigh,theterminationresistorisenabled

and the differential resistance from A to B is 120Ω. Figure

10 shows the I/V characteristics between pins A and B

with the termination resistor enabled and disabled. The

resistance is maintained over the entire RS485 common

mode range of –7V to +12V as shown in Figure 11.

Proper cable termination is very important for good

signal ﬁdelity. If the cable is not terminated with its char-

acteristic impedance, reﬂections will result in distorted

waveforms.

The LTC2854/LTC2855 are the ﬁrst 3.3V RS485/RS422

transceivers to offer integrated switchable termination

resistors on the receiver input pins. This provides the

advantage of being able to easily change, through logic

control,thelineterminationforoptimalperformancewhen

conﬁguring transceiver networꢀs.

The integrated termination resistor has a high frequency

responsewhichdoesnotlimitperformanceatthemaximum

speciﬁed data rate. Figure 12 shows the magnitude and

phase of the termination impedance vs frequency.

150

V

= 2V

AB

140

130

120

110

100

–10

–5

0

5

10

15

COMMON MODE VOLTAGE (V)

Figure 10. Curve Trace Hetween A and H

with Termination Enabled and Disabled

285455 F11

Figure 11. Typical Resistance of the Enabled

Terminator vs Voltage on H Pin

185

170

155

140

30

15

PHASE

0

–15

–30

–45

–60

MAGNITUDE

125

110

95

80

10

–75

1

–1

0

10

FREQUENCY (MHz)

10

285455 F12

Figure 12. Termination Magnitude

and Phase vs Frequency

285455fb

11

LTC2854/LTC2855

APPLICATIONS INFORMATION

Supply Current

the full duplex LTC2855, DI and A/B should not be routed

near the driver or receiver outputs.

The unloaded static supply currents in the LTC2854/

LTC2855 are very low —typically under 500μA for all

modes of operation. In applications with resistively ter-

minated cables, the supply current is dominated by the

driver load. For example, when using two 120Ω termina-

tors with a differential driver output voltage of 2V, the DC

current is 33mA, which is sourced by the positive voltage

supply. This is true whether the terminators are external

or internal such as in the LTC2854/LTC2855. Power sup-

ply current increases with toggling rate due to capacitive

loading and this term can increase signiﬁcantly at high

data rates. Figure 13 shows supply current vs data rate for

two different capacitive loads for the circuit conﬁguration

of Figure 4.

The logic inputs of the LTC2854/LTC2855 have 150mV of

hysteresis to provide noise immunity. Fast edges on the

outputscancauseglitchesinthegroundandpowersupplies

whichareexacerbatedbycapacitiveloading.Ifalogicinput

is held near its threshold (typically 1.5V), a noise glitch

from a driver transition may exceed the hysteresis levels

on the logic and data input pins causing an unintended

state change. This can be avoided by maintaining normal

logic levels on the pins and by slewing inputs through

their thresholds by faster than 1V/μs when transitioning.

Good supply decoupling and proper line termination also

reduces glitches caused by driver transitions.

Cable Length vs Data Rate

80

R

= 54Ω

DIFF

For a given data rate, the maximum transmission distance

is bounded by the cable properties. A typical curve of

cablelengthvsdataratecompliantwiththeRS485/RS422

standards is shown in Figure 14. Three regions of this

curve reﬂect different performance limiting factors in data

transmission. In the ﬂat region of the curve, maximum

distanceisdeterminedbyresistivelossesinthecable. The

downwardslopingregionrepresentslimitsindistanceand

data rate due to AC losses in the cable. The solid vertical

line represents the speciﬁed maximum data rate in the

RS485/RS422 standards. The dashed lines at 20Mbps

show the maximum data rates of the LTC2854/LTC2855.

70

60

50

40

30

20

C

= 1000pF

L

C

= 100pF

L

0.1

1

10

100

DATA RATE (Mbps)

285455 F13

Figure 13. Supply Current vs Data Rate

10ꢀ

ꢁigh Speed Considerations

A ground plane layout is recommended for the LTC2854/

LTC2855. A 0.1μF bypass capacitor less than one-quarter

1ꢀ

inch away from the V pin is also recommended. The PC

CC

LTC2854/LTC2855

MAX DATA RATE

board traces connected to signals A/B and Z/Y (LTC2855)

shouldbesymmetricalandasshortaspossibletomaintain

good differential signal integrity. To minimize capacitive

effects,thedifferentialsignalsshouldbeseparatedbymore

than the width of a trace and should not be routed on top

of each other if they are on different signal planes.

100

RS485/RS422

MAX DATA RATE

10

10ꢀ

100ꢀ

1M

10M

100M

DATA RATE (bps)

285455 F14

Care should be taꢀen to route outputs away from any

sensitive inputs to reduce feedbacꢀ effects that might

cause noise, jitter, or even oscillations. For example, in

Figure 14. Cable Length vs Data Rate (RS48ꢀ/

RS422 Standards Shown in Vertical Solid Line)

285455fb

12

LTC2854/LTC2855

TYPICAL APPLICATION

Failsafe “0” Application (Idle State = Logic “0”)

V

CC

100ꢀΩ

LTC2854

R

RO

DI

I1

B

A

"A"

"B"

D

I2

285455 TA03

PACKAGE DESCRIPTION

DD Pac5age

10-Lead Plastic DFN (3mm × 3mm)

(Reference LTC DWG # 05-08-1699)

R = 0.115

0.38 ± 0.10

TYP

6

10

0.675 ±0.05

3.50 ±0.05

2.15 ±0.05 (2 SIDES)

1.65 ±0.05

3.00 ±0.10

(4 SIDES)

1.65 ± 0.10

(2 SIDES)

PIN 1

PACKAGE

OUTLINE

TOP MARK

(SEE NOTE 6)

(DD) DFN 1103

5

1

0.25 ± 0.05

0.50 BSC

0.75 ±0.05

0.200 REF

0.25 ± 0.05

0.50

BSC

2.38 ±0.10

(2 SIDES)

2.38 ±0.05

(2 SIDES)

0.00 – 0.05

BOTTOM VIEW—EXPOSED PAD

RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS

NOTE:

1. DRAWING TO BE MADE A JEDEC PACKAGE OUTLINE M0-229 VARIATION OF (WEED-2).

CHECK THE LTC WEBSITE DATA SHEET FOR CURRENT STATUS OF VARIATION ASSIGNMENT

2. DRAWING NOT TO SCALE

3. ALL DIMENSIONS ARE IN MILLIMETERS

4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE

MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE

5. EXPOSED PAD SHALL BE SOLDER PLATED

6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION ON THE

TOP AND BOTTOM OF PACKAGE

285455fb

13

LTC2854/LTC2855

PACKAGE DESCRIPTION

DE/UE Package

12-Lead Plastic DFN (4mm × 3mm)

(Reference LTC DWG # 05-08-1695 Rev D)

0.70 ±0.05

3.30 ±0.05

3.60 ±0.05

2.20 ±0.05

1.70 ± 0.05

PACKAGE OUTLINE

0.25 ± 0.05

0.50 BSC

2.50 REF

RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS

APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED

0.40 ± 0.10

4.00 ±0.10

(2 SIDES)

R = 0.115

TYP

7

12

R = 0.05

TYP

3.30 ±0.10

3.00 ±0.10

(2 SIDES)

1.70 ± 0.10

PIN 1

TOP MARK

(NOTE 6)

PIN 1 NOTCH

R = 0.20 OR

0.35 × 45°

CHAMFER

(UE12/DE12) DFN 0806 REV D

6

1

0.25 ± 0.05

0.75 ±0.05

0.200 REF

0.50 BSC

2.50 REF

BOTTOM VIEW—EXPOSED PAD

0.00 – 0.05

NOTE:

1. DRAWING PROPOSED TO BE A VARIATION OF VERSION

(WGED) IN JEDEC PACKAGE OUTLINE M0-229

2. DRAWING NOT TO SCALE

3. ALL DIMENSIONS ARE IN MILLIMETERS

4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE

MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE

5. EXPOSED PAD SHALL BE SOLDER PLATED

6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION

ON THE TOP AND BOTTOM OF PACKAGE

285455fb

14

LTC2854/LTC2855

PACKAGE DESCRIPTION

GN Pac5age

16-Lead Plastic SSOP (Narrow .1ꢀ0 Inch)

(Reference LTC DWG # 05-08-1641)

.189 – .196*

(4.801 – 4.978)

.045 ±.005

.009

(0.229)

REF

16 15 14 13 12 11 10 9

.254 MIN

.150 – .165

.229 – .244

.150 – .157**

(5.817 – 6.198)

(3.810 – 3.988)

.0165 ±.0015

.0250 BSC

RECOMMENDED SOLDER PAD LAYOUT

1

2

3

4

5

6

7

8

.015 ± .004

(0.38 ± 0.10)

× 45°

.0532 – .0688

(1.35 – 1.75)

.004 – .0098

(0.102 – 0.249)

.007 – .0098

(0.178 – 0.249)

0° – 8° TYP

.016 – .050

(0.406 – 1.270)

.0250

(0.635)

BSC

.008 – .012

GN16 (SSOP) 0204

(0.203 – 0.305)

TYP

NOTE:

*DIMENSION DOES NOT INCLUDE MOLD FLASH. MOLD FLASH

SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE

1. CONTROLLING DIMENSION: INCHES

INCHES

2. DIMENSIONS ARE IN

(MILLIMETERS)

**DIMENSION DOES NOT INCLUDE INTERLEAD FLASH. INTERLEAD

FLASH SHALL NOT EXCEED 0.010" (0.254mm) PER SIDE

3. DRAWING NOT TO SCALE

285455fb

Information furnished by Linear Technology Corporation is believed to be accurate and reliable.

However, no responsibility is assumed for its use. Linear Technology Corporation maꢀes no representa-

tion that the interconnection of its circuits as described herein will not infringe on existing patent rights.

15

LTC2854/LTC2855

TYPICAL APPLICATION

Multi-Node Networ5 with End Termination Using the LTC28ꢀ4

TE = 0V

D

R

LTC2854

R

TE = 3.3V

D

285455 TA04

RELATED PARTS

PART NUMHER

LTC485

DESCRIPTION

COMMENTS

Low Power RS485 Interface Transceiver

Differential Driver and Receiver Pair

3.3V Ultralow Power RS485 Transceiver

I

= 300μA (Typ)

CC

LTC491

I

= 300μA

LTC1480

LTC1483

LTC1485

LTC1487

3.3V Operation

Ultralow Power RS485 Low EMI Transceiver

Differential Bus Transceiver

Controlled Driver Slew Rate

10Mbps Operation

Ultralow Power RS485 with Low EMI, Shutdown and High

Input Impedance

Up to 256 Transceivers on the Bus

LTC1520

LTC1535

LTC1685

LT1785

50Mbps Precision Quad Line Receiver

Isolated RS485 Full-Duplex Transceiver

52Mbps RS485 Transceiver with Precision Delay

60V Fault Protected RS485 Transceiver

Channel-to-Channel Sꢀew 400ps (Typ)

2500V

Isolation in Surface Mount Pacꢀage

RMS

Propagation Delay Sꢀew 500ps (Typ)

60V Tolerant, 15ꢀV ESD

LTC2856/LTC2857/ 20Mbps and Slew Rate-Limited, 15ꢀV RS485/RS422

LTC2858 Transceiver

Up to 256 Transceivers on the Bus

LTC2859/LTC2861 20Mbps RS485 Transceiver with Integrated Switchable

Termination

5V Integrated, Switchable, 120Ω Termination Resistor, 15ꢀV ESD

285455fb

LT 0308 REV B • PRINTED IN USA

LinearTechnology Corporation

1630 McCarthy Blvd., Milpitas, CA 95035-7417

16

●

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


型号：	LTC2855HDE#TRPBF
厂家：	Linear
描述：	LTC2855 - 3.3V 20Mbps RS485/RS422 Transceiver with Integrated Switchable Termination; Package: DFN; Pins: 12; Temperature Range: -40°C to 125°C 驱动光电二极管接口集成电路驱动器
文件：	总16页 (文件大小：215K)
中文：	中文翻译
下载：	下载PDF数据表文档文件

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