ltc2872 - rs232/rs485 dual multiprotocol transceiver with … · 2020-02-01 · ltc2872 1 2872f...

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LTC2872 1 2872f TYPICAL APPLICATIONS DESCRIPTION RS232/RS485 Dual Multiprotocol Transceiver with Integrated Termination The LTC ® 2872 is a robust pin-configurable transceiver that supports RS232, RS485, and RS422 standards while operating on a single 3V to 5.5V supply. The LTC2872 can be configured as four RS232 single-ended transceivers or two RS485 differential transceivers, or combinations of both, on shared I/O lines. Pin-controlled integrated termination resistors allow for easy interface reconfiguration, eliminating external resistors and control relays. Half-duplex switches allow four-wire and two-wire RS485 configurations. Loopback mode steers the driver inputs to the receiver outputs for diagnostic self-test. The RS485 receivers support up to 256 nodes per bus, and feature full failsafe operation for floating, shorted or terminated inputs. An integrated DC/DC boost converter uses a small induc- tor and one capacitor, eliminating the need for multiple supplies for driving RS232 levels. L, LT, LTC, LTM, Linear Technology, the Linear logo and µModule are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. RS485 Mode with Duplex Control FEATURES APPLICATIONS n Four RS232 and Two RS485 Transceivers n 3V to 5.5V Supply Voltage n 20Mbps RS485 and 500kbps RS232 n Automatic Selection of Integrated RS485 (120Ω) and RS232 (5kΩ) Termination Resistors n Half-/Full-Duplex RS485 Switching n Logic Loopback Mode n High ESD: ±16kV on Line I/O n 1.7V to 5.5V Logic Interface n Supports Up to 256 RS485 Nodes n RS485 Receiver Full Failsafe Eliminates UART Lockup n Available in 38-Pin 5mm × 7mm QFN Package n Flexible RS232/RS485/RS422 Interface n Software Selectable Multiprotocol Interface Ports n Point-of-Sale Terminals n Cable Repeaters n Protocol Translators n PROFIBUS-DP Networks RS232 Mode Mixed Mode with RS485 Termination 2872 TA01 LTC2872 3V TO 5.5V 1.7V TO V CC RS485 TERMINATION H/F DY1 2.2μF Y1 Z1 A2 B2 A1 B1 RA1 DY2 RA2 V DD V EE DY1 DY2 RB2 RA2 DZ2 RB1 RA1 DZ1 Y1 B2 A2 Z1 Y2 Z2 A1 B1 LTC2872 DY2 RB2 RA2 DZ2 B2 A2 Y2 Z2 Y1 Z1 DY1 TE485-1 LTC2872 120Ω ON OFF RS485 DUPLEX HALF FULL Y2 Z2 A1 B1 RA1 120Ω V L V CC CAP 470nF 22μH 2.2μF 2.2μF 2.2μF 2.2μF 2.2μF 2.2μF 2.2μF 2.2μF 2.2μF SW 3V TO 5.5V 1.7V TO V CC 2.2μF V DD V EE V L V CC CAP 470nF 22μH SW 3V TO 5.5V 1.7V TO V CC 2.2μF V DD V EE V L V CC CAP 470nF 22μH SW

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Page 1: LTC2872 - RS232/RS485 Dual Multiprotocol Transceiver with … · 2020-02-01 · LTC2872 1 2872f Typical applicaTions DescripTion RS232/RS485 Dual Multiprotocol Transceiver with Integrated

LTC2872

12872f

Typical applicaTions

DescripTion

RS232/RS485 Dual Multiprotocol Transceiver

with Integrated Termination

The LTC®2872 is a robust pin-configurable transceiver that supports RS232, RS485, and RS422 standards while operating on a single 3V to 5.5V supply. The LTC2872 can be configured as four RS232 single-ended transceivers or two RS485 differential transceivers, or combinations of both, on shared I/O lines.

Pin-controlled integrated termination resistors allow for easy interface reconfiguration, eliminating external resistors and control relays. Half-duplex switches allow four-wire and two-wire RS485 configurations. Loopback mode steers the driver inputs to the receiver outputs for diagnostic self-test. The RS485 receivers support up to 256 nodes per bus, and feature full failsafe operation for floating, shorted or terminated inputs.

An integrated DC/DC boost converter uses a small induc-tor and one capacitor, eliminating the need for multiple supplies for driving RS232 levels.L, LT, LTC, LTM, Linear Technology, the Linear logo and µModule are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners.

RS485 Mode with Duplex Control

FeaTures

applicaTions

n Four RS232 and Two RS485 Transceiversn 3V to 5.5V Supply Voltagen 20Mbps RS485 and 500kbps RS232n Automatic Selection of Integrated RS485 (120Ω)

and RS232 (5kΩ) Termination Resistorsn Half-/Full-Duplex RS485 Switchingn Logic Loopback Moden High ESD: ±16kV on Line I/On 1.7V to 5.5V Logic Interfacen Supports Up to 256 RS485 Nodesn RS485 Receiver Full Failsafe Eliminates UART Lockupn Available in 38-Pin 5mm × 7mm QFN Package

n Flexible RS232/RS485/RS422 Interfacen Software Selectable Multiprotocol Interface Portsn Point-of-Sale Terminalsn Cable Repeatersn Protocol Translatorsn PROFIBUS-DP Networks

RS232 Mode Mixed Mode with RS485 Termination

2872 TA01

LTC2872

3V TO 5.5V1.7V TO VCC

RS485

TERMINATION

H/F

DY1

2.2µF

Y1

Z1

A2

B2

A1

B1RA1

DY2

RA2

VDD VEE

DY1

DY2

RB2

RA2

DZ2

RB1

RA1

DZ1

Y1

B2

A2

Z1

Y2

Z2

A1

B1

LTC2872

DY2

RB2

RA2

DZ2

B2

A2

Y2

Z2

Y1

Z1DY1

TE485-1LTC2872

120Ω

ONOFF

RS485

DUPLEX

HALFFULL

Y2

Z2

A1

B1RA1 120Ω

VL VCCCAP

470nF 22µH

2.2µF

2.2µF

2.2µF

2.2µF

2.2µF

2.2µF2.2µF 2.2µF 2.2µF

SW

3V TO 5.5V1.7V TO VCC

2.2µF

VDD VEE

VL VCCCAP

470nF 22µH

SW

3V TO 5.5V1.7V TO VCC

2.2µF

VDD VEE

VL VCCCAP

470nF 22µH

SW

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LTC2872

22872f

pin conFiguraTionabsoluTe MaxiMuM raTings

Input Supplies VCC, VL ..................................................... –0.3V to 7V

Generated Supplies VDD ................................................ VCC – 0.3V to 7.5V VEE .........................................................0.3V to –7.5V VDD – VEE ..............................................................15V SW ........................................... –0.3V to (VDD + 0.3V) CAP ............................................. 0.3V to (VEE – 0.3V)

A1, A2, B1, B2, Y1, Y2, Z1, Z2 ......................–15V to 15VDY1, DY2, DZ1, DZ2, RXEN1, RXEN2, DXEN1, DXEN2,LB, H/F, TE485_1, TE485_2, 485/232_1, 485/232_2 ................................ –0.3V to 7VFEN, RA1, RA2, RB1, RB2 ...............–0.3V to (VL + 0.3V)Differential Enabled Terminator Voltage

(A1-B1 or A2-B2 or Y1-Z1 or Y2-Z2) .....................±6VOperating Temperature

LTC2872C ................................................ 0°C to 70°C LTC2872I .............................................–40°C to 85°C

Storage Temperature Range .................. –65°C to 125°C

(Note 1)

13 14 15 16

TOP VIEW

39VEE

UHF PACKAGE38-LEAD (5mm × 7mm) PLASTIC QFN

17 18 19

38 37 36 35 34 33 32

24

25

26

27

28

29

30

31

8

7

6

5

4

3

2

1VCC

A1

B1

Y1

GND

Z1

DY1

DZ1

RXEN1

DXEN1

TE485_1

TE485_2

VCC

A2

B2

Y2

GND

Z2

DY2

DZ2

RXEN2

DXEN2

VCC

VDD

RB1

RA1

LB V L GND

RA2

RB2

485/232_

1

485/232_

2

H/F

FEN

CAP

GND

SW

23

22

21

20

9

10

11

12

TJMAX = 125°C, θJA = 34.7°C/W

EXPOSED PAD (PIN #39) IS VEE, MUST BE SOLDERED TO PCB

orDer inForMaTionLEAD FREE FINISH TAPE AND REEL PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE

LTC2872CUHF#PBF LTC2872CUHF#TRPBF 2872 38-Lead (5mm × 7mm) Plastic QFN 0°C to 70°C

LTC2872IUHF#PBF LTC2872IUHF#TRPBF 2872 38-Lead (5mm × 7mm) Plastic QFN –40°C to 85°C

Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container.Consult LTC Marketing for information on non-standard lead based finish parts.For more information on lead free part marking, go to: http://www.linear.com/leadfree/ For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/

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LTC2872

32872f

elecTrical characTerisTics The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VCC = VL = 3.3V, TE485_1 = TE485_2 = 0V, LB = 0V unless otherwise noted.

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

Power Supply

VCC Supply Voltage Operating Range l 3 5.5 V

VL Logic Supply Voltage Operating Range VL ≤ VCC l 1.7 VCC V

VCC Supply Current in Shutdown Mode RXEN1 = RXEN2 = VL, DXEN1 = DXEN2 = FEN = H/F = 0V

l 8 60 µA

VCC Supply Current in RS485 Transceiver Mode (Outputs Unloaded) (Note 3)

485/232_1 = 485/232_2 = DXEN1 = DXEN2 = VL, RXEN1 = RXEN2 = 0V

l 4.5 7 mA

VCC Supply Current in RS232 Transceiver Mode (Outputs Unloaded) (Note 3)

DXEN1 = DXEN2 = VL; 485/232_1 = 485/232_2 = RXEN1 = RXEN2 = 0V

l 5.5 8 mA

VL Supply Current in RS485 or RS232 Transceive Mode (Outputs Unloaded)

DXEN1 = DXEN2 = VL, RXEN1 = RXEN2 = 0V l 0 5 µA

RS485 Drivers

|VOD| Differential Output Voltage RL = ∞, VCC = 3V (Figure 1) RL = 27Ω, VCC = 4.5V (Figure 1) RL = 27Ω, VCC = 3V (Figure 1) RL = 50Ω, VCC = 3.13V (Figure 1)

l

l

l

l

2.1 1.5 2

6 VCC VCC VCC

V V V V

∆|VOD| Difference in Magnitude of Differential Output Voltage for Complementary Output States

RL = 27Ω, VCC = 3V (Figure 1) RL = 50Ω, VCC = 3.13V (Figure 1)

l

l

0.2 0.2

V V

VOC Common Mode Output Voltage RL = 27Ω or 50Ω (Figure 1) l 3 V

∆|VOC| Difference in Magnitude of Common Mode Output Voltage for Complementary Output States

RL = 27Ω or 50Ω (Figure 1) l 0.2 V

IOZD485 Three-State (High Impedance) Output Current VOUT = 12V or –7V, VCC = 0V or 3.3V (Figure 2)

l –100 125 µA

IOSD485 Maximum Short-Circuit Current –7V ≤ VOUT ≤ 12V (Figure 2) l –250 250 mA

RS485 Receiver

IIN485 Input Current VIN = 12V or –7V, VCC = 0V or 3.3V (Figure 3) (Note 5)

l –100 125 µA

RIN485 Input Resistance VIN = 12V or –7V, VCC = 0V or 3.3V (Figure 3) (Note 5)

125 kΩ

Differential Input Signal Threshold Voltage (A–B) –7V ≤ (A or B) ≤ 12 (Note 5) l ±200 mV

Differential Input Signal Hysteresis B = 0V (Notes 3, 5) 190 mV

Differential Input DC Failsafe Threshold Voltage (A–B)

–7V ≤ (A or B) ≤ 12 (Note 5) l –200 –65 0 mV

Differential Input DC Failsafe Hysteresis B = 0V (Note 5) 30 mV

VOL Output Low Voltage Output Low, I(RA) = 3mA (Sinking), 3V ≤ VL ≤ 5.5V

l 0.4 V

Output Low, I(RA) = 1mA (Sinking), 1.7V ≤ VL < 3V

l 0.4 V

VOH Output High Voltage Output High, I(RA) = –3mA (Sourcing), 3V ≤ VL ≤ 5.5V

l VL – 0.4 V

Output High, I(RA) = –1mA (Sourcing), 1.7V ≤ VL < 3V

l VL – 0.4 V

Three-State (High Impedance) Output Current 0V ≤ RA ≤ VL, VL = 5.5V l 0 ±5 μA

Short-Circuit Output Current 0V ≤ RA ≤ VL, VL = 5.5V l ±135 mA

RTERM Terminating Resistor TE485 = VL, A–B = 2V, B = –7V, 0V, 10V (Figure 8) (Note 5)

l 108 120 156 Ω

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LTC2872

42872f

elecTrical characTerisTics The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VCC = VL = 3.3V, TE485_1 = TE485_2 = 0V, LB = 0V unless otherwise noted.

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

RS232 Driver

VOLD Output Low Voltage RL = 3kΩ, VEE ≤ –6V l –5 –5.7 VEE V

VOHD Output High Voltage RL = 3kΩ, VDD ≥ 6.5V l 5 6.2 VDD V

Three-State (High Impedance) Output Current Y or Z = ±15V l ±156 µA

Output Short-Circuit Current Y or Z = 0V l ±35 ±90 mA

RS232 Receiver

Input Threshold Voltage l 0.6 1.5 2.5 V

Input Hysteresis l 0.1 0.4 1.0 V

Output Low Voltage I(RA, RB) = 1mA (Sinking), 1.7V ≤ VL ≤ 5.5V

l 0.4 V

Output High Voltage I(RA, RB) = –1mA (Sourcing), 1.7V ≤ VL ≤ 5.5V

l VL – 0.4 V

Input Resistance –15V ≤ (A, B) ≤ 15V, Receiver Enabled l 3 5 7 kΩ

Three-State (High Impedance) Output Current 0V ≤ (RA, RB) ≤ VL l 0 ±5 μA

Output Short-Circuit Current VL = 5.5V, 0V ≤ (RA, RB) ≤ VL l ±25 ±50 mA

Logic Inputs

Threshold Voltage l 0.4 0.75•VL V

Input Current l 0 ±5 µA

Power Supply Generator

VDD Regulated VDD Output Voltage RS232 Drivers Enabled, Outputs Loaded with RL = 3kΩ to GND, DY1 = DY2 = VL, DZ1 = DZ2 = 0V (Note 3)

7 V

VEE Regulated VEE Output Voltage –6.3 V

ESD

Interface Pins (A, B, Y, Z) Human Body Model to GND or VCC, Powered or Unpowered (Note 7)

±16 kV

All Other Pins Human Body Model (Note 7) ±4 kV

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LTC2872

52872f

swiTching characTerisTics The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VCC = VL = 3.3V, TE485_1 = TE485_2 = 0V, LB = 0V unless otherwise noted. VL ≤ VCC.

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

RS485 AC Characteristics

Maximum Data Rate (Note 3) l 20 Mbps

tPLHD485 tPHLD485

Driver Propagation Delay RDIFF = 54Ω, CL = 100pF (Figure 4) l 20 70 ns

Driver Propagation Delay Difference |tPLHD485 – tPHLD485|

RDIFF = 54Ω, CL = 100pF (Figure 4) l 1 6 ns

tSKEWD485 Driver Skew (Y to Z) RDIFF = 54Ω, CL = 100pF (Figure 4) l 1.5 ±8 ns

tRD485, tFD485 Driver Rise or Fall Time RDIFF = 54Ω, CL = 100pF (Figure 4) l 7.6 15 ns

tZLD485, tZHD485, tLZD485, tHZD485

Driver Output Enable or Disable Time FEN = VL, RL = 500Ω, CL = 50pF (Figure 5) l 120 ns

tZHSD485, tZLSD485 Driver Enable from Shutdown FEN = 0V, RL = 500Ω, CL = 50pF (Figure 5) l 0.2 2 ms

tPLHR485, tPHLR485 Receiver Input to Output CL = 15pF, VCM = 1.5V, |A–B| = 1.5V, (Figure 6) (Note 5)

l 55 85 ns

tSKEWR485 Differential Receiver Skew |tPLHR485 – tPHLR485|

CL = 15pF (Figure 6) l 1 9 ns

tRR485, tFR485 Receiver Output Rise or Fall Time CL = 15pF (Figure 6) l 3 15 ns

tZLR485, tZHR485 tLZR485, tHZR485

Receiver Output Enable or Disable Time FEN = VL, RL = 1k, CL = 15pF (Figure 7) l 30 85 ns

tRTEN485, tRTZ485 Termination Enable or Disable Time FEN = VL, VB = 0V, VAB = 2V (Figure 8) (Note 5) l 100 µs

RS232 AC Characteristics

Maximum Data Rate RL = 3kΩ, CL = 2500pF, RL = 3kΩ, CL = 500pF (Note 3)

l

l

100 500

kbps kbps

Driver Slew Rate (Figure 9) RL = 3kΩ, CL = 2500pF RL = 3kΩ, CL = 50pF

l

l

4 30

V/µs V/µs

tPHLD232, tPLHD232 Driver Propagation Delay RL = 3kΩ, CL = 50pF (Figure 9) l 1 2 µs

tSKEWD232 Driver Skew RL = 3kΩ, CL = 50pF (Figure 9) 50 ns

tZLD232, tZHD232 tLZD232, tHZD232

Driver Output Enable or Disable Time FEN = VL, RL = 3kΩ, CL = 50pF (Figure 10) l 0.4 2 µs

tPHLR232, tPLHR232 Receiver Propagation Delay CL = 150pF (Figure 11) l 60 200 ns

tSKEWR232 Receiver Skew CL = 150pF (Figure 11) 25 ns

tRR232, tFR232 Receiver Rise or Fall Time CL = 150pF (Figure 11) l 60 200 ns

tZLR232, tZHR232, tLZR232, tHZR232

Receiver Output Enable or Disable Time FEN = VL, RL = 1kΩ, CL = 150pF (Figure 12) l 0.7 2 µs

Power Supply Generator

VDD/VEE Supply Rise Time FEN = , (Notes 3 and 4) l 0.2 2 ms

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.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. Guaranteed by other measured parameters and not tested directly.

Note 4. Time from FEN until VDD ≥ 5V and VEE ≤ –5V. External components as shown in typical application.Note 5. Condition applies to A, B for H/F = 0V, and Y, Z for H/F = VL.Note 6. 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.Note 7. Guaranteed by design and not subject to production test.

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LTC2872

62872f

Typical perForMance characTerisTics

VCC Supply Current vs RS232 Data Rate

VCC Supply Current vs Supply Voltage for RS485 at Maximum Data Rate

RS485 Driver Differential Output Voltage vs Temperature

RS485 Driver Propagation Delay vs Temperature

RS485 Driver Short-Circuit Current vs Short-Circuit Voltage

RS485 Driver and Receiver Skew vs Temperature

VCC Supply Current vs Supply Voltage in Shutdown Mode

VCC Supply Current vs Supply Voltage in Fast Enable Mode

VCC Supply Current vs RS485 Data Rate

INPUT VOLTAGE (V)3

INPU

T CU

RREN

ET (µ

A)

30

25

20

15

10

5

04.5 53.5

2872 G01

5.54

H/F LOW

H/F HIGH

SUPPLY VOLTAGE (V)3

SUPP

LY C

URRE

NT (m

A)

4.6

4.2

3.8

3.4

4.4

4.0

3.6

3.2

3.053.5

2872 G02

5.54.54

–40°C

25°C

85°C

DATA RATE (Mbps)0.1

SUPP

LY C

URRE

NT (m

A)

200

140

100

60

160

180

120

80

40

20

01

2872 G03

10010

DRIVER AND RECEIVERTERMINATION ENABLED

TERMINATION DISABLED

ALL RS485 DRIVERS AND RECEIVERS SWITCHING.Y TIED TO A; Z TIED TO B, H/F = 0V, CL = 100pF ON Y AND Z TO GND

VCC = 5VVCC = 3.3V

DATA RATE (kbps)0

INPU

T CU

RREN

T (m

A)

50

45

35

25

40

30

20

15

1050

2872 G04

500100 150 200 250 300 350 450400

ALL RS232 DRIVERS AND RECEIVERS SWITCHING

2.5nF

2.5nF

0.5nF

VCC = 5VVCC = 3.3V

0.5nF

0.05nF

0.05nF

SUPPLY VOLTAGE (V)3

SUPP

LY C

URRE

NT (m

A)

240

220

180

140

200

160

120

100

803.5

2872 G05

5.54 4.5 5

BOTH RS485 DRIVERS ANDRECEIVERS SWITCHING.Y TIED TO A, Z TIED TO BH/F = 0VDRIVER AND RECEIVERTERMINATION ENABLED20Mbps, CL = 100pF ONY AND Z TO GND

85°C25°C–40°C

TEMPERATURE (°C)–50

VOLT

AGE

(V)

4.5

3.5

2.5

1.5

4.0

3.0

2.0

1.0

0.5

050 75–25

2872 G06

100250

RL = 100Ω

RL = 54Ω

RL = 100Ω

RL = 54Ω

VCC = 5VVCC = 3.3V

TEMPERATURE (°C)–50

DELA

Y (n

s)

50

40

30

20

10

050 75–25

2872 G07

100250

VCC = 3.3V, VL = 1.7VVCC = 5V, VL = 1.7VVCC = 3.3V, VL = 3.3VVCC = 5V, VL = 5V

SHORT-CIRCUIT VOLTAGE (V)–10

SHOR

T-CI

RCUI

T CU

RREN

T (m

A)

150

100

0

50

–50

–100

–15010–5

2872 G08

1550

OUTPUT LOW

OUTPUT HIGH

VCC = 5VVCC = 3.3V

TEMPERATURE (°C)–50

SKEW

(ns)

3.0

2.5

1.5

2.0

1.0

0.5

050 75–25

2872 G09

100250

DRIVER

RECEIVER

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LTC2872

72872f

Typical perForMance characTerisTics

RS485 Termination Resistance vs Temperature RS232 Operation at 500kbps

RS485 Operation at 20Mbps

RS485 Receiver Output Voltage vs Load Current

RS232 Receiver Input Threshold vs Temperature

RS232 Receiver Output Voltage vs Load Current

RS232 Driver Outputs Enabling and Disabling VDD and VEE Powering Up

RS485 Receiver Propagation Delay vs Temperature

TEMPERATURE (°C)–50

DELA

Y (n

s)

80

70

60

50

4050 75–25

2872 G10

100250

VCC = 3.3V, VL = 1.7VVCC = 5V, VL = 1.7VVCC = 3.3V, VL = 3.3VVCC = 5V, VL = 5V

OUTPUT CURRENT (mA)0

OUTP

UT V

OLTA

GE (V

)

6

5

4

2

3

1

082

2872 G11

1064

VL = 5VVL = 3.3VVL = 1.7V

TEMPERATURE (°C)–50

THRE

SHOL

D VO

LTAG

E (V

)

2.0

1.8

1.6

1.4

1.2

1.050 75–25

2872 G12

100250

VCC = 5VVCC = 3.3V

INPUT HIGH

INPUT LOW

OUTPUT CURRENT (mA)0

OUTP

UT V

OLTA

GE (V

)

6

5

4

2

3

1

082

2872 G13

1064

VL = 5VVL = 3.3VVL = 1.7V

TEMPERATURE (°C)–50

RESI

STAN

CE (Ω

)

130

118

116

114

112

110

128

126

124

122

120

50 75–25

2872 G14

100250

VCM = –7VVCM = 2VVCM = 12V

1µs/DIVWRAPPING DATADOUT LOADS: 5kΩ + 50pF

5V/DIV

2872 G15

DY

DZ

RA

RB

Z

Y

20ns/DIVH/F HIGHY, Z LOADS: 120Ω (DIFF) + 50pF

1V/DIV

5V/DIV

5V/DIV

2872 G16

DY

RA

Z

Y

40ns/DIV

5V/DIV

2872 G17

DXEN

FEN = VL

FEN = 0VZ

Y

Z

Y

100µs/DIV

5V/DIV

2V/DIV

2872 G18

FEN

VDD

VEE

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pin FuncTionsVCC (Pins 1, 21, 31): Input Supply (3.0V to 5.5V). Tie all three pins together and connect 2.2µF capacitor between VCC and GND.

VL (Pin 35): Logic Supply (1.7V to 5.5V) for the receiver outputs, driver inputs, and control inputs. This pin should be bypassed to GND with a 0.1µF capacitor if it is not tied to VCC. VL must be less than or equal to VCC for proper operation.

VDD (Pin 20): Generated Positive Supply Voltage for RS232 Driver (7V). Connect 2.2µF capacitor between VDD and GND.

VEE (Pin 39):Generated Negative Supply Voltage for RS232 Driver (–6.3V). Tie all pins together and connect 2.2µF capacitor between VEE and GND.

GND (Pins 5, 18, 27, 34): Ground. Tie all four pins together.

CAP (Pin 17): Charge Pump Capacitor for Generated Nega-tive Supply Voltage. Connect a 470nF capacitor between CAP and SW.

SW (Pin 19): Switch Pin. Connect 22µH inductor between SW and VCC.

A1 (Pin 2): RS485 Differential Receiver #1 Positive Input (Full-Duplex Mode) or RS232 Receiver #1a Input.

A2 (Pin 30): RS485 Differential Receiver #2 Positive Input (Full-Duplex Mode) or RS232 Receiver #2a Input.

B1 (PIn 3): RS485 Differential Receiver #1 Negative Input (Full-Duplex Mode) or RS232 Receiver #1b Input.

B2 (Pin 29): RS485 Differential Receiver #1 Negative Input (Full-Duplex Mode) or RS232 Receiver #2b Input.

RA1 (Pin 37): RS485 Differential Receiver #1 Output or RS232 Receiver #1a Output.

RA2 (Pin 33): RS485 Differential Receiver #2 Output or RS232 Receiver #2a Output.

RB1 (Pin 38): RS232 Receiver #1b Output.

RB2 (Pin 32): RS232 Receiver #2b Output.

DY1 (Pin 7): RS485 Differential Driver #1 Input or RS232 Driver #1y Input.

DY2 (Pin 25): RS485 Differential Driver #2 Input or RS232 Driver #2y Input.

DZ1 (Pin 8): RS232 Driver #1z Input.

DZ2 (Pin 24): RS232 Driver #2z Input.

Y1 (Pin 4): RS485 Differential Driver #1 Positive Output or RS232 Driver #1y Output, RS485 Differential Receiver #1 Positive Input (Half-Duplex Mode).

Y2 (Pin 28): RS485 Differential Driver #2 Positive Output or RS232 Driver #2y Output, RS485 Differential Receiver #2 Positive Input (Half-Duplex Mode).

Z1 (Pin 6): RS485 Differential Driver #1 Negative Output or RS232 Driver #1z Output, RS485 Differential Receiver #1 Negative Input (Half-Duplex Mode).

Z2 (Pin 26): RS485 Differential Driver #2 Negative Output or RS232 Driver #2z Output, RS485 Differential Receiver #2 Negative Input (Half-Duplex Mode).

485/232_1 (Pin 13): Interface Select #1 Input. A logic low enables RS232 mode and a high enables RS485 mode for transceiver #1. The mode determines which transceiver inputs and outputs are accessible at the LTC2872 pins as well as which is controlled by the driver and receiver enable pins.

485/232_2 (Pin 14): Interface Select #2 Input. A logic low enables RS232 mode and a high enables RS485 mode for transceiver #2. The mode determines which transceiver inputs and outputs are accessible at the LTC2872 pins as well as which is controlled by the driver and receiver enable pins.

RXEN1 (Pin 9): Receivers #1 Enable. A logic high disables RS232 and RS485 receivers in transceiver #1, leaving their outputs Hi-Z. A logic low enables the RS232 or RS485 receivers in transceiver #1, depending on the state of the Interface Select Input 485/232_1.

RXEN2 (Pin 23): Receivers #2 Enable. A logic high disables RS232 and RS485 receivers in transceiver #2, leaving their outputs Hi-Z. A logic low enables the RS232 or RS485 receivers in transceiver #2, depending on the state of the Interface Select Input 485/232_2.

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pin FuncTionsDXEN1 (Pin 10): Drivers #1 Enable. A logic low disables the RS232 and RS485 drivers in transceiver #1, leaving their outputs in a Hi-Z state. A logic high enables the RS232 or RS485 drivers in transceiver #1, depending on the state of the Interface Select Input 485/232_1.

DXEN2 (Pin 22): Drivers #2 Enable. A logic low disables the RS232 and RS485 drivers in transceiver #2, leaving their outputs in a Hi-Z state. A logic high enables the RS232 or RS485 drivers in transceiver #2, depending on the state of the Interface Select Input 485/232_2.

TE485_1 (Pin 11): RS485 Termination Enable for Trans-ceiver #1. A logic high enables a 120Ω resistor between pins A1 and B1. If DZ1 is also high, a 120Ω resistor is enabled between pins Y1 and Z1. A logic low on TE485_1 opens the resistors, leaving A1/B1 and Y1/Z1 unterminated, independent of DZ1. The differential termination resistors are never enabled in RS232 mode.

TE485_2 (Pin 12): RS485 Termination Enable for Trans-ceiver #2. A logic high enables a 120Ω resistor between pins A2 and B2. If DZ2 is also high, a 120Ω resistor is enabled between pins Y2 and Z2. A logic low on TE485_2 opens the resistors, leaving A2/B2 and Y2/Z2 unterminated, independent of DZ2. The differential termination resistors are never enabled in RS232 mode.

H/F (Pin 15): RS485 Half-duplex Select Input for Trans-ceivers #1 and #2. A logic low is used for full duplex operation where pins A and B are the receiver inputs and pins Y and Z are the driver outputs. A logic high is used for half duplex operation where pins Y and Z are both the receiver inputs and driver outputs and pins A and B do not serve as the receiver inputs. The impedance on A and B and state of differential termination between A and B is independent of the state of H/F. The H/F pin has no effect on RS232 operation.

FEN (Pin 16): Fast Enable. A logic high enables Fast Enable Mode. In fast enable mode the integrated DC/DC converter is active independent of the state of driver, receiver, and termination enable pins allowing faster circuit enable times than are otherwise possible. A logic low disables Fast Enable Mode leaving the state of the DC/DC converter dependent on the state of driver, receiver, and termina-tion enable control inputs. The DC/DC converter powers down only when FEN is low and all drivers, receivers, and terminators are disabled (refer to Table 1).

LB (Pin 36): Loopback Enable for Transceivers #1 and #2. A logic high enables Logic Loopback diagnostic mode, internally routing the driver input logic levels to the receiver output pins within the same transceiver. This applies to both RS232 channels as well as the RS485 driver/receiver. The targeted receiver must be enabled for the loopback signal to be available on its output. A logic low disables loopback mode. In loopback mode, signals are not inverted from driver inputs to receiver outputs.

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block DiagraM

2872 BD

CONTROLLOGIC

DRIVERS

TRANSCEIVER #1

RECEIVERSLOOPBACK

PATH

0.1µF

DXEN

RXEN1

TE485_1

H/F

485/232_1

DXEN2

DY2

DZ2

RA2

RB2

RXEN2

TE485_2

485/232_2

FEN

LB

DY1

DZ1

RA1

RB1

GND

B1

Y2

Z2

A2

B2

A1

PORT 1

PORT 2

Z1

Y1

VEE

VDD

GND

VCC

GND

1.7V TO 5.5V(≤ VCC)

PULSE-SKIPPINGBOOST REGULATOR

f = 1.2MHzRT232

RT485

2.2µF

22µH 470nF3V TO 5.5V

2.2µF

2.2µF

CAPSWVCCVL

232

485

232

232

485

232

5k

RT232

RT485

125k

5k

125k

125k 120Ω

RT485

120Ω

H/F

125k

TRANSCEIVER #2

35 21 19 17

20

39

18

4

1

5

6

2

3

28

VCC

GND31

27

26

30

29

34

32

33

24

25

14

12

23

22

38

37

8

7

13

11

9

10

15

36

16

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TesT circuiTs

Figure 1. RS485 Driver DC Characteristics Figure 2. RS485 Driver Output Short-Circuit Current

Figure 3. RS485 Receiver Input Current and Resistance (Note 5)

Figure 4. RS485 Driver Timing Measurement

2872 F01

DRIVERDYGND

ORVL

YRL

RLZ

VOD

+

–VOC

+

2872 F03

RECEIVER

A OR B

B OR A

VINIIN485

RIN485 =

VIN

IIN485

+–

2872 F04

DRIVERDY

Y

Z

RDIFF

CL

CL

tPLHD485

tSKEWD485

tPLHD485

tRD485 tFD485

90%0V

VOD ½VOD

VL

0V

DY

Y, Z

Y - Z10%

90%0V

10%

2872 F02

DRIVER

Y OR Z

DYGND

ORVL

Z OR Y VOUT

IOZD485, IOSD485

+–

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TesT circuiTs

Figure 5. RS485 Driver Enable and Disable Timing Measurements

Figure 6. RS485 Receiver Propagation Delay Measurements (Note 5)

Figure 7. RS485 Receiver Enable and Disable Timing Measurements (Note 5)

2872 F07

tZLR485

tLZR485

tHZR485tZHR485

½VL

½VL

VL

VOL

VL

VOH

0V

0V

0.5V

RXEN

RA

RA

RECEIVERRA

RXEN

0V TO 3V

3V TO 0V

ARL

B

VLOR

GNDCL

½VL

0.5V

½VL

2872 F05

tZLD485,tZLSD485 tLZD485

tHZD485tZHD485,tZHSD485

½VCC

½VCC

VL

VOL

VCC

VOH

0V

0V

0.5V

DXEN

Y OR Z

Z OR Y

DRIVERDY

DXEN

VLOR

GND

Y

RL

Z

GNDORVCC

VCCOR

GND

RL

CL

CL

½VL

0.5V

½VL

2872 F06

RECEIVERVCM

±VAB/2 A

B

RA

±VAB/2CL

tPLHR485

tSKEWR485 = tPLHR485 – tPHLR485

tRR485

90%

0V

½VL

A–B

RA10%

tPHLR485

tFR485

90%

0V

–VAB

VAB

½VL 10%

VCC

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TesT circuiTs

Figure 8. RS485 Termination Resistance and Timing Measurements (Note 5)

Figure 9. RS232 Driver Timing and Slew Rate Measurements

Figure 10. RS232 Driver Enable and Disable Times

2872 F08

RECEIVER VAB

A

B

VAB

IARTERM =

VB

½VL ½VLTE485

IA

IA

90%10%

tRTZ485tRTEN4850V

VL

TE485

+–

+–

2872 F09

DRIVERINPUT

DRIVEROUTPUT

CLRL

tPHLD232

tPLHD232

tSKEWD232 = |tPHLD232 – tPLHD232|

tF tR

DRIVERINPUT

DRIVERINPUT

SLEW RATE = 6V

tF OR tR

3V

–3V

0V

VOLD

VL

½VL ½VL

0V3V

–3V0V

VOHD

2872 F10

0V OR VL

DXEN

DRIVEROUTPUT

CLRLtHZD232

tLZD232

DXEN

DRIVEROUTPUT

DRIVEROUTPUT

0.5V

tZHD232

tZLD232

5V

5V 0.5V

0V

0V

0V

VOHD

VL

½VL ½VL

VOLD

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TesT circuiTs

Figure 11. RS232 Receiver Timing Measurements

Figure 12. RS232 Receiver Enable and Disable Times

2872 F11

tPHLR232

tSKEWR232 = |tPLHR232 – tPHLR232|

tPLHR232

tRR232

90%

1.5V 1.5V

½VL 10%tFR232

90%

0V

–3V

VL

+3V

½VL10% 0V

VL

RECEIVEROUTPUT

RECEIVEROUTPUT

RECEIVERINPUT

RECEIVERINPUT

CL

2872 F12

–3V OR +3V

RXEN

RECEIVEROUTPUT

GNDOR VL

CL

RL

tHZR232

tLZR232

RXEN

RECEIVEROUTPUT

RECEIVEROUTPUT

0.5V

tZHR232

tZLR232

½VL

½VL0.5V

0V

0V

VL

VOHR

VL

½VL ½VL

VOLR

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FuncTion TablesTable 1. Shutdown and Fast Enable Modes

FEN485/232_1 AND

485/232_2RXEN1 AND

RXEN2DXEN1 AND

DXEN2TE485_1 AND

TE485_2 H/F LBDC/DC

CONVERTER MODE AND COMMENTS

0 X 1 0 0 X X OFF Shutdown: All Main Functions Off

1 X 1 0 0 X X ON Fast-Enable: DC/DC Converter On Only

Table 2. Mode Selection Table for a Given Port (FEX = X)485/232 RXEN DXEN TE485 H/F LB DC/DC CONVERTER MODE AND COMMENTS

0 X 1 X X 0 ON RS232 Drivers On

0 0 X X X 0 ON RS232 Receivers On

1 X 1 X X 0 ON RS485 Driver On

1 0 X X X 0 ON RS485 Receiver On

1 X X 1 X X ON RS485 Termination Mode (See Table 7)

1 X X X 0 0 X RS485 Full Duplex Mode

1 X X X 1 0 X RS485 Half Duplex Mode

1 0 X X X 1 ON RS485 Loopback Mode

0 0 X X X 1 ON RS232 Loopback Mode

Table 3. RS232 Receiver Mode for a Given Port (485/232 = 0)RXEN RECEIVER INPUT (A, B) CONDITIONS RECEIVER OUTPUTS (RA, RB) RECEIVER INPUTS (A, B)

1 X No Fault Hi-Z 125kΩ

0 0 No Fault 1 5kΩ

0 1 No Fault 0 5kΩ

0 X Thermal Fault Hi-Z 5kΩ

Table 4. RS232 Driver Mode for a Given Port (485/232 = 0)DXENX DRIVER INPUT (DY, DZ) CONDITIONS DRIVER OUTPUT (Y, Z)

0 X No Fault 125kΩ

1 0 No Fault 1

1 1 No Fault 0

X X Thermal Fault 125kΩ

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FuncTion TablesTable 5. RS485 Driver Mode for a Given Port (485/232 = 1, TE485 = 0)

DXEN DY CONDITIONS Y Z

0 X No Fault 125kΩ 125kΩ

1 0 No Fault 0 1

1 1 No Fault 1 0

X X Thermal Fault 125kΩ 125kΩ

Table 6. RS485 Receiver Mode for a Given Port (485/232 = 1, LB = 0)RXEN A–B (NOTE 5) CONDITIONS RA

1 X No Fault Hi-Z

0 < –200mV No Fault 0

0 > 200mV No Fault 1

0 Inputs Open or Shorted Together (DC) No Fault 1

X X Thermal Fault Hi-Z

Table 7. RS485 Termination for a Given Port (485/232 = 1)TE485 DZ H/F, LB CONDITIONS R(A TO B) R(Y TO Z)

0 X X No Fault Hi-Z Hi-Z

1 0 X No Fault 120Ω Hi-Z

1 1 X No Fault 120Ω 120Ω

X X X Thermal Fault Hi-Z Hi-Z

Table 8. RS485 Duplex Control for Given Port (485/232 = 1)H/F RS485 DRIVER OUTPUTS RS485 RECEIVER INPUTS

0 Y, Z A, B

1 Y, Z Y, Z

Table 9. Loopback Functions for a Given PortLB RXEN TRANSCEIVER MODE

0 X Not Loopback

1 1 Not Loopback

1 0 Loopback (RA = DY, RB = DZ)

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Overview

The LTC2872 is a flexible multiprotocol transceiver sup-porting RS485/RS422 and RS232 protocols. It can be powered from a single 3.0V to 5.5V supply with optional logic interface supply as low as 1.7V. An integrated DC/DC converter provides the positive and negative supply rails needed for RS232 operation. Automatically selected integrated termination resistors for both RS232 and RS485 protocols are included, eliminating the need for external components and switching relays. Both parts include loopback control for self-test and debug as well as logically-switchable half- and full-duplex control of the RS485 bus interface.

The LTC2872 offers two ports that can be independently configured as either two RS232 receivers and drivers or one RS485/RS422 receiver and driver depending on the state of its 485/232 pins. Control inputs DXEN and RXEN provide independent control of driver and receiver opera-tion for either RS232 or RS485 transceivers, depending on the selected operating protocol.

The LTC2872 features rugged operation with an ESD rating of ±15kV HBM on the receiver inputs and driver outputs, both powered and unpowered. All other pins offer protec-tion exceeding ±4kV.

DC/DC Converter

The on-chip DC/DC converter operates from the VCC input, generating a 7V VDD supply and a charge pumped –6.3V VEE supply, as shown in Figure 13. VDD and VEE power the output stage of the RS232 drivers and are regulated to levels that guarantee greater than ±5V output swing. The DC/DC converter requires a 22µH inductor (L1) and a bypass capacitor (C4) of 2.2µF or larger. The charge pump capacitor (C1) is 470nF and the storage capacitors (C2 and C3) are 2.2µF. Larger storage capacitors up to 4.7µF may be used if C1 and C4 are scaled proportionately. Locate C1-C4 close to their associated pins.

Bypass capacitor C5 on the logic supply pin can be omitted if VL is connected to VCC. See the VL Logic Supply section for more details about the VL logic supply.

Inductor Selection

An inductor with a value of 22µH ±20% is required. It must have a saturation current (ISAT) rating of at least 200mA and a DCR (copper wire resistance) of less than 1.3Ω. Some small inductors meeting these requirements are listed in Table 10.

Table 10. Recommended Inductors

PART NUMBERL

(µH)ISAT

(mA)

MAX DCR (Ω) SIZE (mm) MANUFACTURER

BRC2016T220M CBC2518T220M

22 22

310 320

1.3 1.0

2 × 1.6 × 1.6 2.5 × 1.8 × 1.8

Taiyo Yuden t-yuden.com

LQH32CN220K53 22 250 0.92 3.2 × 2.5 × 1.6 Murata murata.com

Capacitor Selection

The small size of ceramic capacitors makes them ideal for the LTC2872. Use X5R or X7R dielectric types; their ESR is low and they retain their capacitance over relatively wide voltage and temperature ranges. Use a voltage rating of at least 10V.

applicaTions inForMaTion

Figure 13. DC/DC Converter with Required External Components

2872 F13

BOOSTREGULATOR

VCC3V TO 5.5V

VL1.7V TO VCC

C1470nF

L122µH

21

18

C42.2µF

VCC

VDD

VEE

SW

GNDGND

CAP

C50.1µF

C22.2µF

19

C32.2µF

17

20

3934

VL35

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Inrush Current and Supply Overshoot Precaution

In certain applications fast supply slew rates are gener-ated when power is connected. If VCC’s voltage is greater than 4.5V and its rise time is faster than 10μs, the pins VDD and SW can exceed their Absolute Maximum values during start-up. When supply voltage is applied to VCC, the voltage difference between VCC and VDD generates inrush current flowing through inductor L1 and capacitors C1 and C2. The peak inrush current must not exceed 2A. To avoid this condition, add a 1Ω resistor as shown in Figure 14. This precaution is not relevant for supply voltages below 4.5V or rise times longer than 10μs.

by more than 1V for proper operation. Logic input pins do not have internal biasing devices to pull them up or down. They must be driven high or low to establish valid logic levels; do not float.

RS485 Driver

The RS485 driver provides full RS485/RS422 compat-ibility. When enabled, if DI is high, Y–Z is positive. When the driver is disabled, Y and Z output resistance is greater than 96k (typically 125k) to ground over the entire common mode range of –7V to 12V. This resistance is equivalent to the input resistance on these lines when the driver is configured in half-duplex mode and Y and Z act as the RS485 receiver inputs.

Driver Overvoltage and Overcurrent Protection

The RS232 and RS485 driver outputs are protected from short circuits to any voltage within the Absolute Maximum range ±15V. The maximum current in this condition is 90mA for the RS232 driver and 250mA for the RS485 driver.

If an RS485 driver output is shorted to a voltage greater than VCC, when active high, positive current of about 100mA can flow from the driver output back to VCC. If the system power supply or loading cannot sink this excess current, clamp VCC to GND with a Zener diode (e.g., 5.6V, 1W, 1N4734) to prevent an overvoltage condition on VCC.

All devices also feature thermal shutdown protection that disables the drivers, receivers, and RS485 terminators in case of excessive power dissipation (see Note 6).

RS485 Balanced Receiver with Full Failsafe Support

The LTC2872 RS485 receiver has a differential threshold voltage that is about 80mV for signals that are rising and –80mV for signals that are falling, as illustrated in Figure 15. If a differential input signal lingers in the win-dow between these thresholds for more than about 2µs, the rising threshold changes from 80mV to –50mV, while the falling threshold remains at –80mV. Thus, differential inputs that are shorted, open, or terminated but not driven for more than 2µs produce a high on the receiver output, indicating a failsafe condition.

applicaTions inForMaTion

Figure 14. Supply Current Overshoot Protection for Input Supplies of 4.5V of Higher

VL Logic Supply

A separate logic supply pin VL allows the LTC2872 to interface with any logic signal from 1.7V to 5.5V. All logic I/Os use VL as their high supply. For proper operation, VL should not be greater than VCC. During power-up, if VL is higher than VCC, the device will not be damaged, but behavior of the device is not guaranteed. If VL is not con-nected to VCC, bypass VL with a 0.1µF capacitor.

RS232 and RS485 driver outputs are undriven and the RS485 termination resistors are disabled when VL or VCC is grounded or VCC is disconnected.

Although all logic input pins reference VL as their high supply, they can be driven up to 7V, independent of VL and VCC, with the exception of FEN, which must not exceed VL

2872 F14

0V

5V

≤10µs

C1470nF

L122µH

INRUSHCURRENTC4

2.2µF

R11Ω1/8W

VCC

VDD GND

SW19 17

CAP

20

21

18

C22.2µF

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Figure 15. RS485 Receiver Input Threshold Characteristics with Typical Values Shown

applicaTions inForMaTion

The benefit of this dual threshold architecture is that it supports full failsafe operation yet offers a balanced threshold, centered on 0V, for normal data signals. This balance preserves duty cycle for small input signals with heavily slewed edges, typical of what might be seen at the end of a very long cable. This performance is highlighted in Figure 16, where a signal is driven through 4000 feet of CAT5e cable at 3Mbps. Even though the differential signal peaks at just over 100mV and is heavily slewed, the output maintains a nearly perfect signal with almost no duty cycle distortion.

lines, which establishes a logic-high state when all the transmitters on the network are disabled. The values of the biasing resistors depend on the number and type of transceivers on the line and the number and value of terminating resistors. Therefore, the values of the biasing resistors must be customized to each specific network installation, and may change if nodes are added to or removed from the network.

The internal failsafe feature of the LTC2872 eliminates the need for external network biasing resistors provided they are used in a network of transceivers with similar internal failsafe features. This also allows the network to support a high number of nodes, up to 256, by eliminating the bias resistor loading. The LTC2872 transceivers will operate correctly on biased, unbiased, or under-biased networks.

Receiver Outputs

The RS232 and RS485 receiver outputs are internally driven high (to VL) or low (to GND) with no external pull-up needed. When the receivers are disabled, the output pin becomes Hi-Z with leakage of less than ±5μA for voltages within the VL supply range.

RS485 Receiver Input Resistance

The RS485 receiver input resistance from A or B to GND (Y or Z to GND in half-duplex mode with driver disabled) is greater than 96k (typically 125k) when the integrated termination is disabled. This permits up to a total of 256 receivers per system without exceeding the RS485 receiver loading specification. The input resistance of the receiver is unaffected by enabling/disabling the receiver or whether the part is in half-duplex, full-duplex, loopback mode, or even unpowered. The equivalent input resistance looking into the RS485 receiver pins is shown in Figure 17.

Figure 16. A 3Mbps Signal Driven Down 4000ft of CAT5e Cable. Top Traces: Received Signals After Transmission Through Cable; Middle Trace: Math Showing Differences of Top Two Signals; Bottom Trace: Receiver Output

An additional benefit of the balanced architecture is excel-lent noise immunity due to the wide effective differential input signal hysteresis of 160mV for signals transitioning through the window region in less than 2μs. Increasingly slower signals will have increasingly less effective hyster-esis, limited by the DC failsafe hysteresis of about 30mV.

RS485 Biasing Network Not Required

RS485 networks are often biased with a resistive divider to generate a differential voltage of ≥200mV on the data

0.1V/DIV

0.1V/DIV

5V/DIV

2872 F16200ns/DIV

RA

(A-B)

A

B

Figure 17. Equivalent RS485 Receiver Input Resistance Into A and B (Note 5)

2872 F17

A

B

TE485

60Ω

60Ω

125k

125k

2872 F15

–80mV –50mV 0V

RA

80mVVAB

(NOTE 5)

RISING THRESHOLDSHIFTS IF SIGNAL ISIN WINDOW > ~2µs

TO SUPPORTFAILSAFE

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applicaTions inForMaTionSelectable RS485 Termination

Proper cable termination is important for good signal fidel-ity. When the cable is not terminated with its characteristic impedance, reflections cause waveform distortion.

The LTC2872 offers integrated switchable 120Ω termination resistors between the differential receiver inputs and also between the differential driver outputs. This provides the advantage of being able to easily change, through logic control, the proper line termination for correct operation when configuring transceiver networks. Termination should be enabled on transceivers positioned at both ends of a network bus.

Termination on the driver nodes is important for cases where the driver is disabled but there is communication on the connecting bus from another node. Driver termination across Y and Z can be disabled independently from the termination across A and B by setting DZ low. See Table 7 for details.

The termination resistance is maintained over the entire RS485 common mode range of –7V to 12V as shown in Figure 18. The voltage across pins with the terminating resistor enabled should not exceed 6V as indicated in the Absolute Maximum Ratings table.

Figure 18. Typical Resistance of the Enabled RS485 Terminator vs Common Mode Voltage of A and B

the differential receiver inputs. With the H/F pin set to a logic-high, the Y and Z pins serve as the differential inputs. In either configuration, the RS485 driver outputs are always on Y and Z. The impedance looking into the A and B pins is not affected by H/F control, including the differential termination resistance. The H/F control does not affect RS232 operation.

Logic Loopback

A loopback mode connects the driver inputs to the re-ceiver outputs (noninverting) for self test. This applies to both RS232 and RS485 transceivers. Loopback mode is entered when the LB pin is set to a logic-high and the relevant receiver is enabled.

In loopback mode, the drivers function normally. They can be disabled with output in a Hi-Z state or left enabled to allow loopback testing in normal operation. Loopback works in half- or full-duplex modes and does not affect the termination resistors.

RS485 Cable Length vs Data Rate

Many factors contribute to the maximum cable length that can be used for for RS485 or RS422 communication, including driver transition times, receiver threshold, duty cycle distortion, cable properties and data rate. A typical curve of cable length versus maximum data rate is shown in Figure 19. Various regions of this curve reflect different performance limiting factors in data transmission.

Figure 19. Cable Length vs Data Rate (RS485/RS422 Standard Shown in Vertical Solid Line)

RS485 Half- and Full-Duplex Control

The LTC2872 is equipped with a control to change the RS485 transceiver operation from full-duplex to half-duplex. With the H/F pin set to a logic-low, the A and B pins serve as

DATA RATE (bps)

CABL

E LE

NGTH

(FT)

2872 F19

10k

1k

100

1010k 10M 100M1M100k

LTC2872MAX DATA RATE

RS485/RS422MAX DATA RATE

VOLTAGE (V)–10

RESI

STAN

CE (Ω

)

126

124

122

118

120

11610–5

2872 F18

1550

VCC = 5.0VVCC = 3.3V

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applicaTions inForMaTionAt frequencies below 100kbps, the maximum cable length is determined by DC resistance in the cable. In this ex-ample, a cable longer than 4000ft will attenuate the signal at the far end to less than what can be reliably detected by the receiver.

For data rates above 100kbps, the capacitive and inductive properties of the cable begin to dominate this relation-ship. The attenuation of the cable is frequency and length dependent, resulting in increased rise and fall times at the far end of the cable. At high data rates or long cable lengths, these transition times become a significant part of the signal bit time. Jitter and intersymbol interference aggravate this so that the time window for capturing valid data at the receiver becomes impossibly small.

The boundary at 20Mbps in Figure 19 represents the guaranteed maximum operating rate of the LTC2872. The dashed vertical line at 10Mbps represents the specified maximum data rate in the RS485 standard. This boundary is not a limit, but reflects the maximum data rate that the specification was written for.

It should be emphasized that the plot in Figure 19 shows a typical relation between maximum data rate and cable length. Results with the LTC2872 will vary, depending on cable properties such as conductor gauge, characteristic impedance, insulation material, and solid versus stranded conductors.

Layout Considerations

All VCC pins must be connected together and all ground pins must be connected together on the PC board with very low impedance traces or dedicated planes. A 2.2µF, or larger, bypass capacitor should be placed less than 0.7cm away from VCC Pin 21. This VCC pin, as well as GND Pin 18, mainly service the DC/DC converter. Additional bypass capacitors of 0.1µF or larger, can be added to VCC

Pins 1 and 31 if the traces back to the 2.2µF capacitor are indirect or narrow. These VCC pins mainly service the transceivers #1 and #2, respectively. Table 11 summarizes the bypass capacitor requirements. The capacitors listed in the table should be placed closest to their respective supply and ground pin.

Table 11. Bypass Capacitor RequirementsCAPACITOR SUPPLY (PIN) RETURN (PIN) COMMENT

2.2µF VCC (21) GND (18) Required

2.2 µF VDD (20) GND (18) Required

2.2uF VEE (39) GND (18) Required

0.1µF VL (35) GND (34) Required*

0.1µF VCC (1) GND (5) Optional

0.1µF VCC (31) GND (27) Optional

* If VL is not connected to VCC.

Place the charge pump capacitor, C1, directly adjacent to the SW and CAP pins, with no more than one centimeter of total trace length to maintain low inductance. Close placement of the inductor, L1, is of secondary importance compared to the placement of C1 but should include no more than two centimeters of total trace length.

The PC board traces connected to high speed signals A/B and Y/Z should be symmetrical and as short as possible to minimize capacitive imbalance and to maintain good differential signal integrity. To minimize capacitive loading effects, the differential signals should be separated by more than the width of a trace and should not be routed on top of each other if they are on different signal planes.

Care should be taken to route outputs away from any sen-sitive inputs to reduce feedback effects that might cause noise, jitter, or even oscillations. For example, DI and A/B should not be routed near the driver or receiver outputs.

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Typical applicaTions

Figure 20. LTC2872 in Various Basic Port Configurations

Figure 21. Loopback in RS232 and RS485 Modes

Figure 22. Half-Duplex RS485 Mode with Driver and Receiver Line Termination on Each Port

Figure 23. Full-Duplex RS485 Mode with Driver and Receiver Line Termination on Port 1, and Receiver-Only Termination on Port 2

VL

DY1

DZ1

Y1

Z1

RA1

RB1

A1

B1

DY2Y2

Z2

RA2A2

B2

2872 F21

LTC2872

LB485/232_2

485/232_1RXEN1RXEN2

H/FGND

VL

2872 F22

LTC2872H/FTE485_1TE485_2485/232_1485/232_2DZ1DZ2

LBGND

DY1

120Ω

Y1

Z1

RA1A1

B1

120Ω

DY2

120Ω

Y2

Z2

RA2A2

B2

120Ω

VL

2872 F23

LTC2872TE485_1TE485_2DZ1485/232_1485/232_2

DZ2H/FLB

GND

DY2Y2

Z2

RA2A2

B2120Ω

DY1

Y1

Z1

RA1A1

B1120Ω

120Ω

VCC = 3V to 5.5V, VL = 1.7V to VCC. Logic input pins not shown are tied to a valid logic state. External components necessary for operation are not shown.

2872 F20

485/232_1485/232_2

LB

DY1

DZ1

Y1

Z1

RA1

RB1

A1

B1

DY2

DZ2

Y2

Z2

RA2

RB2

A2

B2

LTC2872

PORT 1: RS232PORT 2: RS232

PORT 1: RS232PORT 2: RS485

PORT 1: RS485PORT 2: RS232

PORT 1: RS485PORT 2: RS485

DY1

DZ1

Y1

Z1

RA1

RB1

A1

B1

DY2Y2

Z2

RA2A2

B2

LTC2872

485/232_2 485/232_1H/FLB

GND

DY2

DZ2

Y2

Z2

RA2

RB2

A2

B2

DY1Y1

Z1

RB1A1

B1

LTC2872

485/232_1 485/232_2H/FLB

GND

DY1

VLVLVL

Y1

Z1

RA1A1

B1

LTC2872

485/232_1485/232_2

H/FLB

GND

DY2Y2

Z2

RA2A2

B2

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Typical applicaTions

Figure 24. Typical RS485 Half Duplex Network

2872 F25

½ LTC2872

MASTER

120Ω

½ LTC2872

½ LTC2872

SLAVE

TE485H/F

TE485DZH/F

TE485DZH/F

VL VL

120Ω

120Ω

Figure 25. Typical RS485 Full Duplex Network

2872 F24

½ LTC2872

120Ω

½ LTC2872

½ LTC2872

TE485

H/F

TE485DZH/F

TE485DZH/F

VL

VL

VL

120Ω

VCC = 3V to 5.5V, VL = 1.7V to VCC. Logic input pins not shown are tied to a valid logic state. External components necessary for operation are not shown.

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Typical applicaTions

Figure 26. RS485 Receiver with 4-Way Selectable Inputs

2872 F26

H/F

RA1

RXEN1

RS485INTERFACE

INPUT1

INPUT2

Y1

Z1

A1

B1

LTC2872

S3

H/F

RA2

RXEN2

INPUT3

INPUT4

Y2

Z2

A2

B2

S1

OUTPUT

S2

S2

1

1

0

0

1

0

S1

0

0

1

1

1

0

SELECTED INPUT

INPUT1

INPUT2

INPUT3

INPUT4

NONE/Hi-Z

INVALID

S3

1

0

1

0

X

X

VCC = 3V to 5.5V, VL = 1.7V to VCC. Logic input pins not shown are tied to a valid logic state. External components necessary for operation are not shown.

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LTC2872

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Typical applicaTions

Figure 27. Sharing RS232 Receiver Inputs

Figure 28. Low Voltage Microprocessor Interface

2872 F27

RA1 A1

A2

RXEN1

RS232INPUT

* DOES NOT MEET RS232 SPECIFICATIONS

RIN

RA2

RXEN2

S1

OUT1

OUT2

LTC2872

–OR–

S2

S2

1

0

1

0

S1

0

1

1

0

ACTIVE OUTPUT

OUT1

OUT2

NONE (Hi-Z)

OUT1, OUT2

RIN

5k

5k

62.5k

2.5k*

RB1 B1

B2

RXEN1

RS232INPUT

RIN

RB2

RXEN2

S1

OUT1

OUT2

LTC2872

S2

2872 F28

3V TO 5.5V

1.7V TO VCC

µP

LTC2872

LOGICLEVEL

SIGNALS

LINELEVEL

SIGNALSRS232AND/ORRS485

VCC

VL

GND

VCC = 3V to 5.5V, VL = 1.7V to VCC. Logic input pins not shown are tied to a valid logic state. External components necessary for operation are not shown.

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Figure 29. RS232 ↔ RS485 Conversion

Figure 30. RS485 Repeater

Typical applicaTions

2872 F29

LTC2872 DY2RA1

RA2

Y2A1 RS485

OUT

RS485IN

RS232IN

RS232OUT

Y1

Z2

A2

B2

DY1

120Ω

2872 F29

LTC2872 DY2RA1

RA2

Y2

Z2

A2

B2

A1

B1

Y1

Z1

DY1

120Ω

120Ω

120Ω

120Ω

VCC = 3V to 5.5V, VL = 1.7V to VCC. Logic input pins not shown are tied to a valid logic state. External components necessary for operation are not shown.

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LTC2872

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

package DescripTion

UHF Package38-Lead Plastic QFN (5mm × 7mm)

(Reference LTC DWG # 05-08-1701 Rev C)

5.00 ± 0.10

NOTE:1. DRAWING CONFORMS TO JEDEC PACKAGE OUTLINE M0-220 VARIATION WHKD2. DRAWING NOT TO SCALE3. ALL DIMENSIONS ARE IN MILLIMETERS

PIN 1TOP MARK(SEE NOTE 6)

37

1

2

38

BOTTOM VIEW—EXPOSED PAD

5.50 REF5.15 ± 0.10

7.00 ± 0.10

0.75 ± 0.05

R = 0.125TYP

R = 0.10TYP

0.25 ± 0.05

(UH) QFN REF C 1107

0.50 BSC

0.200 REF

0.00 – 0.05

RECOMMENDED SOLDER PAD LAYOUTAPPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED

3.00 REF

3.15 ± 0.10

0.40 ±0.10

0.70 ± 0.05

0.50 BSC5.5 REF

3.00 REF 3.15 ± 0.05

4.10 ± 0.05

5.50 ± 0.05 5.15 ± 0.05

6.10 ± 0.05

7.50 ± 0.05

0.25 ± 0.05

PACKAGEOUTLINE

4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.20mm ON ANY SIDE5. EXPOSED PAD SHALL BE SOLDER PLATED6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION ON THE TOP AND BOTTOM OF PACKAGE

PIN 1 NOTCHR = 0.30 TYP OR0.35 × 45° CHAMFER

Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings.

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LTC2872

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Linear Technology Corporation1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 FAX: (408) 434-0507 www.linear.com LINEAR TECHNOLOGY CORPORATION 2012

LT 0312 • PRINTED IN USA

relaTeD parTs

Typical applicaTion

Figure 31. LTC2872 on Left: RS485 Half-Duplex and Terminated, Plus RS232. LTC2872 on Right: Dual RS485 Half-Duplex and Terminated. All External Components Shown

PART NUMBER DESCRIPTION COMMENTS

LTC2870/LTC2871 RS232/RS485 Multiprotocol Transceivers with Integrated Termination

3V to 5.5V Supply, Automatic Selection of Termination Resistors, Duplex Control, Logic Supply Pin, ±26kV ESD

LTC1334 Single 5V RS232/RS485 Multiprotocol Transceiver Dual Port, Single 5V Supply, Configurable, ±10kV ESD

LTC1387 Single 5V RS232/RS485 Multiprotocol Transceiver Single Port, Configurable

LTC2801/LTC2802/LTC2803/LTC2804

1.8V to 5.5V RS232 Single and Dual Transceivers Up to 1Mbps, ±10kV ESD, Logic Supply Pin, Tiny DFN Packages

LTC2854/LTC2855 3.3V 20Mbps RS485 Transceiver with Integrated Switchable Termination

3.3V Supply, Integrated, Switchable, 120Ω Termination Resistor, ±25kV ESD

LTC2859/LTC2861 20Mbps RS485 Transceiver with Integrated Switchable Termination

5V Supply, Integrated, Switchable, 120Ω Termination Resistor, ±15kV ESD

LTM2881 Complete Isolated RS485/RS422 μModule® Transceiver + Power

20Mbps, 2500VRMS Isolation with Integrated DC/DC Converter, Integrated Switchable 120Ω Termination Resistor, ±15kV ESD

LTM2882 Dual Isolated RS232 µModule Transceiver + Power 1Mbps, 2500VRMS Isolation with Integrated DC/DC Converter, ±10kV ESD

2872 F31

5V

1.8V

VL485/232_1TE485_1DZ1H/F

485/232_2TE485_2

LBGND

VCC SW

22µH 470nF

CAP

LTC2872 LTC2872

RS485

CAT5eCABLE

DYI

RA1

Z1

Y1

Z1

A1

B1

DY1

RA1

DY2

DZ2

RA2

RA2

VDD VEE

Y2

Z2

A2

A2

Y1

120Ω120Ω

2.2µF

0.1µF

2.2µF 2.2µF

RS232

3.3VINTERFACE1.8V

INTERFACE

VDD VEE

2.2µF 2.2µF

VL485/232_1485/232_2TE485_1TE485_2DZ1H/F

DZ2LB

GND

3.3V

VCC SW

22µH 470nF

CAP2.2µF

120Ω

A1

B1120Ω

DY2

RA2

Z2

Y2

A2

B2120Ω