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LTC2870/LTC2871

28701f

TYPICAL APPLICATIONS

DESCRIPTION

RS232/RS485 Multiprotocol

Transceivers with

Integrated Termination

The LTC

2870/LTC2871 are robust pin-configurable mul-

tiprotocol transceivers, supporting RS232, RS485, and

RS422 protocols, operating on a single 3V to 5.5V supply.

The LTC2870 can be configured as two RS232 single-ended

transceivers or one RS485 differential transceiver on shared

I/O lines. The LTC2871 offers independent control of two

RS232 transceivers and one RS485 transceiver, each on

dedicated 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 and the Linear logo are registered trademarks of Linear

Technology Corporation. All other trademarks are the property of their respective owners.

Protocol Switching with

Automatic Termination Selection

FEATURES

APPLICATIONS

One RS485 and Two RS232 Transceivers

3V to 5.5V Supply Voltage

20Mbps RS485 and 500kbps RS232

Automatic Selection of Integrated RS485 (120Ω)

and RS232 (5kΩ)Termination Resistors

Half-/Full-Duplex RS485 Switching

High ESD: ±26kV (LTC2870), ±16kV (LTC2871)

Logic Loopback Mode

1.7V to 5.5V Logic Interface

Supports Up to 256 RS485 Nodes

RS485 Receiver Failsafe Eliminates UART Lockup

Available in 28-Pin 4mm × 5mm QFN and

TSSOP (LTC2870), and 38-Pin 5mm × 7mm QFN

and TSSOP (LTC2871)

Flexible RS232/RS485/RS422 Interface

Software Selectable Multiprotocol Interface Ports

Point-of-Sale Terminals

Cable Repeaters

Protocol Translators

Simultaneous Protocols and

RS485 Termination Switching

RS485 Duplex Switching

28701 TA01

LTC2870

1.7V TO V

3V TO 5.5V

1.7V TO V

3V TO 5.5V

1.7V TO V

3V TO 5.5V

RS485

TERMINATION

RS485

DUPLEX

485/232

TE485

DIN1

ROUT2

DIN2

ROUT1

DOUT1

RIN2

DOUT2

RIN1

LTC2871

120

DI,

RO,

H/F

LTC2870,

LTC2871

HALFFULL

RS485RS232

ONOFF

Summary of content (36 pages)

PAGE 1
LTC2870/LTC2871 RS232/RS485 Multiprotocol Transceivers with Integrated Termination DESCRIPTION FEATURES n n n n n n n n n n n One RS485 and Two RS232 Transceivers 3V to 5.5V Supply Voltage 20Mbps RS485 and 500kbps RS232 Automatic Selection of Integrated RS485 (120Ω) and RS232 (5kΩ)Termination Resistors Half-/Full-Duplex RS485 Switching High ESD: ±26kV (LTC2870), ±16kV (LTC2871) Logic Loopback Mode 1.7V to 5.
PAGE 2
LTC2870/LTC2871 ABSOLUTE MAXIMUM RATINGS (Notes 1 and 2) 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.
PAGE 3
LTC2870/LTC2871 PIN CONFIGURATIONS LTC2871 LTC2871 TOP VIEW VL 1 38 RO 35 RIN1 34 RIN2 VCC 4 RO TE485 VL 36 GND 38 37 36 35 34 33 32 LB 27 VCC 3 H/F 2 H/F TE485 LB GND TOP VIEW VEE 1 31 RIN1 VEE 5 ROUT1 2 30 RIN2 ROUT1 6 33 A ROUT2 3 29 A ROUT2 7 32 B CH2 4 28 B CH2 8 RX485 9 31 VCC 30 Y RX485 5 27 VCC DX485 6 26 Y 39 VEE DI 7 DX485 10 25 GND DI 11 24 Z DIN1 8 23 DOUT1 DIN2 9 DX232 10 22 DOUT2 RX232 11 21 VCC 20 VDD VEE 12 VEE SW GND VEE CAP FEN
PAGE 4
LTC2870/LTC2871 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 = 0V, LB = 0V unless otherwise noted.
PAGE 5
LTC2870/LTC2871 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 = 0V, LB = 0V unless otherwise noted. SYMBOL PARAMETER RTERM Terminating Resistor CONDITIONS MIN TYP MAX UNITS TE485 = VL, A – B = 2V, B = –7V, 0V, 10V (Figure 8) (Note 5) l 108 120 156 Ω RS232 Driver VOLD Output Low Voltage RL = 3kΩ; VEE ≤ –5.9V l –5 –5.7 –7.
PAGE 6
LTC2870/LTC2871 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 = 0V, LB = 0V unless otherwise noted. VL ≤ VCC.
PAGE 7
LTC2870/LTC2871 TYPICAL PERFORMANCE CHARACTERISTICS VCC Supply Current vs Supply Voltage in Shutdown Mode TA = 25°C, VCC = VL = 3.3V, unless otherwise noted. VCC Supply Current vs Supply Voltage in Fast Enable Mode 30 5 VCC Supply Current vs RS485 Data Rate 100 ALL DRIVERS AND RECEIVERS DISABLED TE485 LOW 80 H/F HIGH 20 15 H/F LOW 10 4 SUPPLY CURRENT (mA) SUPPLY CURRENT (mA) INPUT CURRENET (μA) 25 85°C 3 25°C –40°C 2 VCC = 5V VCC = 3.3V ALL RS485 DRIVERS AND RECEIVERS SWITCHING.
PAGE 8
LTC2870/LTC2871 TYPICAL PERFORMANCE CHARACTERISTICS RS485 Receiver Propagation Delay vs Temperature RS485 Receiver Skew vs Temperature VCC = 3.3V, VL = 1.7V VCC = 5V, VL = 1.7V VCC = 3.3V, VL = 3.3V VCC = 5V, VL = 5V 70 RS485 Receiver Output Voltage vs Load Current 3.0 6 2.5 5 OUTPUT VOLTAGE (V) 80 TA = 25°C, VCC = VL = 3.3V, unless otherwise noted. SKEW (ns) DELAY (ns) 2.0 60 1.5 1.0 50 0.5 40 –50 0 25 50 TEMPERATURE (°C) –25 0 25 50 TEMPERATURE (°C) –25 6 INPUT LOW 1.
PAGE 9
LTC2870/LTC2871 TYPICAL PERFORMANCE CHARACTERISTICS RS232 Driver Outputs Enabling and Disabling TA = 25°C, VCC = VL = 3.3V, unless otherwise noted. VDD and VEE Powering Up VDD and VEE Ripple VDD RIPPLE DX232 2V/DIV DOUT1 FEN FEN = 1 10mV/DIV DOUT2 5V/DIV VEE RIPPLE VDD DOUT1 5V/DIV FEN = 0 DOUT2 40μs/DIV VEE 28701 G19 40μs/DIV 28701 G20 TOP CURVES: FAST ENABLE j DX232 BOTTOM CURVES: SHUTDOWN j DX232 28701 G21 40μs/DIV FAST ENABLE MODE, ALL DRIVERS AND RECEIVERS DISABLED.
PAGE 10
LTC2870/LTC2871 PIN FUNCTIONS PIN NAME LTC2870 QFN LTC2870 TSSOP LTC2871 QFN LTC2871 TSSOP DESCRIPTION DOUT1 23 27 RS232 Driver Output 1. DOUT2 22 26 RS232 Driver Output 2. DI 7 11 RS485 Driver Input. DY 7 10 RS485 Driver Input or RS232 Driver Input 1. DZ 8 11 RS232 Driver Input 2. Y 19 22 26 30 RS485 Positive Driver Output. RS232 Driver Output 1 (LTC2870). RS485 Positive Receiver Input (LTC2870 or LTC2871 in Half-Duplex Mode).
PAGE 11
LTC2870/LTC2871 BLOCK DIAGRAM LTC2870 1.7V TO 5.5V (≤ VCC) 3V TO 5.5V 10μH 0.1μF 220nF 2.2μF VL VCC SW CAP DXEN VDD RXEN VEE TE485 RT232 CONTROL LOGIC H/F RT485 485/232 PULSE-SKIPPING BOOST REGULATOR f = 1.
PAGE 12
LTC2870/LTC2871 BLOCK DIAGRAM LTC2871 1.7V TO 5.5V (≤ VCC) 3V TO 5.5V 220nF 10μH 2.2μF 0.1μF VL VCC SW CAP DX232 DX485 VDD RX232 RX485 RT232 CONTROL LOGIC TE485 RT485 H/F VEE PULSE-SKIPPING BOOST REGULATOR f = 1.
PAGE 13
LTC2870/LTC2871 TEST CIRCUITS Y OR Z Y GND OR VL DY/DI + RL GND DY/DI OR VL VOD DRIVER Z – RL + IOZD485, IOSD485 DRIVER + – Z OR Y VOUT VOC – 28701 F02 28701 F01 Figure 1. RS485 Driver DC Characteristics Figure 2. RS485 Driver Output Current IIN485 + – VIN RIN485 = A OR B B OR A RECEIVER VIN IIN485 28701 F03 Figure 3.
PAGE 14
LTC2870/LTC2871 TEST CIRCUITS RL Y VL DY/DI OR GND GND OR VCC DXEN/ DX485 VL ½VL ½VL tZLD485, tZLSD485 CL ½VCC Y OR Z DRIVER 0V tLZD485 VCC 0.5V VOL Z RL DXEN/DX485 VCC OR GND ½VCC Z OR Y 0V tHZD485 tZHD485, tZHSD485 CL VOH 0.5V 28701 F05 Figure 5.
PAGE 15
LTC2870/LTC2871 TEST CIRCUITS IA A RECEIVER TE485 RTERM = VAB VL IA TE485 + – ½VL ½VL 0V VAB tRTEN485 B 90% IA + – tRTZ485 10% VB 28701 F08 Figure 8. RS485 Termination Resistance and Timing Measurements (Note 5) DRIVER INPUT DRIVER OUTPUT DRIVER INPUT tPHLD232 ½VL tPLHD232 tF RL CL 3V DRIVER INPUT –3V SLEW RATE = VL ½VL 0V tR 0V 6V 0V 3V –3V VOHD VOLD tSKEWD232 = |tPHLD232 – tPLHD232| tF OR tR 28701 F09 Figure 9.
PAGE 16
LTC2870/LTC2871 TEST CIRCUITS RECEIVER OUTPUT RECEIVER INPUT RECEIVER INPUT +3V 1.5V 1.5V tPHLR232 CL RECEIVER OUTPUT 90% 10% –3V tPLHR232 ½VL ½VL VL 90% 10% tRR232 tFR232 tSKEWR232 = |tPLHR232 – tPHLR232| 0V 28701 F11 Figure 11. RS232 Receiver Timing Measurements RECEIVER OUTPUT RL –3V OR +3V RXEN/RX232 GND OR VL RXEN/ RX232 VL ½VL ½VL 0V tHZR232 tZHR232 CL RECEIVER OUTPUT tZLR232 RECEIVER OUTPUT 0.5V ½VL 0V tLZR232 ½VL VOHR VL 0.5V VOLR 28701 F12 Figure 12.
PAGE 17
LTC2870/LTC2871 FUNCTION TABLES Table 1.
PAGE 18
LTC2870/LTC2871 FUNCTION TABLES Table 5.
PAGE 19
LTC2870/LTC2871 APPLICATIONS INFORMATION Overview The LTC2870 and LTC2871 are flexible multiprotocol transceivers supporting RS485/RS422 and RS232 protocols. These parts can be powered from a single 3V 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.
PAGE 20
LTC2870/LTC2871 APPLICATIONS INFORMATION Inrush Current and Supply Overshoot Precaution VL Logic Supply and Logic Pins In certain applications fast supply slew rates are generated when power is connected. If the VCC 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.
PAGE 21
LTC2870/LTC2871 APPLICATIONS INFORMATION All devices also feature thermal shutdown protection that disables the drivers, receivers, and RS485 terminators in case of excessive power dissipation (see Note 6). less than approximately 2μs. Increasingly slower signals will have increasingly less effective hysteresis, limited by the DC failsafe value of about 25mV.
PAGE 22
LTC2870/LTC2871 APPLICATIONS INFORMATION The RS232 and RS485 receiver outputs are internally driven high (to VL) or low (to GND) with no external pullup 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.
PAGE 23
LTC2870/LTC2871 APPLICATIONS INFORMATION Logic Loopback A loopback mode connects the driver inputs to the receiver outputs (non-inverting) for self test. This applies to both RS232 and RS485 transceivers. Loopback mode is entered when the LB pin is high and the relevant receiver is enabled. In loopback mode, the drivers function normally. They can be disabled with outputs in a Hi-Z state or left enabled to allow loopback testing in normal operation.
PAGE 24
LTC2870/LTC2871 TYPICAL APPLICATIONS VCC = 3V to 5.5V, VL = 1.7V to VCC. Logic input pins not shown are tied to a valid logic state. VL VL DXEN LTC2870 485/232 VL DXEN RXEN LTC2870 485/232 DXEN RXEN LB LB DY DY Y LTC2870 H/F TE485 LB Y Y DY RA A RA A DZ Z DZ Z 120Ω Z A RA RB RB B Figure 20. LTC2870 in RS232 Mode GND DXEN 485/232 RXEN H/F LB 28701 F22 Figure 21. LTC2870 in RS232 Mode with Loopback TE485 Figure 22.
PAGE 25
LTC2870/LTC2871 TYPICAL APPLICATIONS VCC = 3V to 5.5V, VL = 1.7V to VCC. Logic input pins not shown are tied to a valid logic state. VL VL VL DXEN LTC2870 RXEN DX485 H/F RX232 TE485 LTC2871 LB RS RS 232 485 485/232 DY DX232 DX232 RX485 RX485 LTC2871 DX485 RX232 CH2 CH2 TE485 TE485 H/F H/F LB LB Y Y DI 120Ω DZ Z RA A Z DIN1 A ROUT1 B DIN2 DOUT1 RIN1 RO ROUT2 120Ω RIN2 B RB GND Figure 26.
PAGE 26
LTC2870/LTC2871 TYPICAL APPLICATIONS VCC = 3V to 5.5V, VL = 1.7V to VCC. Logic input pins not shown are tied to a valid logic state.
PAGE 27
LTC2870/LTC2871 TYPICAL APPLICATIONS 1.7V TO VCC VL VL 3V TO 5.5V LTC2870 VCC DXEN RXEN 485/232 H/F RS485 FULL-DUPLEX RS485 HALF-DUPLEX RS232 FULL-DUPLEX 485/232 = 1 H/F = 0 485/232 = 1 H/F = 1 485/232 = 0 H/F = X RS485 RS485 RS232 RS485 RS485 RS232 TE485 DY Y CONTROLLER Z DZ CONNECTOR RA A B RS485 RS232 RS485 RS232 RB GND 28701 F38 Figure 38. LTC2870: Making Use of Shared I/O for Various Communication Configurations 1.7V TO VCC VL VL 3V TO 5.
PAGE 28
LTC2870/LTC2871 TYPICAL APPLICATIONS 1.7V TO VCC VL DX232 RX232 DX485 RX485 H/F TE485 VL 3V TO 5.5V LTC2871 VCC RS232 FULL-DUPLEX RS232 FULL-DUPLEX RS485 FULL-DUPLEX RS485 HALF-DUPLEX H/F = 0 H/F = 1 RS232 RS232 RS485 RS485 RS485 RS485 RS232 RS232 RS232 RS232 DOUT1 DIN1 Y DI Z CONTROLLER DOUT2 DIN2 CONNECTOR ROUT1 RIN1 RS485 A RO RS485 B ROUT2 RS232 RIN2 RS232 GND 28701 F40 Figure 40. LTC2871: Various Communication Configurations 1.7V TO VCC VL VL 3V TO 5.
PAGE 29
LTC2870/LTC2871 TYPICAL APPLICATIONS VCC = 3V to 5.5V, VL = 1.7V to VCC. Logic input pins not shown are tied to a valid logic state. VL VL DX232 LTC2871 RX232 RX232 LB DX485 LB RX485 CH2 DI 120Ω CH2 H/F Y A Z B TE485 RO 120Ω ROUT1 RXIN DIN1 RIN1 DOUT1 RS485 ROUT1 RIN1 ROUT1 DIN1 RO 120Ω RXOUT RS232 UP TO 4000 FT CAT5e CABLE RS232 DRIVER OUT DX232 DX485 RX485 H/F TE485 LTC2871 A Y B Z DI 120Ω GND DRIVER IN GND 28701 F42 Figure 42.
PAGE 30
LTC2870/LTC2871 TYPICAL APPLICATIONS LTC2871 DIN1 PORT 1 LOGIC INTERFACE ROUT1 RIN1 DIN2 PORT 2A/2B LOGIC INTERFACE LTC2871 DOUT1 DIN1 PORT 1 LINE INTERFACE PORT 1 LOGIC INTERFACE PORT 2A LINE INTERFACE PORT 2A LOGIC INTERFACE DOUT2 ROUT2 RIN2 RIN1 PORT 1 LINE INTERFACE DOUT2 ROUT2 PORT 2A/2B LINE INTERFACE RIN2 CH2 SELECT LINE 2A SELECT LINE 2A LTC2871 SELECT LINE 2B CH2 DIN2 ROUT2 DIN1 PORT 3 LOGIC INTERFACE ROUT1 DIN2 CH2 SELECT LINE 2B DOUT1 ROUT1 DOUT2 RIN2 DIN2 PORT 2B L
PAGE 31
LTC2870/LTC2871 TYPICAL APPLICATIONS 3V TO 5.5V 220nF 10μH 2.2μF 1.7V TO VCC VCC SW 220nF CAP CAP VCC SW LTC2870 LTC2871 H/F H/F 2.2μF 0.1μF TE485 TE485 RS485 INTERFACE Y DI 120Ω RO 485/232 Y Z Z A A RA 120Ω B DIN1 DY 120Ω 120Ω B DOUT1 ROUT1 RIN1 DIN2 1μF 1.7V TO VCC VL VL 0.1μF 3V TO 5.5V 10μH RS232 INTERFACE DOUT2 ROUT2 RIN2 VDD GND VDD VEE 1μF VEE GND 28701 F47 1μF 1μF Figure 47. Typical Supply Connections with External Components Shown 3V TO 5.
PAGE 32
LTC2870/LTC2871 PACKAGE DESCRIPTION FE Package 28-Lead Plastic TSSOP (4.4mm) (Reference LTC DWG # 05-08-1663) Exposed Pad Variation EB 9.60 – 9.80* (.378 – .386) 4.75 (.187) 4.75 (.187) 28 2726 25 24 23 22 21 20 19 18 1716 15 6.60 ±0.10 2.74 (.108) 4.50 ±0.10 SEE NOTE 4 0.45 ±0.05 EXPOSED PAD HEAT SINK ON BOTTOM OF PACKAGE 6.40 2.74 (.252) (.108) BSC 1.05 ±0.10 0.65 BSC RECOMMENDED SOLDER PAD LAYOUT 4.30 – 4.50* (.169 – .177) 0.09 – 0.20 (.0035 – .0079) 0.50 – 0.75 (.020 – .030) NOTE: 1.
PAGE 33
LTC2870/LTC2871 PACKAGE DESCRIPTION FE Package 38-Lead Plastic TSSOP (4.4mm) (Reference LTC DWG # 05-08-1772 Rev A) Exposed Pad Variation AA 4.75 REF 38 9.60 – 9.80* (.378 – .386) 4.75 REF (.187) 20 6.60 ±0.10 2.74 REF 4.50 REF SEE NOTE 4 6.40 2.74 REF (.252) (.108) BSC 0.315 ±0.05 1.05 ±0.10 0.50 BSC RECOMMENDED SOLDER PAD LAYOUT 4.30 – 4.50* (.169 – .177) 0.09 – 0.20 (.0035 – .0079) 0.50 – 0.75 (.020 – .030) NOTE: 1. CONTROLLING DIMENSION: MILLIMETERS 2.
PAGE 34
LTC2870/LTC2871 PACKAGE DESCRIPTION UFD Package 28-Lead Plastic QFN (4mm × 5mm) (Reference LTC DWG # 05-08-1712 Rev B) 0.70 ±0.05 4.50 ± 0.05 3.10 ± 0.05 2.50 REF 2.65 ± 0.05 3.65 ± 0.05 PACKAGE OUTLINE 0.25 ±0.05 0.50 BSC 3.50 REF 4.10 ± 0.05 5.50 ± 0.05 RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED 4.00 ± 0.10 (2 SIDES) 0.75 ± 0.05 R = 0.05 TYP PIN 1 NOTCH R = 0.20 OR 0.35 × 45° CHAMFER 2.50 REF R = 0.115 TYP 27 28 0.40 ± 0.
PAGE 35
LTC2870/LTC2871 PACKAGE DESCRIPTION UHF Package 38-Lead Plastic QFN (5mm × 7mm) (Reference LTC DWG # 05-08-1701 Rev C) 0.70 p 0.05 5.50 p 0.05 5.15 ± 0.05 4.10 p 0.05 3.00 REF 3.15 ± 0.05 PACKAGE OUTLINE 0.25 p 0.05 0.50 BSC 5.5 REF 6.10 p 0.05 7.50 p 0.05 RECOMMENDED SOLDER PAD LAYOUT APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED 5.00 p 0.10 0.75 p 0.05 PIN 1 NOTCH R = 0.30 TYP OR 0.35 s 45o CHAMFER 3.00 REF 37 0.00 – 0.05 38 0.40 p0.10 PIN 1 TOP MARK (SEE NOTE 6) 1 2 5.15 ± 0.10 7.
PAGE 36
LTC2870/LTC2871 TYPICAL APPLICATION Quad RS232 Transceiver with RS485 Communication Over Half-Duplex, Terminated Bus 3V TO 5.5V 470nF 22μH 2.2μF VCC VL CAP SW CAP VCC LTC2871 T1IN TE485 DOUT1 DIN1 RIN1 ROUT1 DOUT2 DIN2 RIN2 ROUT2 VL LTC2804 SW T1OUT1 ROUT1 RIN1 T2IN T2OUT ROUT2 VEE RIN2 VDD GND RO 120Ω B DI GND 3.3V A VDD LTC2854 Y A 120Ω Z B VCC 0.1μF TE DI 120Ω RO VEE GND 2.2μF 2.