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LTC2872

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

Four RS232 and Two RS485 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

Logic Loopback Mode

High ESD: ±16kV on Line I/O

1.7V to 5.5V Logic Interface

Supports Up to 256 RS485 Nodes

RS485 Receiver Full Failsafe Eliminates UART Lockup

Available in 38-Pin 5mm × 7mm QFN Package

Flexible RS232/RS485/RS422 Interface

Software Selectable Multiprotocol Interface Ports

Point-of-Sale Terminals

Cable Repeaters

Protocol Translators

PROFIBUS-DP Networks

RS232 Mode Mixed Mode with RS485 Termination

2872 TA01

LTC2872

3V TO 5.5V1.7V TO V

RS485

TERMINATION

H/F

DY1

2.2µF

RA1

DY2

RA2

DY1

DY2

RB2

RA2

DZ2

RB1

RA1

DZ1

LTC2872

DY2

RB2

RA2

DZ2

DY1

TE485-1

LTC2872

120Ω

ONOFF

RS485

DUPLEX

HALFFULL

RA1

120Ω

CAP

470nF

22µH

2.2µF

2.2µF2.2µF 2.2µF 2.2µF

3V TO 5.5V1.7V TO V

2.2µF

CAP

470nF

22µH

3V TO 5.5V1.7V TO V

2.2µF

CAP

470nF

22µH

Summary of content (28 pages)

PAGE 1
Features n n n n n n n n n n n Four RS232 and Two RS485 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 Logic Loopback Mode High ESD: ±16kV on Line I/O 1.7V to 5.
PAGE 2
LTC2872 Absolute Maximum Ratings Pin Configuration (Note 1) RB2 RA2 GND VL LB RA1 RB1 TOP VIEW 38 37 36 35 34 33 32 VCC 1 31 VCC A1 2 30 A2 B1 3 29 B2 Y1 4 28 Y2 GND 5 27 GND Z1 6 26 Z2 39 VEE DY1 7 25 DY2 DZ1 8 24 DZ2 RXEN1 9 23 RXEN2 DXEN1 10 22 DXEN2 21 VCC TE485_1 11 20 VDD TE485_2 12 SW GND CAP FEN H/F 485/232_2 13 14 15 16 17 18 19 485/232_1 Input Supplies VCC, VL...................................................... –0.3V to 7V Generated Supplies VDD...........
PAGE 3
LTC2872 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 VL Logic Supply Voltage Operating Range VCC Supply Current in Shutdown Mode VL ≤ VCC l 1.
PAGE 4
LTC2872 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 VEE V RS232 Driver VOLD Output Low Voltage RL = 3kΩ, VEE ≤ –6V l –5 –5.7 VOHD Output High Voltage RL = 3kΩ, VDD ≥ 6.5V l 5 6.
PAGE 5
LTC2872 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.
PAGE 6
LTC2872 Typical Performance Characteristics VCC Supply Current vs Supply Voltage in Shutdown Mode VCC Supply Current vs Supply Voltage in Fast Enable Mode 4.6 30 H/F HIGH 20 15 H/F LOW 10 5 4.2 SUPPLY CURRENT (mA) SUPPLY CURRENT (mA) INPUT CURRENET (µA) 200 4.4 25 0 85°C 4.0 25°C 3.8 3.6 –40°C 3.4 3.5 3 4.5 4 INPUT VOLTAGE (V) 5 3.0 5.5 3.5 4 4.5 SUPPLY VOLTAGE (V) 5 0.5nF 30 0.05nF 25 20 2.5nF 0.5nF 0.05nF 15 0 3.
PAGE 7
LTC2872 Typical Performance Characteristics RS485 Receiver Propagation Delay vs Temperature 60 50 4 3 2 1 40 –50 –25 50 0 25 TEMPERATURE (°C) 75 0 100 0 2 8 4 6 OUTPUT CURRENT (mA) RS232 Receiver Output Voltage vs Load Current 130 1.6 1.4 4 3 2 1.
PAGE 8
LTC2872 Pin Functions VCC (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.
PAGE 9
LTC2872 Pin Functions DXEN1 (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.
PAGE 10
LTC2872 Block Diagram 1.7V TO 5.5V (≤ VCC) 3V TO 5.5V 470nF 22µH 2.2µF 0.1µF 35 21 VL VCC 19 SW 17 CAP 15 H/F 16 FEN 36 LB VDD TRANSCEIVER #1 10 DXEN 9 RXEN1 RT232 CONTROL LOGIC 11 TE485_1 13 485/232_1 VEE PULSE-SKIPPING BOOST REGULATOR f = 1.2MHz GND VCC DRIVERS DY1 2.2µF 39 18 2.
PAGE 11
LTC2872 Test Circuits Y OR Z Y GND DY OR VL RL + GND OR VL VOD DRIVER Z – RL + DY IOZD485, IOSD485 DRIVER VOC + – Z OR Y VOUT – 2872 F02 2872 F01 Figure 1. RS485 Driver DC Characteristics Figure 2. RS485 Driver Output Short-Circuit Current IIN485 + – A OR B B OR A VIN RIN485 = RECEIVER VIN IIN485 2872 F03 Figure 3.
PAGE 12
LTC2872 test circuits RL Y VL OR GND DY GND OR VCC DXEN ½VL tZLD485, tZLSD485 CL Z RL DXEN CL VCC OR GND VCC 0.5V VOL VOH 0.5V ½VCC Z OR Y 0V tLZD485 ½VCC Y OR Z DRIVER VL ½VL 0V tHZD485 tZHD485, tZHSD485 2872 F05 Figure 5. RS485 Driver Enable and Disable Timing Measurements VAB ±VAB/2 A–B A VCM B 0V tPLHR485 RA RECEIVER CL ±VAB/2 RA 90% 10% tPHLR485 ½VL ½VL tRR485 90% 10% tFR485 tSKEWR485 = tPLHR485 – tPHLR485 –VAB VCC 0V 2872 F06 Figure 6.
PAGE 13
LTC2872 test circuits IA RECEIVER TE485 A RTERM = + – VAB IA VL TE485 ½VL ½VL VAB tRTEN485 B 0V 90% IA + – tRTZ485 10% VB 2872 F08 Figure 8. RS485 Termination Resistance and Timing Measurements (Note 5) DRIVER INPUT DRIVER OUTPUT RL DRIVER INPUT CL tPHLD232 ½VL tPLHD232 tF 3V DRIVER INPUT 6V 0V tR 0V –3V SLEW RATE = VL ½VL 0V 3V –3V VOHD VOLD tSKEWD232 = |tPHLD232 – tPLHD232| tF OR tR 2872 F09 Figure 9.
PAGE 14
LTC2872 test circuits RECEIVER INPUT RECEIVER INPUT RECEIVER OUTPUT 1.5V tPHLR232 CL +3V 1.5V 90% RECEIVER OUTPUT ½VL 10% –3V tPLHR232 ½VL VL 90% 10% tRR232 tFR232 tSKEWR232 = |tPLHR232 – tPHLR232| 0V 2872 F11 Figure 11. RS232 Receiver Timing Measurements RECEIVER OUTPUT RL –3V OR +3V RXEN GND OR VL RXEN VL ½VL ½VL tZHR232 CL RECEIVER OUTPUT 0.5V ½VL tZLR232 RECEIVER OUTPUT 0V tHZR232 0V tLZR232 ½VL VOHR VL 0.5V VOLR 2872 F12 Figure 12.
PAGE 15
LTC2872 Function Tables Table 1. Shutdown and Fast Enable Modes 485/232_1 AND RXEN1 AND DXEN1 AND 485/232_2 RXEN2 DXEN2 FEN TE485_1 AND TE485_2 H/F LB DC/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.
PAGE 16
LTC2872 function tables Table 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.
PAGE 17
LTC2872 Applications Information Overview The LTC2872 is a flexible multiprotocol transceiver supporting 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.
PAGE 18
LTC2872 Applications Information Inrush Current and Supply Overshoot Precaution In certain applications fast supply slew rates are generated 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.
PAGE 19
LTC2872 Applications Information RA RISING THRESHOLD SHIFTS IF SIGNAL IS IN WINDOW > ~2µs TO SUPPORT FAILSAFE –80mV –50mV 0V VAB (NOTE 5) 80mV 2872 F15 Figure 15. RS485 Receiver Input Threshold Characteristics with Typical Values Shown 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.
PAGE 20
LTC2872 Applications Information Selectable RS485 Termination Proper cable termination is important for good signal fidelity. 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.
PAGE 21
LTC2872 Applications Information At frequencies below 100kbps, the maximum cable length is determined by DC resistance in the cable. In this example, 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 relationship.
PAGE 22
LTC2872 Typical Applications 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.
PAGE 23
LTC2872 typical Applications 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. ½ LTC2872 VL H/F TE485 ½ LTC2872 ½ LTC2872 120Ω 120Ω VL VL TE485 DZ H/F TE485 DZ H/F 2872 F24 Figure 24. Typical RS485 Half Duplex Network ½ LTC2872 TE485 H/F MASTER SLAVE ½ LTC2872 ½ LTC2872 120Ω 120Ω 120Ω VL TE485 DZ H/F VL TE485 DZ H/F 2872 F25 Figure 25.
PAGE 24
LTC2872 typical Applications 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. LTC2872 H/F S3 Y1 RS485 INTERFACE Z1 INPUT1 RA1 OUTPUT A1 RXEN1 S1 H/F B1 INPUT2 Y2 Z2 INPUT3 RA2 A2 RXEN2 S2 B2 INPUT4 2872 F26 S1 S2 S3 SELECTED INPUT 0 1 1 INPUT1 0 1 0 INPUT2 1 0 1 INPUT3 1 0 0 INPUT4 1 1 X NONE/Hi-Z 0 0 X INVALID Figure 26.
PAGE 25
LTC2872 typical Applications 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. LTC2872 OUT1 S1 OUT2 S2 LTC2872 RA1 A1 RS232 INPUT RXEN1 RB2 OUT2 RXEN2 RS232 INPUT RIN –OR– A2 B1 RXEN1 S1 RIN RA2 RB1 OUT1 B2 RXEN2 S2 2872 F27 S1 S2 RIN ACTIVE OUTPUT 0 1 5k OUT1 1 0 5k OUT2 1 1 62.5k NONE (Hi-Z) 0 0 2.
PAGE 26
LTC2872 typical Applications 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. RA1 LTC2872 DY2 Y2 RS232 IN A1 RS232 OUT Y1 Z2 RS485 OUT A2 120Ω DY1 B2 RS485 IN RA2 2872 F29 Figure 29. RS232 ↔ RS485 Conversion RA1 LTC2872 DY2 Y2 A1 B1 120Ω 120Ω A2 Y1 Z1 Z2 120Ω 120Ω DY1 B2 RA2 2872 F29 Figure 30.
PAGE 27
LTC2872 Package Description Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings. UHF Package 38-Lead Plastic QFN (5mm × 7mm) (Reference LTC DWG # 05-08-1701 Rev C) 0.70 ± 0.05 5.50 ± 0.05 5.15 ± 0.05 4.10 ± 0.05 3.00 REF 3.15 ± 0.05 PACKAGE OUTLINE 0.25 ± 0.05 0.50 BSC 5.5 REF 6.10 ± 0.05 7.50 ± 0.05 RECOMMENDED SOLDER PAD LAYOUT APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED 5.00 ± 0.10 0.75 ± 0.05 PIN 1 NOTCH R = 0.30 TYP OR 0.35 × 45° CHAMFER 3.
PAGE 28
LTC2872 Typical Application 5V 2.2µF 1.8V 0.1µF VCC 3.3V 470nF 22µH SW 2.2µF CAP VL 485/232_2 485/232_1 TE485_2 TE485_1 LB LTC2872 DZ1 GND H/F RS485 Z1 CAT5e CABLE A1 RA1 SW CAP DZ2 LB GND Y1 120Ω Z1 1.8V INTERFACE VCC VL 485/232_1 485/232_2 LTC2872 TE485_1 TE485_2 DZ1 H/F Y1 DY1 470nF 22µH 120Ω A1 DYI RA1 120Ω 120Ω B1 B1 DY2 Y2 Y2 DZ2 Z2 Z2 RA2 A2 3.3V INTERFACE DY2 RS232 A2 RA2 120Ω RA2 A2 VDD VEE B2 VDD VEE 2872 F31 2.2µF 2.2µF 2.2µF 2.2µF Figure 31.