LTC2870/LTC2871 RS232/RS485 Multiprotocol Transceivers with Integrated Termination Description Features The LTC®2870/LTC2871 are robust pin-configurable multiprotocol transceivers, supporting RS232, RS485, and RS422 protocols, operating on a single 3V to 5.5V supply. The LTC2870 can be configured as two RS232 singleended transceivers or one RS485 differential transceiver on shared I/O lines.
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 SW............................................ –0.3V to (VDD + 0.3V) CAP.............................................. 0.3V to (VEE – 0.3V) A, B, Y, Z, RIN1, RIN2, DOUT1, DOUT2.....
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 37 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 DIN1 8 24 Z DIN2 9 23 DOUT1 DX232 10 22 DOUT2 RX232 11 21 VCC DI 11 20 VDD VEE 12 28 Z DIN1 12 27 DOUT1
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.
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 CONDITIONS Terminating Resistor 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.
LTC2870/LTC2871 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 = 0V, LB = 0V unless otherwise noted. VL ≤ VCC.
LTC2870/LTC2871 Typical Performance Characteristics VCC Supply Current vs Supply Voltage in Shutdown Mode VCC Supply Current vs Supply Voltage in Fast Enable Mode 5 SUPPLY CURRENT (mA) INPUT CURRENET (µA) 15 H/F LOW 10 100 ALL DRIVERS AND RECEIVERS DISABLED TE485 LOW 25 H/F HIGH VCC Supply Current vs RS485 Data Rate 80 4 SUPPLY CURRENT (mA) 30 20 TA = 25°C, VCC = VL = 3.3V, unless otherwise noted. 85°C 3 25°C –40°C 2 4.5 4 INPUT VOLTAGE (V) 5 5.5 3.5 3 4.
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 –25 50 0 25 TEMPERATURE (°C) 75 –25 50 0 25 TEMPERATURE (°C) 75 28701 G10 6 1.
LTC2870/LTC2871 Typical Performance Characteristics RS232 Operation at 500kbps RS485 Operation at 20Mbps DIN1 5V/DIV DIN2 LTC2870 Drivers Changing Modes DI 485/232 Y Z DOUT2 5V/DIV TA = 25°C, VCC = VL = 3.3V, unless otherwise noted.
LTC2870/LTC2871 Pin Functions PIN NAME VCC LTC2870 QFN LTC2870 TSSOP LTC2871 QFN LTC2871 TSSOP DESCRIPTION 16, 20, 24 19, 23, 27 21, 27, 33 25, 31, 37 Input Supply (3V to 5.5V). Tie all three pins together and connect a 2.2µF or larger capacitor between VCC (adjacent to VDD) and GND. VL 25 28 35 1 Logic Supply (1.7V to 5.5V) for the receiver outputs, driver inputs, and control inputs. Bypass this pin to GND with a 0.1µF capacitor if not tied tot VCC. Keep VL ≤ VCC for proper operation.
LTC2870/LTC2871 PIN FUNCTIONS LTC2870 QFN LTC2870 TSSOP 485/232 4 7 Interface Select Input. A logic low enables RS232 mode and a high enables RS485 mode. The mode determines which transceiver inputs and outputs are accessible at the LTC2870 pins as well as which is controlled by the driver and receiver enable pins. Do not float. RXEN 5 8 Receiver Enable. A logic high disables RS232 and RS485 receivers leaving receiver outputs Hi-Z.
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 TE485 RT232 CONTROL LOGIC H/F RT485 485/232 FEN VEE PULSE-SKIPPING BOOST REGULATOR f = 1.2MHz 1µF 1µF DRIVERS LB 232 DY Y RT485 485 120Ω Z 232 DZ LOOPBACK PATH 125k 125k H/F RECEIVERS RT232 232 5k A 125k RA RT485 485 5k 125k 120Ω B RB 232 GND 12 Downloaded from Arrow.com. 2870 BD 28701fb For more information www.linear.
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 H/F RT485 VEE PULSE-SKIPPING BOOST REGULATOR f = 1.2MHz 1µF 1µF CH2 FEN DRIVERS LB 232 DIN1 DOUT1 Y RT485 DI 485 120Ω Z DOUT2 232 DIN2 LOOPBACK PATH 125k 125k H/F RECEIVERS RT232 ROUT1 232 RIN1 5k A 125k RO RT485 485 5k ROUT2 125k 120Ω B RIN2 232 GND 2871 BD 28701fb For more information www.linear.
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 VOC – VOUT 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.
LTC2870/LTC2871 test circuits RL Y VL DY/DI OR GND GND OR VCC DXEN/ DX485 ½VL tZLD485, tZLSD485 CL Z RL DXEN/DX485 CL VCC OR GND 0V tLZD485 ½VCC Y OR Z DRIVER VL ½VL 0.5V VOL VOH 0.5V ½VCC Z OR Y VCC 0V tHZD485 tZHD485, tZHSD485 28701 F05 Figure 5.
LTC2870/LTC2871 test circuits IA RECEIVER TE485 A RTERM = + – VAB VL IA TE485 ½VL ½VL VAB tRTEN485 B 0V 90% IA + – tRTZ485 10% VB 28701 F08 Figure 8. RS485 Termination Resistance and Timing Measurements (Note 5) DRIVER INPUT DRIVER OUTPUT RL DRIVER INPUT tPHLD232 ½VL tPLHD232 tF CL 3V DRIVER OUTPUT –3V SLEW RATE = VL ½VL 0V 6V 0V tR 0V 3V –3V VOHD VOLD tSKEWD232 = |tPHLD232 – tPLHD232| tF OR tR 28701 F09 Figure 9.
LTC2870/LTC2871 test circuits RECEIVER INPUT RECEIVER INPUT RECEIVER OUTPUT 1.5V tPHLR232 CL RECEIVER OUTPUT +3V 1.5V 90% 10% –3V tPLHR232 ½VL ½VL VL 90% 10% tRR232 tFR232 tSKEWR232 = |tPLHR232 – tPHLR232| 0V 28701 F11 Figure 11. RS232 Receiver Timing Measurements –3V OR +3V RECEIVER OUTPUT RL RXEN/RX232 GND OR VL RXEN/ RX232 VL ½VL ½VL tZHR232 CL RECEIVER OUTPUT 0.5V ½VL tZLR232 RECEIVER OUTPUT 0V tHZR232 0V tLZR232 ½VL VOHR VL 0.5V VOLR 28701 F12 Figure 12.
LTC2870/LTC2871 Function Tables Table 1.
LTC2870/LTC2871 function tables Table 5.
LTC2870/LTC2871 Applications Information Overview VCC 3V TO 5.5V 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.
LTC2870/LTC2871 Applications Information Multiple LTC2870 or LTC2871 devices can be powered using the boost regulator from only one of the devices, requiring only one inductor and flying cap. Since the RS232 drivers provide the primary load to the circuit, the following guidelines apply: 1. No more than four RS232 drivers can be supplied from a single device. 2.
LTC2870/LTC2871 Applications Information VL Logic Supply and Logic Pins A separate logic supply pin VL allows the LTC2870 and LTC2871 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 connected to VCC, bypass VL with a 0.1µF capacitor to GND.
LTC2870/LTC2871 Applications Information RS485 Biasing Resistors Not Required B 100mV/DIV A (A-B) 200mV/DIV RO 5V/DIV 200ns/DIV 28701 F16 Figure 17. A 3Mbps Signal Driven Down 4000ft of CAT 5e Cable.
LTC2870/LTC2871 Applications Information RS485 Receiver Input Resistance 126 125k 60Ω B 28701 F17 118 116 –10 –5 0 5 VOLTAGE (V) 15 10 28701 F18 Figure 19. Typical Resistance of the Enabled RS485 Terminator vs Common Mode Voltage on A /B RS485 Half- and Full-Duplex Control Figure 18. Equivalent RS485 Receiver Input Resistance Into A and B (Note 5) Selectable RS485 Termination Proper cable termination is important for good signal fidelity.
LTC2870/LTC2871 Applications Information Because the LTC2870 was not designed to be a direct replacement of the LTC1387, the polarity of the RS485/422 signaling was updated to the more modern convention. For LTC2870 signal compatibility with the LTC1387 or LTC1334, if the polarity of the RS485 cannot be reversed in the software, exclusive-OR logic gates can be used at the driver input and receiver outputs. Tie one of the logic gate inputs to RS485/RS232# and the other to DY or RA.
LTC2870/LTC2871 Applications Information Layout Considerations All VCC pins must be connected together on the PC board with very low impedance traces or with a dedicated plane. A 2.2µF or larger decoupling capacitor (C4 in Figure 13) must be placed less than 0.7cm away from the VCC pin that is adjacent to the VDD pin. 0.1µF capacitors to GND can be added on the VCC pins adjacent to the B and VL pins if the connection to the 2.2µF decoupling capacitor is not direct or if the trace is very narrow.
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. External Components not shown. 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 RB B Figure 21. LTC2870 in RS232 Mode 28701 F22 Figure 22. LTC2870 in RS232 Mode with Loopback VL Figure 23.
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. External Components not shown. 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 RIN1 RO DOUT2 ROUT2 120Ω RIN2 B RB GND GND GND 28701 F26 Figure 27.
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. External Components not shown.
LTC2870/LTC2871 typical Applications Logic input pins not shown are tied to a valid logic state. External Components not shown. 1.7V TO VCC VL VL 3V TO 5.5V LTC2870 VCC DXEN RXEN 485/232 H/F TE485 DY Y CONTROLLER Z 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 DZ CONNECTOR RA A B RS485 RS232 RS485 RS232 RB GND 28701 F38 Figure 39.
LTC2870/LTC2871 typical Applications Logic input pins not shown are tied to a valid logic state. External Components not shown. 1.7V TO VCC VL DX232 RX232 DX485 RX485 H/F TE485 VL 3V TO 5.
LTC2870/LTC2871 typical Applications Logic input pins not shown are tied to a valid logic state. External Components not shown. 1.7V TO VCC VL DX232 RX232 DX485 RX485 H/F TE485 VL 3V TO 5.
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. External Components not shown. VL VL DX232 LTC2871 RX232 RX232 LB DX485 LB RX485 CH2 DI 120Ω CH2 H/F Y A Z B TE485 RO 120Ω DIN1 ROUT1 RXIN RS232 RIN1 DOUT1 RS485 UP TO 4000 FT CAT5e CABLE DOUT1 DRIVER OUT DX232 DX485 RX485 H/F TE485 LTC2871 ROUT1 120Ω RS232 RIN1 DIN1 RO RXOUT A Y B Z DI 120Ω GND DRIVER IN GND 28701 F42 Figure 45.
LTC2870/LTC2871 typical Applications Logic input pins not shown are tied to a valid logic state. External Components not shown.
LTC2870/LTC2871 typical Applications Logic input pins not shown are tied to a valid logic state. 3V TO 5.5V 220nF 10µH 2.2µF 1.7V TO VCC VCC VL SW 220nF CAP CAP VCC SW TE485 DI 120Ω RO 120Ω DIN1 H/F 0.1µF 485/232 Y Z Z A A B DY 120Ω RA 120Ω B DOUT1 ROUT1 RIN1 DIN2 1µF 2.2µF TE485 RS485 INTERFACE Y 1.7V TO VCC VL LTC2870 LTC2871 H/F 0.1µF 3V TO 5.5V 10µH RS232 INTERFACE DOUT2 ROUT2 RIN2 VDD GND VDD VEE 1µF VEE GND 28701 F47 1µF 1µF Figure 50.
LTC2870/LTC2871 typical Applications L1 22µH C4 2.2µF DX485-A RX485-A DX232-A RX232-A TE485-A DIN1 DI RO ROUT1 DI-A RO-A ROUT2 C2 2.2µF DX485-B RX485-B DX232-B RX232-B TE485-B ROUT2 C3 2.2µF DX485-C RX485-C DX232-C RX232-C TE485-C ROUT2 C5 2.
LTC2870/LTC2871 Package Description Please refer to http://www.linear.com/product/LTC2870#packaging for the most recent package drawings. FE Package 28-Lead Plastic TSSOP (4.4mm) FE Package (Reference LTC DWG # 05-08-1663 Rev K) 28-Lead Plastic TSSOP (4.4mm) Exposed Pad Variation EB (Reference LTC DWG # 05-08-1663 Rev K) Exposed Pad Variation EB 9.60 – 9.80* (.378 – .386) 4.75 (.187) 4.75 (.187) 28 27 26 2524 23 22 21 20 1918 17 16 15 6.60 ±0.10 4.50 ±0.10 2.74 (.108) SEE NOTE 4 0.45 ±0.
LTC2870/LTC2871 Package Description Please refer to http://www.linear.com/product/LTC2870#packaging for the most recent package drawings. FE Package FE Package 38-Lead Plastic TSSOP (4.4mm) 38-LeadLTC Plastic (4.4mm) (Reference DWG TSSOP # 05-08-1772 Rev C) (Reference LTC DWG 05-08-1772 Exposed Pad #Variation AA Rev C) Exposed Pad Variation AA 4.75 REF 38 9.60 – 9.80* (.378 – .386) 4.75 REF (.187) 20 6.60 ±0.10 4.50 REF 2.74 REF SEE NOTE 4 6.40 2.74 REF (.252) (.108) BSC 0.315 ±0.05 1.05 ±0.10 0.
LTC2870/LTC2871 Package Description Please refer to http://www.linear.com/product/LTC2870#packaging for the most recent package drawings. UFD Package UFD Package 28-Lead Plastic QFN (4mm × 5mm) 28-Lead Plastic QFN (4mm × 5mm) (Reference LTC DWG # 05-08-1712 Rev C) (Reference LTC DWG # 05-08-1712 Rev C) 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.
LTC2870/LTC2871 Package Description Please refer to http://www.linear.com/product/LTC2870#packaging for the most recent package drawings. Package UHFUHF Package 38-Lead Plastic (5mm × 7mm) 38-Lead Plastic QFNQFN (5mm × 7mm) (Reference LTC DWG # 05-08-1701 Rev C) (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.
LTC2870/LTC2871 Revision History REV DATE DESCRIPTION A 11/14 Added H-Grade (–40°C to 125°C) PAGE NUMBER Removed Absolute Maximum Ratings for VDD – VEE Increased TJMAX to 150°C 3 10-11 Expanded DC/DC Converter Applications section 20-21 21 Expanded RS485 Biasing Resistors Not Required Applications section 23 Added RS485 Polarity and the LTC1334, LTC1387 Applications section 24-25 Added Standards Compatibility Applications section 08/17 2 Noted which pins should not float Added Running with
LTC2870/LTC2871 Typical Application Quad RS232 Transceiver with RS485 Communication Over Half-Duplex, Terminated Bus 3V TO 5.5V 2.2µF 470nF 22µH VCC VL CAP SW CAP VCC VL LTC2804 LTC2871 T1IN TE485 DIN1 DOUT1 ROUT1 RIN1 DIN2 DOUT2 ROUT2 RIN2 SW T1OUT1 ROUT1 RIN1 T2IN T2OUT ROUT2 VEE RIN2 VDD GND 3.3V A RO 120Ω B DI GND Logic input pins not shown are tied to a valid logic state. VDD LTC2854 Y A 120Ω Z B VCC TE 0.1µF DI 120Ω RO VEE GND 2.2µF 2.
