LTC2870/LTC2871
RS232/RS485 Multiprotocol
Transceivers with
Integrated Termination
Description
Features
n
n
n
n
n
n
n
n
n
n
n
n
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 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.
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
H-Grade Available (–40°C to 125°C)
Available in 28-Pin 4mm × 5mm QFN and
TSSOP (LTC2870), and 38-Pin 5mm × 7mm QFN
and TSSOP (LTC2871)
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.
Applications
n
n
n
n
An integrated DC/DC boost converter uses a tiny
2mm × 1.6mm inductor and one capacitor, eliminating
the need for multiple supplies for driving RS232 levels.
Software Selectable RS232/RS485/RS422 Interface
Point-of-Sale Terminals
Cable Repeaters
Protocol Translators
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.
Typical Applications
Protocol Switching with
Automatic Termination Selection
1.7V TO VCC
0.1µF
RS232 RS485
VL
485/232
220nF
CAP
SW
Simultaneous Protocols and
RS485 Termination Switching
3V TO 5.5V
10µH
VCC
2.2µF
LTC2870
0.1µF
RS485
TERMINATION
DY
1.7V TO VCC
220nF
VL
CAP SW
TE485
3V TO 5.5V
10µH
1.7V TO VCC
2.2µF
VCC
120Ω
RS485
1µF
GND
1µF
1µF
B
RA
RIN2
ROUT2
VDD
A
RO,
DOUT2
DIN2
H/F
DUPLEX
RIN1
ROUT1
VEE
FULL HALF
DOUT1
DIN1
RB
Z
DY
B
B
2.2µF
120Ω
RO
A
VCC
Y
DI,
Z
A
RA
CAP SW
3V TO 5.5V
10µH
Y
DI
DZ
VL
220nF
LTC2870,
LTC2871
Z
VDD
0.1µF
LTC2871
OFF ON
Y
RS485 Duplex Switching
GND
VEE
VDD
1µF
1µF
GND
VEE
1µF
28701 TA01
28701fa
For more information www.linear.com/LTC2870
1
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.....–15V to 15V
DI, DZ, DY, RXEN, DXEN, LB, H/F, TE485, RX485,
DX485, RX232, DX232, DIN1, DIN2,
485/232, CH2............................................ –0.3V to 7V
FEN, RA, RB, RO, ROUT1, ROUT2....–0.3V to (VL + 0.3V)
Differential Enabled Terminator Voltage
(A-B or Y-Z)...........................................................±6V
Operating Temperature
LTC2870C/LTC2871C................................ 0°C to 70°C
LTC2870I/LTC2871I.............................. –40°C to 85°C
LTC2870H........................................... –40°C to 125°C
Storage Temperature Range................... –65°C to 125°C
Lead Temperature (Soldering, 10 sec)
FE package........................................................ 300°C
Pin Configuration
LTC2870
LTC2870
TOP VIEW
LB
1
28 VL
H/F
2
27 VCC
TE485
3
26 GND
VEE
4
25 A
RA
5
24 B
RB
6
23 VCC
485/232
7
18 GND
RXEN
8
17 Z
DXEN
9
GND
VCC
VL
LB
H/F
TE485
TOP VIEW
28 27 26 25 24 23
VEE 1
22 A
RA 2
21 B
RB 3
20 VCC
19 Y
485/232 4
29
VEE
RXEN 5
21 GND
20 Z
16 VCC
DY 10
19 VCC
15 VDD
DZ 11
18 VDD
9 10 11 12 13 14
FEN 12
17 SW
GND 13
16 GND
CAP 14
15 VEE
DY 7
GND
VEE
CAP
FEN
GND
DZ 8
UFD PACKAGE
28-LEAD (4mm × 5mm) PLASTIC QFN
FE PACKAGE
28-LEAD PLASTIC TSSOP
TJMAX = 150°C, θJA = 43°C/W
EXPOSED PAD (PIN 29) IS VEE,
MUST BE SOLDERED TO PCB
2
22 Y
SW
DXEN 6
29
VEE
TJMAX = 150°C, θJA = 30°C/W
EXPOSED PAD (PIN 29) IS VEE,
MUST BE SOLDERED TO PCB
28701fa
For more information www.linear.com/LTC2870
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
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
DIN2 13
26 DOUT2
25 VCC
RX232 15
VEE 16
24 VDD
23 VEE
FEN 17
22 SW
GND 18
21 GND
CAP 19
20 VEE
VEE
SW
GND
VEE
CAP
FEN
29 GND
DX232 14
13 14 15 16 17 18 19
GND
39
VEE
UHF PACKAGE
38-LEAD (5mm × 7mm) PLASTIC QFN
FE PACKAGE
38-LEAD PLASTIC SSOP
TJMAX = 150°C, θJA = 28°C/W
EXPOSED PAD (PIN 39) IS VEE,
MUST BE SOLDERED TO PCB
TJMAX = 150°C, θJA = 34°C/W
EXPOSED PAD (PIN 39) IS VEE,
MUST BE SOLDERED TO PCB
Order Information
LEAD FREE FINISH
TAPE AND REEL
PART MARKING
PACKAGE DESCRIPTION
TEMPERATURE RANGE
LTC2870CFE#PBF
LTC2870IFE#PBF
LTC2870CFE#TRPBF
LTC2870IFE#TRPBF
LTC 2870FE
LTC 2870FE
28-Lead Plastic TSSOP
28-Lead Plastic TSSOP
0°C to 70°C
–40°C to 85°C
LTC2870CUFD#PBF
LTC2870IUFD#PBF
LTC2870HUFD#PBF
LTC2870CUFD#TRPBF
LTC2870IUFD#TRPBF
LTC2870HUFD#TRPBF
2870
2870
2870
28-Lead (4mm × 5mm) Plastic QFN
28-Lead (4mm × 5mm) Plastic QFN
28-Lead (4mm × 5mm) Plastic QFN
0°C to 70°C
–40°C to 85°C
–40°C to 125°C
LTC2871CFE#PBF
LTC2871IFE#PBF
LTC2871CFE#TRPBF
LTC2871IFE#TRPBF
LTC2871FE
LTC2871FE
38-Lead Plastic TSSOP
38-Lead Plastic TSSOP
0°C to 70°C
–40°C to 85°C
LTC2871CUHF#PBF
LTC2871IUHF#PBF
LTC2871CUHF#TRPBF
LTC2871IUHF#TRPBF
2871
2871
38-Lead (5mm × 7mm) Plastic QFN
38-Lead (5mm × 7mm) Plastic QFN
0°C to 70°C
–40°C to 85°C
Consult LTC Marketing for parts specified with wider operating temperature ranges.
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/
Product Selection Guide
PART NUMBER
CONFIGURABLE TRANSCEIVER COMBINATIONS (RS485 + RS232)
PACKAGES
LTC2870
(0 + 0), (1 + 0), (0 + 2)
28-Lead QFN, 28-Lead TSSOP
LTC2871
(0 + 0), (1 + 0), (1 + 1), (1 + 2), (0 + 1), (0 + 2)
38-Lead QFN, 38-Lead TSSOP
28701fa
For more information www.linear.com/LTC2870
3
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
CONDITIONS
MIN
TYP
MAX
UNITS
Power Supply
VCC
Supply Voltage Operating Range
VL
Logic Supply Voltage Operating Range
VL ≤ VCC
VCC Supply Current in Shutdown Mode
RXEN = VL, DXEN = TE485 = FEN = 0V,
(LTC2870)
DX485 = DX232 = TE485 = FEN = H/F = 0V,
RX485 = RX232 = VL (LTC2871)
VCC Supply Current in Transceiver Mode
(Outputs Unloaded) (Note 3)
485/232 = DXEN = VL, RXEN = 0V,
DY/DZ = 0V or VL (LTC2870)
DX485 = DX232 = VL, RX485 = RX232 = 0V,
DI/DIN1/DIN2 = 0V or VL (LTC2871)
VL Supply Current in Transceiver Mode
(Outputs Unloaded)
3
5.5
V
1.7
VCC
V
60
µA
8
l
3.3
0
l
mA
5
µA
6
VCC
VCC
V
V
V
RS485 Driver
|VOD|
Differential Output Voltage
RL = ∞, VCC = 3V (Figure 1)
RL = 27Ω, VCC = 3V (Figure 1)
RL = 50Ω, VCC = 3.13V (Figure 1)
l
l
l
1.5
1.5
2
∆|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
125
µA
RS485 Receiver
IIN485
Input Current
VIN = 12V or –7V, VCC = 0V or 3.3V (Figure 3)
(Note 5)
l
–100
RIN485
Input Resistance
VIN = 12V or –7V, VCC = 0V or 3.3V (Figure 3)
(Note 5)
l
96
Differential Input Signal Threshold Voltage
(A-B)
–7V ≤ (A or B) ≤ 12V (Note 5)
l
Input Hysteresis
B = 0V (Notes 3, 5)
Differential Input Failsafe Threshold Voltage
–7V ≤ (A or B) ≤ 12V (Note 5)
VOL
VOH
4
125
kΩ
±200
130
l
–200
–50
mV
mV
0
Input DC Failsafe Hysteresis
B = 0V (Note 5)
Output Low Voltage
Output Low, I(RA, RO) = 3mA (Sinking),
3V ≤ VL ≤ 5.5V
l
0.4
V
Output Low, I(RA, RO) = 1mA (Sinking),
1.7V ≤ VL < 3V
l
0.4
V
Output High, I(RA, RO) = –3mA (Sourcing),
3V ≤ VL ≤ 5.5V
l
VL – 0.4
V
Output High, I(RA, RO) = –1mA (Sourcing),
1.7V ≤ VL < 3V
l
VL – 0.4
V
Three-State (High Impedance) Output Current
0V ≤ (RA, RO) ≤VL, VL = 5.5V
l
Short-Circuit Output Current
0V ≤ (RA, RO) ≤VL, VL = 5.5V
l
Output High Voltage
25
mV
0
mV
±5
µA
±125
mA
28701fa
For more information www.linear.com/LTC2870
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
–7.5
V
VOHD
Output High Voltage
RL = 3kΩ; VDD ≥ 6.5V
l
5
6.2
7.5
V
Three-State (High Impedance) Output Current
Y or Z (LTC2870) = ±15V
RS232 Receiver Enabled
DOUT1 or DOUT2 (LTC2871) = ±15V
l
±156
µA
l
±10
µA
Driver Output = 0V
l
±35
±90
mA
Output Short-Circuit Current
RS232 Receiver
Input Threshold Voltage
l
0.6
1.5
2.5
V
Input Hysteresis
l
0.1
0.4
1.0
V
0.4
V
Output Low Voltage
I(RA, RB, ROUT1, ROUT2) = 1mA (Sinking)
1.7V ≤ VL ≤ 5.5V
l
Output High Voltage
I(RA, RB, ROUT1, ROUT2) = –1mA (Sourcing)
1.7V ≤ VL ≤ 5.5V
l
VL – 0.4
Input Resistance
–15V ≤ (A, B, RIN1, RIN2) ≤ 15V,
RS232 Receiver Enabled
l
3
Three-State (High Impedance) Output Current
0V ≤ (RA, RB, ROUT1, ROUT2) ≤ VL
Output Short-Circuit Current
VL = 5.5V
0V ≤ (RA, RB, ROUT1, ROUT2) ≤ VL
V
5
7
kΩ
l
0
±5
µA
l
±25
±50
mA
Logic Inputs
Threshold Voltage
l
Input Current
l
0.4
0
0.75 • VL
V
±5
µA
Power Supply Generator
VDD
Regulated VDD Output Voltage
VEE
Regulated VEE Output Voltage
RS232 Drivers Enabled, Outputs Loaded with
RL = 3kΩ to GND, DIN1/DY = VL, DIN2/DZ = 0V
(Note 3)
7
V
–6.3
V
Human Body Model to GND or VCC, Powered or
Unpowered (Note 7)
±26
kV
±16
kV
Human Body Model (Note 7)
±4
kV
ESD
LTC2870 Interface Pins (A, B, Y, Z)
LTC2871 Interface Pins (A, B, Y, Z, RIN1,
RIN2, DOUT1, DOUT2)
All Other Pins
28701fa
For more information www.linear.com/LTC2870
5
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.
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
RS485 AC Characteristics
20
Mbps
Maximum Data Rate
(Note 3)
l
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
±6
ns
tRD485, tFD485
Driver Rise or Fall Time
RDIFF = 54Ω, CL = 100pF (Figure 4)
l
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
RL = 500Ω, CL = 50pF (Figure 5)
l
8
µs
tPLHR485, tPHLR485 Receiver Input to Output
CL = 15pF, VCM = 1.5V, |A – B| = 1.5V
(Figure 6) (Note 5)
l
65
85
ns
tPLHD485
tPHLD485
tSKEWR485
Differential Receiver Skew
|tPLHR485 – tPHLR485|
CL = 15pF (Figure 6)
l
1
6
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 = 1kΩ, CL = 15pF (Figure 7)
l
50
ns
tRTEN485, tRTZ485
Termination Enable or Disable Time
FEN = VL, VB = 0V, VAB = 2V (Figure 8) (Note
5)
l
100
µs
Maximum Data Rate
RL = 3kΩ, CL = 2500pF
RL = 3kΩ, CL = 500pF
(Note 3)
l
l
100
500
Driver Slew Rate (Figure 9)
RL = 3kΩ, CL = 2500pF
RL = 3kΩ, CL = 50pF
l
l
4
RL = 3kΩ, CL = 50pF (Figure 9)
l
1
RS232 AC Characteristics
tPHLD232, tPLHD232 Driver Propagation Delay
kbps
kbps
30
V/µs
V/µs
2
µs
tSKEWD232
Driver Skew
RL = 3kΩ, CL = 50pF (Figure 9)
tZLD232, tZHD232,
tLZD232, tHZD232
Driver Output Enable or Disable Time
FEN = VL, RL = 3kΩ, CL = 50pF (Figure 10)
l
0.4
2
µs
CL = 150pF (Figure 11)
l
60
200
ns
tPHLR232, tPLHR232 Receiver Propagation Delay
50
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
l
0.2
2
ms
VDD /VEE Supply Rise Time
FEN = , (Notes 3 and 4)
Note 1: Stresses beyond those listed under Absolute Maximum Ratings
Note 4: Time from FEN until VDD ≥ 5V and VEE ≤ –5V. External
may cause permanent damage to the device. Exposure to any Absolute
components as shown in the Typical Application section.
Maximum Rating condition for extended periods may affect device
Note 5: Condition applies to A, B for H/F = 0V, and Y, Z for H/F = VL.
reliability and lifetime.
Note 6: This IC includes overtemperature protection that is intended
Note 2: All currents into device pins are positive; all currents out of device
to protect the device during momentary overload conditions.
pins are negative. All voltages are referenced to device ground unless
Overtemperature protection activates at a junction temperature exceeding
otherwise specified.
150°C. Continuous operation above the specified maximum operating
Note 3: Guaranteed by other measured parameters and not tested directly.
junction temperature may result in device degradation or failure.
Note 7: Guaranteed by design and not subject to production test.
6
28701fa
For more information www.linear.com/LTC2870
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
4.5
4
SUPPLY VOLTAGE (V)
5
ALL RS232 DRIVERS
AND RECEIVERS
SWITCHING.
30
25
20
2.5nF
0.5nF
4.5
100
90
–40°C
85°C
100
200
300
DATA RATE (kbps)
400
3.5
25°C
70
500
3
3.5
4
4.5
SUPPLY VOLTAGE (V)
150
2.5
100
SHORT-CIRCUIT CURRENT (mA)
SKEW (ns)
75
100
28701 G07
–25
0
25
50
TEMPERATURE (°C)
1.5
0
–50
–25
50
0
25
TEMPERATURE (°C)
100
75
28701 G06
3.0
0.5
0
25
50
TEMPERATURE (°C)
VCC = 5V
VCC = 3.3V
RS485 Driver Short-Circuit
Current vs Short-Circuit Voltage
1.0
–25
0
–50
5.5
5
2.0
10
RL = 54Ω
1.5
28701 G05
VCC = 3.3V, VL = 1.7V
VCC = 5V, VL = 1.7V
VCC = 3.3V, VL = 3.3V
VCC = 5V, VL = 5V
20
0
–50
2.0
RS485 Driver Skew
vs Temperature
30
RL = 100Ω
2.5
0.5
RS485 Driver Propagation Delay
vs Temperature
40
RL = 54Ω
3.0
1.0
80
28701 G04
50
RL = 100Ω
4.0
0.05nF
0
100
RS485 Driver Differential Output
Voltage vs Temperature
ALL DRIVERS AND
RECEIVERS SWITCHING.
DRIVER OUTPUTS TIED TO
RECEIVER INPUTS.
RS232: 0.5Mbps (CL = 500pF)
RS485: 20Mbps (CL = 100pF)
TE485 HIGH
110
0.05nF
10
120
VCC = 5V
VCC = 3.3V
15
1
10
DATA RATE (Mbps)
28701 G03
VCC Supply Current vs Supply
Voltage, All Transceivers at Max
Rate (LTC2871)
0.5nF
2.5nF
5.5
28701 G02
SUPPLY CURRENT (mA)
35
INPUT CURRENT (mA)
40
0
0.1
VOLTAGE (V)
3.5
3
VCC Supply Current
vs RS232 Data Rate
DELAY (ns)
TE HIGH
TE LOW
1
28701 G01
5
60
20
5
0
VCC = 5V
VCC = 3.3V
ALL RS485 DRIVERS
AND RECEIVERS
SWITCHING.
CL = 100pF ON EACH
DRIVER OUTPUT.
75
100
28701 G08
50
VCC = 5V
VCC = 3.3V
OUTPUT LOW
0
–50
OUTPUT HIGH
–100
–150
–10
–5
10
0
5
SHORT-CIRCUIT VOLTAGE (V)
15
28701 G09
28701fa
For more information www.linear.com/LTC2870
7
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
1.6
INPUT LOW
1.2
VCC = 5V
VCC = 3.3V
75
100
28701 G13
8
0
2
8
4
6
OUTPUT CURRENT (mA)
10
28701 G12
RS485 Termination Resistance
vs Temperature
130
4
128
126
RESISTANCE (Ω)
OUTPUT VOLTAGE (V)
THRESHOLD VOLTAGE (V)
INPUT HIGH
0
25
50
TEMPERATURE (°C)
0
100
VL = 5V
VL = 3.3V
VL = 1.7V
5
1.8
–25
2
RS232 Receiver Output Voltage
vs Load Current
2.0
1.0
–50
3
28701 G11
RS232 Receiver Input Threshold
vs Temperature
1.4
4
1
0
–50
100
VL = 5V
VL = 3.3V
VL = 1.7V
3
2
VCM = –7V
VCM = 2V
VCM = 12V
124
122
120
118
116
114
1
112
0
0
2
8
4
6
OUTPUT CURRENT (mA)
10
28701 G14
110
–50
–25
50
0
25
TEMPERATURE (°C)
75
100
28701 G15
28701fa
For more information www.linear.com/LTC2870
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.
1V/DIV
DOUT1
ROUT1
Y
28701 G16
1µs/DIV
WRAPPING DATA
DOUT LOADS: 5kΩ + 50pF
Z
20ns/DIV
H/F HIGH
Y, Z LOADS: 120Ω (DIFF) + 50pF
RS232 Driver Outputs Enabling
and Disabling
RS232
MODE
RO
5V/DIV
ROUT2
5V/DIV
28701 G17
2µs/DIV
VDD and VEE Powering Up
RS232
MODE
28701 G18
VDD and VEE Ripple
VDD RIPPLE
DX232
2V/DIV
DOUT1
FEN
FEN = 1
5V/DIV
RS485
MODE
10mV/DIV
DOUT2
VDD
DOUT1
VEE RIPPLE
5V/DIV
FEN = 0
DOUT2
40µs/DIV
VEE
28701 G19
40µs/DIV
28701 G20
TOP CURVES: FAST ENABLE ↔ DX232
BOTTOM CURVES: SHUTDOWN ↔ DX232
28701 G21
40µs/DIV
FAST ENABLE MODE,
ALL DRIVERS AND RECEIVERS DISABLED.
28701fa
For more information www.linear.com/LTC2870
9
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.
However, VL > VCC will not damage the device, provided that absolute maximum limits are
respected.
VDD
15
18
20
24
Generated Positive Supply Voltage for RS232 Driver (+7V). Connect 1μF capacitor between
VDD and GND.
VEE
1, 12, 29
4, 15, 29
GND
10, 13,
18, 23
13, 16,
21, 26
14, 17,
25, 32
18, 21,
29, 36
CAP
11
14
15
19
Charge Pump Capacitor for Generated Negative Supply Voltage. Connect a 220nF capacitor
between CAP and SW.
SW
14
17
18
22
Switch Pin. Connect 10µH inductor between SW and VCC.
A
22
25
29
33
RS485 Positive Receiver Input (Full-Duplex Mode) or RS232 Receiver Input 1 (LTC2870).
B
21
24
28
32
RS485 Negative Receiver Input (Full-Duplex Mode) or RS232 Receiver Input 2 (LTC2870).
RA
2
5
RS485 Differential Receiver Output or RS232 Receiver Output 1.
RB
3
6
RS232 Receiver Output 2.
RO
1, 12, 16, 5, 16, 20, Generated Negative Supply Voltage for RS232 Driver (–6.3V). Tie all pins together and connect
19, 39
23, 39 1μF capacitor between VEE (adjacent to the CAP pin) and GND.
34
38
Ground. Tie all four pins together.
RS485 Differential Receiver Output.
RIN1
31
35
RS232 Receiver Input 1.
RIN2
30
34
RS232 Receiver Input 2.
ROUT1
2
6
RS232 Receiver Output 1.
ROUT2
3
7
RS232 Receiver Output 2.
DIN1
8
12
RS232 Driver Input 1. Do not float.
DIN2
9
13
RS232 Driver Input 2. Do not float.
DOUT1
23
27
RS232 Driver Output 1.
DOUT2
22
26
RS232 Driver Output 2.
DI
7
11
RS485 Driver Input. Do not float.
DY
7
10
RS485 Driver Input or RS232 Driver Input 1. Do not float.
DZ
8
11
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).
Z
17
20
24
28
RS485 Negative Driver Output or RS232 Driver Output 2 (LTC2870).
RS485 Negative Receiver Input (LTC2870 or LTC2871 in Half-Duplex Mode).
10
RS232 Driver Input 2. Do not float.
28701fa
For more information www.linear.com/LTC2870
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 HiZ. A logic low enables the RS232 or RS485 receivers, depending on the state of the interface
select input 485/232 . Do not float.
DXEN
6
9
Driver Enable. A logic low disables the RS232 and RS485 drivers leaving the driver output in
a Hi-Z state. A logic high enables the RS232 or RS485 drivers, depending on the state of the
interface select input 485/232. Do not float.
PIN NAME
LTC2871
QFN
LTC2871
TSSOP DESCRIPTION
RX232
11
15
RS232 Receiver Enable. A logic high disables the RS232 receivers and input termination
resistors leaving the RS232 receiver outputs in a Hi-Z state. A logic low enables the RS232
receivers and resistors, subject to the state of the CH2 pin. Do not float.
RX485
5
9
RS485 Receiver Enable. A logic high disables the RS485 receiver leaving the RS485 receiver
output in a Hi-Z state. A logic low enables the RS485 receiver and resistors, subject to the
state of the CH2 pin. Do not float.
DX232
10
14
RS232 Driver Enable. A logic low disables the RS232 drivers leaving the RS232 driver outputs
in a Hi-Z state. A logic high enables the RS232 drivers. Do not float.
DX485
6
10
RS485 Driver Enable. A logic low disables the RS485 driver leaving the RS485 driver output in
a Hi-Z state. A logic high enables the RS485 driver. Do not float.
H/F
27
2
37
3
RS485 Half-Duplex Select Input. 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 halfduplex 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. Do not float.
TE485
28
3
38
4
RS485 Termination Enable. A logic high enables a 120Ω resistor between pins A and B and
also between pins Y and Z. A logic low opens the resistors, leaving A/B and Y/Z unterminated.
The LTC2870 termination resistors are never enabled in RS232 mode. Do not float.
FEN
9
12
13
17
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 termination 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). Do not
float.
LB
26
1
36
2
Loopback Enable. A logic high enables logic loopback diagnostic mode, internally routing
the driver input logic levels to the receiver output pins. 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. Do not float.
4
8
RS232 Channel 2 Disable. A logic high disables RS232 receiver 2 and RS232 driver 2
independent of the state of RX232 and DX232 pins. In this state, the disabled driver output
becomes Hi-Z and the 5kΩ load resistor on the disabled receiver input is opened.
A logic low allows both RS232 transceiver channels to be enabled or disabled together based
on the RX232 and DX232 pins. Has no effect on RS485 operation. Do not float.
CH2
28701fa
For more information www.linear.com/LTC2870
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
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
2870 BD
28701fa
For more information www.linear.com/LTC2870
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
28701fa
For more information www.linear.com/LTC2870
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
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. RS485 Receiver Input Current and Resistance (Note 5)
tPLHD485
DY/DI
Y
DY/DI
RDIFF
DRIVER
Z
CL
VL
tPLHD485
0V
tSKEWD485
Y, Z
½VOD
VOD
CL
Y-Z
90%
10%
0V
0V
tRD485
90%
10%
tFD485
28701 F04
Figure 4. RS485 Driver Timing Measurement
14
28701fa
For more information www.linear.com/LTC2870
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
VCC
0.5V
VOL
VOH
0.5V
½VCC
Z OR Y
0V
tLZD485
½VCC
Y OR Z
DRIVER
VL
½VL
0V
tHZD485
tZHD485,
tZHSD485
28701 F05
Figure 5. RS485 Driver Enable and Disable Timing Measurements
VAB
±VAB/2
VCM
A-B
A
RECEIVER
B
0V
tPLHR485
RA/RO
CL
±VAB/2
RA/RO
90%
10%
tPHLR485
½VL
½VL
90%
tRR485
10%
tFR485
tSKEWR485 = tPLHR485 – tPHLR485
–VAB
VL
0V
28701 F06
Figure 6. RS485 Receiver Propagation Delay Measurements (Note 5)
RXEN/
RX485
0V TO 3V
3V TO 0V
A
B
RECEIVER
RA/RO
RL
VL
OR
GND
½VL
VL
½VL
tZLR485
½VL
RA/RO
0.5V
½VL
RA/RO
tZHR485
VL
0.5V
CL
RXEN/RX485
0V
tLZR485
tHZR485
VOL
VOH
0V
28701 F07
Figure 7. RS485 Receiver Enable and Disable Timing Measurements (Note 5)
28701fa
For more information www.linear.com/LTC2870
15
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. RS232 Driver Timing and Slew Rate Measurements
DRIVER
OUTPUT
0V OR VL
DXEN/DX232
RL
DXEN/
DX232
VL
½VL
½VL
CL
DRIVER
OUTPUT
0.5V
5V
tZLD232
DRIVER
OUTPUT
0V
tHZD232
tZHD232
0V
tLZD232
5V
VOHD
0V
0.5V
VOLD
28701 F10
Figure 10. RS232 Driver Enable and Disable Times
16
28701fa
For more information www.linear.com/LTC2870
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. RS232 Receiver Enable and Disable Times
28701fa
For more information www.linear.com/LTC2870
17
LTC2870/LTC2871
Function Tables
Table 1. LTC2870 Mode Selection Table
TE485
H/F
LB
DC/DC
CONVERTER MODE AND COMMENTS
0
0
X
X
OFF
Low Power Shutdown: All Main Functions Off
0
X
X
X
OFF
Low Power Shutdown: All Main Functions Off
1
0
0
X
X
ON
Fast-Enable: DC/DC Converter On Only
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
X
1
0
X
X
X
0
ON
RS485 Receiver On
X
1
X
X
1
X
X
ON
RS485 Driver and Receiver 120Ω Termination Enabled
X
1
X
X
X
0
0
X
RS485 Full-Duplex Mode
X
1
X
X
X
1
0
X
RS485 Half-Duplex Mode
X
1
0
X
X
X
1
ON
RS485 Loopback Mode
X
0
0
X
X
X
1
ON
RS232 Loopback Mode
FEN
485/232
RXEN
DXEN
0
X
1
0
0
1
1
X
X
0
X
X
Table 2. LTC2871 Mode Selection Table (CH2 = 0)
FEN
RX232
DX232
RX485
DX485
TE485
H/F
LB
DC/DC
CONVERTER MODE AND COMMENTS
0
1
0
1
0
0
X
X
OFF
Low Power Shutdown: All Main Functions Off
1
1
0
1
0
0
X
X
ON
Fast-Enable: DC/DC Converter On Only
X
X
1
X
X
X
X
0
ON
RS232 Drivers On
X
0
X
X
X
X
X
0
ON
RS232 Receivers On
X
X
X
X
1
X
X
0
ON
RS485 Driver On
X
X
X
0
X
X
X
0
ON
RS485 Receiver On
X
X
X
X
X
X
0
0
X
RS485 Full-Duplex Mode
X
X
X
X
X
X
1
0
X
RS485 Half-Duplex Mode
X
X
X
0
X
X
X
1
ON
RS485 Loopback Mode
X
0
X
X
X
X
X
1
ON
RS232 Loopback Mode
Table 3. RS232 Receiver Mode (485/232 = 0 for LTC2870, CH2 = 0 for LTC2871)
RX232 OR RXEN
RECEIVER INPUTS
(A, B, RIN1, RIN2)
CONDITIONS
RECEIVER OUTPUTS
(RA, RB, ROUT1, ROUT2)
1
X
No Fault
Hi-Z
LTC2870 RECEIVER INPUTS LTC2871 RECEIVER INPUTS
(A, B)
(RIN1, RIN2)
125kΩ
Hi-Z
0
0
No Fault
1
5kΩ
5kΩ
0
1
No Fault
0
5kΩ
5kΩ
0
X
Thermal Fault
Hi-Z
5kΩ
5kΩ
Table 4. RS232 Driver Mode (485/232 = 0 for LTC2870, CH2 = 0 for LTC2871)
DX232 OR DXEN
DRIVER INPUTS
(DY, DZ, DIN1, DIN2)
CONDITIONS
LTC2870 DRIVER OUTPUTS
(Y, Z)
LTC2871 DRIVER OUTPUTS
(DOUT1, DOUT2)
0
X
No Fault
125kΩ
Hi-Z
1
0
No Fault
1
1
1
1
No Fault
0
0
X
X
Thermal Fault
125kΩ
Hi-Z
18
28701fa
For more information www.linear.com/LTC2870
LTC2870/LTC2871
function tables
Table 5. LTC2871 CH2 CONTROL
RS232 RECEIVER INPUTS
RS232 DRIVER OUTPUTS
CH2
DX232
RX232
RIN1
RIN2
DOUT1
DOUT2
X
0
1
Hi-Z
Hi-Z
Hi-Z
Hi-Z
COMMENTS
Both Drivers and Receivers Disabled
0
0
0
5kΩ
5kΩ
Hi-Z
Hi-Z
Both Receivers Enabled, Both Drivers Disabled
0
1
1
Hi-Z
Hi-Z
Driven
Driven
Both Receivers Disabled, Both Drivers Enabled
0
1
0
5kΩ
5kΩ
Driven
Driven
Both Receivers and Drivers Enabled
1
0
0
5kΩ
Hi-Z
Hi-Z
Hi-Z
Channel 2 Drivers and Receivers Disabled
1
1
1
Hi-Z
Hi-Z
Driven
Hi-Z
Channel 2 Drivers and Receivers Disabled
1
1
0
5kΩ
Hi-Z
Driven
Hi-Z
Channel 2 Drivers and Receivers Disabled
Table 6. RS485 Driver Mode (TE485 = 0)
DX485 OR DXEN
DI
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 7. RS485 Receiver Mode (LB = 0)
RXEN OR RX485
A - B (NOTE 5)
CONDITIONS
RA, RO
1
X
No Fault
Hi-Z
0
< –200mV
No Fault
0
0
> 200mV
No Fault
1
0
Inputs Open or Shorted Together (DC)
Failsafe
1
X
X
Thermal Fault
Hi-Z
Table 8. RS485 Termination (485/232 = 1 for LTC2870)
TE485
H/F, LB
CONDITIONS
R (A TO B)
R (Y TO Z)
0
X
No Fault
Hi-Z
Hi-Z
1
X
No Fault
120Ω
120Ω
X
X
Thermal Fault
Hi-Z
Hi-Z
Table 9. RS485 Duplex Control (485/232 = 1 for LTC2870)
H/F
RS485 DRIVER OUTPUTS
RS485 RECEIVER INPUTS
0
Y, Z
A, B
1
Y, Z
Y, Z
Table 10. LTC2870 Loopback Functions
Table 11. LTC2871 Loopback Functions
LB
RXEN
MODE
LB
RX232
RX485
MODE
0
X
Not Loopback
0
X
X
Not Loopback
X
1
Not Loopback
X
1
1
Not Loopback
1
0
Loopback (RA = DY, RB = DZ)
1
0
1
Loopback RS232 (ROUT1 = DIN1, ROUT2 = DIN2)
1
1
0
Loopback RS485 (R0 = DI)
1
0
0
Loopback All (ROUT1 = DIN1, ROUT2 = DIN2, RO = DI)
28701fa
For more information www.linear.com/LTC2870
19
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. 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 LTC2870 offers a single port that can be configured
as either two RS232 receivers and drivers or one RS485/
RS422 receiver and driver depending on the state of the
485/232 pin. Control inputs DXEN and RXEN provide
independent control of driver and receiver operation for
either RS232 or RS485 transceivers, depending on the
selected operating protocol.
The LTC2871 separates the RS232 and RS485 transceivers into independent I/Os allowing simultaneous operation
of two RS232 transceivers and one RS485 transceiver.
Independent control over driver and receiver mode for
each protocol is provided with logic inputs DX232, RX232,
DX485, RX485. Single channel RS232 operation is possible
via the CH2 control pin. The disabled channel maintains a
Hi-Z state on the receiver input and driver output, allowing
these lines to be shared with other transceivers.
L1
10µH
C1
220nF
C4
2.2µF
VL
1.7V TO VCC
VCC
SW
VDD
VL
C5
0.1µF
CAP
BOOST
REGULATOR
GND
C2
1µF
VEE
GND
28701 F13
C3
1µF
Figure 13. DC/DC Converter with Required External Components
The DC/DC converter requires a 10µH inductor (L1) and a
bypass capacitor (C4) of 2.2µF. The charge pump capacitor
(C1) is 220nF and the storage capacitors (C2 and C3) are
1µF. 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.
Figure 14 shows the layout of external components on
the bottom of the demonstration circuit for the LTC2871
(Demo Circuit 1786A). Refer to Layout Considerations
section for further guidance.
Both parts feature rugged operation with ESD ratings
of ±26kV (LTC2870) and ±16kV (LTC2871) HBM on the
RS232 and RS485 receiver inputs and driver outputs, both
unpowered and powered. All other pins offer protection
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.
20
Figure 14. Demo Circuit 1786A (Bottom) Showing Layout of
External Devices. U.S. Penny Included to Demonstrate the Scale
28701fa
For more information www.linear.com/LTC2870
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.If more than two RS232 drivers are being supplied from
a single device, then the inductor, L1, must be increased
to 22µH and the flying cap, C1, must be increased to
470nH, and VDD and VEE bypass caps must be increased
to 2.2µF.
3.Ground the SW pin on devices with inactive boost
converters.
4.Connect CAP pins together for all devices.
5. Connect VEE pins together for all devices.
6. Connect VDD pins together for all devices.
Figures 51 and 52 show examples of connecting two
devices and four devices.
Inductor Selection
A 10μH inductor with a saturation current (ISAT) rating of
at least 220mA and a DCR (copper wire resistance) of less
than 1.3Ω is required. Some very small inductors meeting
these requirements are listed in Table 12.
Table 12. Recommended Inductors
PART NUMBER
ISAT
MAX
(mA) DCR (Ω)
LBC2016T100K
CBC2016T100M
245
380
FSLB2520-100K
220
Running with External VDD and VEE Supplies
The inductor and flying cap, C1, can be omitted only if VDD
and VEE are externally supplied. Bypass caps on VDD and
VEE must remain in place. In this circumstance, ground the
SW pin and float the CAP pin. External supplies must not
exceed the ABSMAX levels of ±7.5V. Ideal supply levels
are 7.3V and –6.5V as these are each just wider than the
regulation points of 7.2V and –6.3V so the internal feedback is satisfied and the switching stops. Lower voltages
can be used even at –6V and +6V but the internal boost
regulator will be switching. This may cause some switching noise but will not harm the part. VDD and VEE supplies
must be present for proper operation in all modes except
shutdown, including RS485-only mode.
Inrush Current and Supply Overshoot Precaution
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. 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 15.
This precaution is not relevant for supply voltages below
4.5V or rise times longer than 10μs.
5V
0V
SIZE(mm)
MANUFACTURER
1.07
1.07
2 × 1.6 × 1.6
2 × 1.6 × 1.6
Taiyo Yuden
www.t-yuden.com
1.1
2.5 × 2 × 1.6
Toko
www.tokoam.com
≤10µs
R1
1Ω
1/8W
C4
2.2µF
L1
10µH
INRUSH
CURRENT
C1
220nF
SW
CAP
VDD
GND
VCC
Capacitor Selection
The small size of ceramic capacitors makes them ideal
for the LTC2870 and LTC2871. 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.
28701 F14
C2
1µF
Figure 15. Supply Current Overshoot Protection
for Input Supplies of 4.5V of Higher
28701fa
For more information www.linear.com/LTC2870
21
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.
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 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.
If the RS485 driver output is shorted to a voltage greater
than VCC, when it is active, positive current of up to 100mA
may 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 Operation
The LTC2870 and LTC2871 receivers use a window comparator with two voltage thresholds centered around zero
for low pulse width distortion. As illustrated in Figure 16, for
a differential signal approaching from a negative direction,
the threshold is typically +65mV. When approaching from
the positive direction, the threshold is typically –65mV.
Each of these thresholds has about 25mV of hysteresis (not
shown in the figure). The state of RO reflects the polarity
of A–B in full-duplex mode or Y–Z in half-duplex mode.
RS485 Driver
The RS485 driver provides full RS485/RS422 compatibility.
When enabled, if DI is high, Y – Z is positive. With the
driver disabled the Y and Z output resistance is greater
than 96kΩ (typically 125kΩ) 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.
RO
RECEIVER
OUTPUT HIGH
RECEIVER
OUTPUT LOW
–200mV
–65mV
0V
65mV
200mV
VAB
28701 F15
Figure 16. RS485 Receiver Input Threshold Characteristics
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.
22
28701fa
For more information www.linear.com/LTC2870
LTC2870/LTC2871
Applications Information
RS485 Biasing Resistors Not Required
B
100mV/DIV
A
(A-B)
100mV/DIV
RO
5V/DIV
200ns/DIV
28701 F16
Figure 17. A 3Mbps Signal Driven Down 4000ft of CAT 5e
Cable. Top Traces: Received Signals After Transmission
Through Cable; Middle Trace: Math Showing Differences
of Top Two Signals; Bottom Trace: Receiver Output
This windowing around 0V preserves pulse width and
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 17,
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.
An additional benefit of the window comparator architecture
is excellent noise immunity due to the wide effective differential hysteresis (or ‘AC’ hysteresis) of about 130mV for
normal signals transitioning through the window region in
less than approximately 2µs. Increasingly slower signals
will have increasingly less effective hysteresis, limited by
the DC failsafe value of about 25mV.
The LTC2870 and LTC2871 provide full failsafe operation
that guarantees the receiver output will be a logic high
state when the inputs are shorted, left open, or terminated
but not driven, for more than about 2µs. The delay allows
normal data signals to transition through the threshold
region without being interpreted as a failsafe condition.
RS485 networks are often biased with a resistive divider
to generate a differential voltage of ≥200mV on the data
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 LTC2870 and LTC2871
eliminates the need for external biasing resistors. The
LTC2870 and LTC2871 transceivers will operate correctly
on unbiased, biased or underbiased networks.
If a twisted pair has unbalanced capacitance from its two
conductors to AC ground, common mode transients can
translate into small differential voltages. If the common
mode event is large and fast enough, the resulting differential voltage can cause a receiver, whose inputs are
undriven, to change state momentarily. In these extreme
conditions, high quality shielded cable is recommended.
If necessary, biasing resistors can be used on the bus to
pull the resting signal farther from the receivers failsafe
threshold.
Receiver Outputs
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.
28701fa
For more information www.linear.com/LTC2870
23
LTC2870/LTC2871
Applications Information
RS485 Receiver Input Resistance
126
124
RESISTANCE (Ω)
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 96kΩ (typically 125kΩ) 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 18.
VCC = 5.0V
VCC = 3.3V
122
120
118
116
–10
–5
0
5
VOLTAGE (V)
15
10
28701 F18
125k
Figure 19. Typical Resistance of the Enabled RS485
Terminator vs Common Mode Voltage on A /B
A
60Ω
When the TE485 pin is high, the termination resistors are
enabled and the differential resistance from A to B and Y
to Z is 120Ω. The resistance is maintained over the entire
RS485 common mode range of –7V to 12V as shown in
Figure 19.
TE485
125k
60Ω
B
28701 F17
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. When the cable is not terminated with its characteristic
impedance, reflections cause waveform distortion.
The LTC2870 and LTC2871 offer 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 the 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. Differential termination resistors are never
enabled in RS232 mode on the LTC2870.
24
RS485 Half- and Full-Duplex Control
The LTC2870 and LTC2871 are equipped with a control to
switch between half- and full-duplex operation. With the
H/F pin set to a logic low, the A and B pins serve as 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.
RS485 Polarity and the LTC1334, LTC1387
The LTC2870/LTC2871 receiver uses A as the positive input
and B as the negative input. The receiver output is a logic
high when A−B is positive. For the RS485 driver, a high
input produces a positive differential voltage on Y – Z. In
contrast, the LTC1334 and LTC1387 dual protocol devices
use the opposite RS485 convention: A is the negative
28701fa
For more information www.linear.com/LTC2870
LTC2870/LTC2871
Applications Information
The TIA/EIA-485 and TIA/EIA-422 standards do not definitively specify the polarity of the driver input (DI) or receiver
output (RO) that correspond to the bus polarity (Y/Z/A/B).
This has caused some contention among manufacturers
of early devices, with some using one polarity and others
using its opposite. However, today nearly all manufacturers,
including Linear Technology, agree on the convention of
A−B and Y−Z being positive for a logic high on DI and RO.
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. When RS232 mode is selected, the signal is passed
without inversion but in RS485 mode the output signal
becomes inverted.
RS485 Cable Length vs Data Rate
For a given data rate, the maximum transmission distance is bounded by the cable properties. A typical curve
of cable length vs data rate compliant with the RS485/
RS422 standards is shown in Figure 20. Three regions
of this curve reflect different performance limiting factors in data transmission. In the flat region of the curve,
maximum distance is determined by resistive losses in
the cable. The downward sloping region represents limits
in distance and data rate due to AC losses in the cable.
The solid vertical line represents the specified maximum
data rate in the RS485/RS422 standards. The dashed lines
at 20Mbps show the maximum data rates of the LTC2870
and LTC2871.
10k
CABLE LENGTH (FT)
receiver input, B is the positive receiver input, Y is the
negative driver output, and Z is the positive driver output.
1k
LTC2870/LTC2871
MAX DATA RATE
100
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. Loopback
works in half- or full-duplex mode and does not affect the
termination resistors.
RS485/RS422
MAX DATA RATE
10
10k
100k
1M
10M
DATA RATE (bps)
100M
28701 F19
Figure 20. Cable Length vs Data Rate (RS485/RS422
Standard Shown in Vertical Solid Line)
28701fa
For more information www.linear.com/LTC2870
25
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. All GND pins must be connected together and
all VEE pins must be connected together, including the
exposed pad on the bottom of the package. The bypass
capacitor at VEE, C3, should be positioned closest to the
VEE pin that is adjacent to the CAP pin, with no more than
1cm of total trace length between the VEE and GND pins.
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. Figure 14,
shows an excellent example of a layout for these external
components on the bottom of the LTC2871 Demo Circuit
1786A.
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 maintain good differential signal integrity. To minimize capacitive loading
effects, the differential signals should be separated by
more than the width of a trace.
Route outputs away from sensitive inputs to reduce
feedback effects that might cause noise, jitter, or even
oscillations. For example, do not route DI or A/B near the
driver or receiver outputs.
Standards Compatibility:
RS485: The LTC2870 and LTC2871 are compliant with
the TIA/EIA-485-A (RS485) standard. The LTC2870 and
26
LTC2871 are production tested to the electrical specifications in the data sheet, ensuring interoperability in RS485
networks.
RS422: The LTC2870 and LTC2871 are fully compatible
with TIA/EIA-422-B (RS422). The internal termination in
the LTC2870 and LTC2871 are designed to work with either
120Ω RS485 cables or 100Ω RS422 cables.
Two characteristics prevent 100% compliance to the
TIA/EIA-422-B specification:
1.Maximum Driver Short Circuit Current: RS422 specifies
±150mA. The LTC2870/71, like the RS485 specification,
is ±250mA.
2 Maximum Receiver Differential Signal: RS422 specifies
that the receiver can operate with a 10V (peak) differential signal and tolerate 12V (peak). The LTC2870/
LTC2871, with internal termination disabled, meets these
requirements. If the termination resistor is enabled,
the differential voltage is limited to 6V, to limit power
dissipation in the terminating resistor.
Neither of these points are practical constraints for use
in RS422 systems.
RS232: The LTC2870 and LTC2871 are fully compatible
with the TIA/EIA-232-F (RS232). However, one characteristic prevents 100% compliance to the TIA/EIA-232-F
specification: The RS232 standard specifies that the receiver inputs meet defined electrical characteristics from
–15V to +15V and can tolerate ±25V without damage.
The LTC2870 and LTC2871 RS232 receiver inputs meet
the specified electrical characteristics from –15V to +15V
but have an absolute maximum rating of ±15V, which is
characteristic of the robust ESD protection on those pins.
This distinction has little impact in most RS232 systems
where loaded driver output voltages cannot exceed 15V,
per the standard, and usually do not even approach these
levels. Like RS485, there is no certification body for the
RS232 standard. The LTC2870 and LTC2871 are production tested to the electrical specifications in the data sheet,
ensuring compatibility with the standard.
28701fa
For more information www.linear.com/LTC2870
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 in RS485
Mode, Terminated
VL
VL
DXEN
485/232
RXEN
485/232
LB
TE485
LTC2870
D R
TE485
485/232
LB
H/F
H/F
LTC2870
H/F
DXEN
TE485
RXEN
LB
Y
Y
DY
DY
Z
Z
D R
DXEN
RXEN
Y
DY
120Ω
A
RA
B
GND
GND
28701 F23
Figure 24. LTC2870 in RS485
Mode in Loopback
Z
A
RA
RA
B
GND
28701 F21
28701 F20
LTC2870
120Ω
B
GND
GND
Z
A
RA
RB
RXEN
485/232
120Ω
B
GND
28701 F24
Figure 25. LTC2870 in RS485
Mode Half-Duplex
28701 F25
Figure 26. LTC2870 in RS485
Mode, Half-Duplex, with
Loopback and Terminated
28701fa
For more information www.linear.com/LTC2870
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.
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
Figure 27. LTC2870 Protocol Switching
VL
LTC2871
GND
GND
28701 F26
DX232
DOUT1
DX485
28701 F28
28701 F27
Figure 28. LTC2871 in RS485 Mode
Figure 29. LTC2871 in RS232 Mode
VL
VL
DX232
LTC2871
DX485
RX232
DX232
CH2
DX485
RX485
RX232
CH2
TE485
RX485
LB
H/F
TE485
TE485
LB
H/F
LTC2871
RX232
CH2
RX485
H/F
LB
Y
Y
DI
120Ω
DI
Z
DIN1
DOUT1
ROUT1
RIN1
RO
B
DIN1
RIN1
DIN2
DOUT2
ROUT2
RIN2
GND
GND
28701 F29
Figure 30. LTC2871 Single
RS232 Channel Active
120Ω
B
DOUT1
DOUT1
ROUT1
28
A
RO
DIN1
Z
A
RIN1
ROUT1
DIN2
DOUT2
RIN2
ROUT2
GND
28701 F30
Figure 31. LTC2871 in
RS485 and RS232 Mode
28701 F31
Figure 32. LTC2871 in RS485 and
RS232 Mode with Loopback and
RS485 Termination
28701fa
For more information www.linear.com/LTC2870
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
H/F
LTC2871
CH2
DX232
LB
DX485
TE485
D R 485
VL
DX485
LTC2871
DX232
RX232
RX485
LB
TE485
D R 485
RX232
CH2
H/F
CH2
H/F
LTC2871
LB
DX485
RX485
RX485
DX232
D R 232
RX232
Y
DI
Y
Y
DI
DI
Z
Z
RO
DIN1
DIN1
RIN1
DIN2
DOUT2
RIN2
ROUT2
DIN1
RIN1
ROUT1
DIN2
B
DOUT1
RIN1
ROUT1
DOUT2
DOUT2
DIN2
RIN2
ROUT2
RIN2
ROUT2
GND
GND
GND
28701 F32
Figure 33. LTC2871 in RS485 and
RS232 Mode, Both Half-Duplex
120Ω
DOUT1
DOUT1
ROUT1
Z
A
RO
RO
120Ω
28701 F34
28701 F33
Figure 34. LTC2871 in RS485 and RS232
Mode, RS485 Half-Duplex, Loopback
Figure 35. LTC2871 in RS485
and RS232 Mode, RS485
Half-Duplex, Terminated
VL
485/232
TE485
LTC2870/
LTC2871
H/F
3V TO 5.5V
LB
1.7V TO VCC
VCC
LTC2870/
LTC2871
VL
LTC2870/
LTC2871
CONTROL
SIGNALS
Y
DI
Z
DY
RS485
FULL HALF
DY, DIN1
Y
RA, ROUT1
A
DZ, DIN2
Z
RB, ROUT2
B
120Ω
H/F
µP
RS232
DUPLEX
A
RO
120Ω
B
RB
GND
28701 F35
GND
28701 F36
Figure 37. Microprocessor Interface
28701 F37
DATA RATE CL
100kbps 5nF
500kbps 1nF
CL
3k
Figure 38. Driving Larger
RS232 Loads
Figure 36. RS485 Duplex Switching
28701fa
For more information www.linear.com/LTC2870
29
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: Making Use of Shared I/O for Various Communication Configurations
1.7V TO VCC
VL
VL
3V TO 5.5V
LTC2870
DXEN
VCC
TE485
RXEN
485/232
H/F
RS485
HALF-DUPLEX
RS232
HALF-DUPLEX
DY
485/232 = 1
H/F = 0
485/232 = 0
H/F = X
RS485
RS232
RS485
RS232
Y
CONTROLLER
Z
CONNECTOR
DZ
RA
A
B
RB
NOTE: THIS CONFIGURATION IS NOT COMPATIBLE
WITH INTERNAL RS485 TERMINATION.
SETTING TE485 HIGH WILL RESULT IN DOUBLE
TERMINATION IN RS485 MODE.
GND
28701 F39
30
Figure 40. LTC2870: Using External Connections for Half-Duplex RS232 or RS485 Operation
For more information www.linear.com/LTC2870
28701fa
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 5.5V
LTC2871
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
CONTROLLER
VCC
Z
DOUT2
DIN2
ROUT1
RIN1
RS485
A
RO
RS485
B
ROUT2
CONNECTOR
RS232
RIN2
RS232
GND
28701 F40
Figure 41. LTC2871: Various Communication Configurations
1.7V TO VCC
VL
VL
3V TO 5.5V
LTC2871
DX232
RX232
DX485
RX485
H/F
TE485
DIN1
DOUT1
Y
DI
CONTROLLER
VCC
Z
DOUT2
DIN2
RS232
HALF-DUPLEX
RS232
HALF-DUPLEX
RS485
FULL-DUPLEX
RS485
HALF-DUPLEX
H/F = 0
H/F = 1
RS232
RS232
RS485
RS485
RS485
RS485
RS232
RS232
CONNECTOR
RIN1
ROUT1
A
RO
B
RS485
RS485
RIN2
ROUT2
GND
28701 F41
Figure 42. LTC2871: More Communication Configurations Using External Connections
28701fa
For more information www.linear.com/LTC2870
31
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 5.5V
LTC2871
VCC
RS485
HALF-DUPLEX
RS232
FULL-DUPLEX
DX232 = 0
RX232 = 1
TE485 = X
H/F = 0
DX232 = 0
RX232 = 1
TE485 = X
H/F = 1
DX232 = 0
RX485 = 1
DX232 = 1
RX232 = 0
TE485 = 0
H/F = X
RS485
RS485
RS232
RS485
RS485
RS232
DOUT1
DIN1
Y
DI
CONTROLLER
RS485
FULL-DUPLEX
Z
DOUT2
DIN2
ROUT1
RIN1
A
RO
B
ROUT2
RS485
RS232
RS485
RS232
RIN2
GND
28701 F50
This configuration allows for a two-wire interface (top two wires) operating in either RS485
half-duplex or RS232 full-duplex mode. A second pair of wires (bottom two wires) allows a
second RS232 interface operating in full duplex mode and also allows for RS485 operating in
full duplex mode. The two-wire application is comparable to the LTC1387 data sheet, Figure 14.
Figure 43. LTC2871: Using External Connections for RS485 (Half or Full Duplex) and RS232 (Full Duplex)
L1
10µH
C4
2.2µF
RS485 DRIVER
ENABLE
RS485
RS232
DI
RO
LTC2871
VCC
VL
H/F
CH
DX485
RX485
DX232
RX232
TE485
DIN1
DI
RO
ROUT1
ROUT2
SW
CAP
VDD
VEE
GND
FEN
LB
DIN2
C1
220nF
C3
1µF
DOUT1
Y
Z
RIN1
DOUT2
A
B
RIN2
C2
1µF
TxAY
RxBZ
28701 F51
Two logic gates were added (optional) to simplify
the control signals needed from the controller.
Figure 44. Schematic Implementation of Figure 43, Above, Using a Single Pair of Wires for the Bus I/O
32
28701fa
For more information www.linear.com/LTC2870
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. RS232 Extension Cord Using RS232 to RS485 Conversion
SLAVE
SLAVE
LTC2852
LTC2852
SLAVE
MASTER
LTC2870/LTC2871
LTC2855
120Ω
120Ω
120Ω
VL
3.3V
TE485
TE
28701 F43
Figure 46. RS485 Full-Duplex Network
28701fa
For more information www.linear.com/LTC2870
33
LTC2870/LTC2871
typical Applications
Logic input pins not shown are tied to a valid logic state. External Components not shown.
LTC2871
DIN1
PORT 1
LOGIC
INTERFACE
PORT 2A/2B
LOGIC
INTERFACE
LTC2871
DOUT1
ROUT1
RIN1
DIN2
DOUT2
ROUT2
RIN2
DIN1
PORT 1
LINE
INTERFACE
PORT 1
LOGIC
INTERFACE
PORT 2A
LINE
INTERFACE
PORT 2A
LOGIC
INTERFACE
CH2
RIN1
DIN2
DOUT2
ROUT2
PORT 2A/2B
LINE
INTERFACE
RIN2
SELECT LINE 2A
LTC2871
SELECT LINE 2B
CH2
DIN2
ROUT2
PORT 3
LOGIC
INTERFACE
ROUT1
PORT 1
LINE
INTERFACE
CH2
SELECT LINE 2A
SELECT LINE 2B
DOUT1
DIN1
ROUT1
DOUT2
RIN2
DOUT1
RIN1
PORT 2B
LINE
INTERFACE
PORT 2B
LOGIC
INTERFACE
PORT 3
LINE
INTERFACE
PORT 3
LOGIC
INTERFACE
LTC2871
CH2
DIN2
ROUT2
DIN1
ROUT1
28701 F44
DOUT2
RIN2
DOUT1
RIN1
PORT 3
LINE
INTERFACE
28701 F45
Figure 47. RS232 Triple Transceiver
with Selectable Line Interface
Figure 48. RS232 Triple Transceiver
with Selectable Logic Interface
LTC2870/
LTC2871
SELECT
H/F
INPUT2 INPUT1
Y
RS485
INTERFACE
Z
RA,
RO
INPUT1
A
B
INPUT2
28701 F46
Figure 49. RS485 Receiver with Multiplexed Inputs
34
28701fa
For more information www.linear.com/LTC2870
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. Typical Supply Connections with External Components Shown
3V TO 5.5V
2.2µF
470nF
22µH
VCC
VL
GND
CAP
SW
CAP
LTC2870/
LTC2871
VDD
VCC
LTC2870/
LTC2871
VEE
VEE
VDD
VL
SW
GND
28701 F48
2.2µF
2.2µF
INDUCTOR: TAIYO YUDEN CBC2518T220M,
MURATA LQH32CN220K53
Figure 51. Running Two LTC2870 or LTC2871 Devices from One Shared Power Source
28701fa
For more information www.linear.com/LTC2870
35
LTC2870/LTC2871
typical Applications
L1
22µH
C4
2.2µF
DX485-A
RX485-A
DX232-A
RX232-A
TE485-A
LTC2871
VCC
SW
VL
CAP
H/F
VDD
CH
VEE
DX485
GND
RX485
FEN
DX232
LB
RX232
DIN2
TE485
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
2.2µF
DX485-D
RX485-D
DX232-D
RX232-D
TE485-D
DI-D
RO-D
TxAY-A
RxBZ-A
DOUT1
Y
Z
RIN1
DOUT2
A
B
RIN2
TxAY-B
RxBZ-B
LTC2871
VCC
SW
VL
CAP
VDD
H/F
VEE
CH
DX485
GND
RX485
FEN
DX232
LB
RX232
DIN2
TE485
DIN1
DI
RO
ROUT1
DI-C
RO-C
DOUT1
Y
Z
RIN1
DOUT2
A
B
RIN2
LTC2871
VCC
SW
VL
CAP
VDD
H/F
VEE
CH
DX485
GND
RX485
FEN
DX232
LB
RX232
DIN2
TE485
DIN1
DI
RO
ROUT1
DI-B
RO-B
C1
470nF
DOUT1
Y
Z
RIN1
DOUT2
A
B
RIN2
TxAY-C
RxBZ-C
LTC2871
VCC
SW
VL
CAP
VDD
H/F
VEE
CH
DX485
GND
RX485
FEN
DX232
LB
RX232
DIN2
TE485
DIN1
DI
RO
ROUT1
ROUT2
DOUT1
Y
Z
RIN1
DOUT2
A
B
RIN2
TxAY-D
RxBZ-D
C3
2.2µF
C2
2.2µF
28701 F51
Each channel supports half-duplex RS485 or full-duplex RS232. Up to four RS232 drivers can be
operated simultaneously using one inductor and flying cap as shown. I/O interface on left can be
simplified with additional logic gates as shown in Figure 44
Figure 52. Quad Transceiver
36
28701fa
For more information www.linear.com/LTC2870
LTC2870/LTC2871
Package Description
Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings.
FE Package
28-Lead Plastic TSSOP (4.4mm)
(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 ±0.05
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)
0.25
REF
1.20
(.047)
MAX
0° – 8°
0.65
(.0256)
BSC
0.50 – 0.75
(.020 – .030)
NOTE:
1. CONTROLLING DIMENSION: MILLIMETERS
2. DIMENSIONS ARE IN MILLIMETERS
(INCHES)
3. DRAWING NOT TO SCALE
1 2 3 4 5 6 7 8 9 10 11 12 13 14
0.195 – 0.30
(.0077 – .0118)
TYP
0.05 – 0.15
(.002 – .006)
FE28 (EB) TSSOP REV K 0913
4. RECOMMENDED MINIMUM PCB METAL SIZE
FOR EXPOSED PAD ATTACHMENT
*DIMENSIONS DO NOT INCLUDE MOLD FLASH. MOLD FLASH
SHALL NOT EXCEED 0.150mm (.006") PER SIDE
28701fa
For more information www.linear.com/LTC2870
37
LTC2870/LTC2871
Package Description
Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings.
FE Package
38-Lead Plastic TSSOP (4.4mm)
(Reference LTC DWG # 05-08-1772 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
0.50 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. DIMENSIONS ARE IN MILLIMETERS
(INCHES)
3. DRAWING NOT TO SCALE
38
1
0.25
REF
19
1.20
(.047)
MAX
0° – 8°
0.50
(.0196)
BSC
0.17 – 0.27
(.0067 – .0106)
TYP
0.05 – 0.15
(.002 – .006)
FE38 (AA) TSSOP REV C 0910
4. RECOMMENDED MINIMUM PCB METAL SIZE
FOR EXPOSED PAD ATTACHMENT
*DIMENSIONS DO NOT INCLUDE MOLD FLASH. MOLD FLASH
SHALL NOT EXCEED 0.150mm (.006") PER SIDE
28701fa
For more information www.linear.com/LTC2870
LTC2870/LTC2871
Package Description
Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings.
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 ± 0.10
PIN 1
TOP MARK
(NOTE 6)
1
2
5.00 ± 0.10
(2 SIDES)
3.50 REF
3.65 ± 0.10
2.65 ± 0.10
(UFD28) QFN 0506 REV B
0.200 REF
0.00 – 0.05
0.25 ± 0.05
0.50 BSC
BOTTOM VIEW—EXPOSED PAD
NOTE:
1. DRAWING PROPOSED TO BE MADE A JEDEC PACKAGE OUTLINE MO-220 VARIATION (WXXX-X).
2. DRAWING NOT TO SCALE
3. ALL DIMENSIONS ARE IN MILLIMETERS
4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE
MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE
5. EXPOSED PAD SHALL BE SOLDER PLATED
6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION
ON THE TOP AND BOTTOM OF PACKAGE
28701fa
For more information www.linear.com/LTC2870
39
LTC2870/LTC2871
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
3.00 REF
37
0.00 – 0.05
38
0.40 ±0.10
PIN 1
TOP MARK
(SEE NOTE 6)
1
2
5.15 ± 0.10
5.50 REF
7.00 ± 0.10
3.15 ± 0.10
(UH) QFN REF C 1107
0.200 REF 0.25 ± 0.05
0.50 BSC
R = 0.125
TYP
R = 0.10
TYP
BOTTOM VIEW—EXPOSED PAD
NOTE:
1. DRAWING CONFORMS TO JEDEC PACKAGE
OUTLINE M0-220 VARIATION WHKD
2. DRAWING NOT TO SCALE
3. ALL DIMENSIONS ARE IN MILLIMETERS
40
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 SIDE
5. EXPOSED PAD SHALL BE SOLDER PLATED
6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION
ON THE TOP AND BOTTOM OF PACKAGE
28701fa
For more information www.linear.com/LTC2870
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
1-40
2
3
Noted which pins should not float
10-11
Expanded DC/DC Converter Applications section
20-21
Added Running with External VDD and VEE Supplies Applications section
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
26
Revised Figures 25, 26, and 35
27, 29
Added Figures 43, 44, and 52
32, 36
28701fa
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 representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
41
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
T1IN
TE485
DIN1
DOUT1
ROUT1
RIN1
DIN2
DOUT2
ROUT2
VL
LTC2804
LTC2871
SW
T1OUT1
ROUT1
RIN1
T2IN
T2OUT
ROUT2
RIN2
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
2.2µF
28701 TA02
Related Parts
PART NUMBER
DESCRIPTION
COMMENTS
LTC2872
RS232/RS485 Dual Multiprotocol Transceiver with
Integrated Termination
Four RS232 Transceivers and Two RS485 Transceivers. 3V to 5.5V Supply with
1.7V to 5.5V Logic Interface, Automatic Selection of Termination Resistors,
Duplex Control, Logic Supply Pin
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/ 1.8V to 5.5V RS232 Single and Dual Transceivers
LTC2803/LTC2804
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
42 Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
For more information www.linear.com/LTC2870
(408) 432-1900 ● FAX: (408) 434-0507
●
www.linear.com/LTC2870
28701fa
LT 1114 REV A • PRINTED IN USA
 LINEAR TECHNOLOGY CORPORATION 2010