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LTC2870, LTC2871 Datasheet

Linear Technology/Analog Devices

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Datasheet

LTC2870/LTC2871
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For more information www.linear.com/LTC2870
Typical applicaTions
DescripTion
RS232/RS485 Multiprotocol
Transceivers with
Integrated Termination
The LTC
®
2870/LTC2871 are robust pin-configurable mul-
tiprotocol transceivers, supporting RS232, RS485, and
RS422 protocols, operating on a single 3V to 5.5V supply.
The LTC2870 can be configured as two RS232 single-
ended transceivers or one RS485 differential transceiver
on shared I/O lines. The LTC2871 offers independent con-
trol of two RS232 transceivers and one RS485 transceiver,
each on dedicated I/O lines.
Pin-controlled integrated termination resistors allow
for easy interface reconfiguration, eliminating external
resistors and control relays. Half-duplex switches allow
four-wire and two-wire RS485 configurations. Loopback
mode steers the driver inputs to the receiver outputs for
diagnostic self-test.The RS485 receivers support up to
256 nodes per bus, and feature full failsafe operation for
floating, shorted or terminated inputs.
An integrated DC/DC boost converter uses a tiny
2mm×1.6mm inductor and one capacitor, eliminating
the need for multiple supplies for driving RS232 levels.
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks of Analog
Devices, Inc. All other trademarks are the property of their respective owners.
Protocol Switching with
Automatic Termination Selection
FeaTures
applicaTions
n One RS485 and Two RS232 Transceivers
n 3V to 5.5V Supply Voltage
n 20Mbps RS485 and 500kbps RS232
n Automatic Selection of Integrated RS485 (120Ω)
and RS232 (5kΩ)Termination Resistors
n Half-/Full-Duplex RS485 Switching
n High ESD: ±26kV (LTC2870), ±16kV (LTC2871)
n Logic Loopback Mode
n 1.7V to 5.5V Logic Interface
n Supports Up to 256 RS485 Nodes
n RS485 Receiver Failsafe Eliminates UART Lockup
n H-Grade Available for the LTC2870 (–40°C to 125°C)
n Available in 28-Pin 4mm × 5mm QFN and
TSSOP (LTC2870), and 38-Pin 5mm × 7mm QFN
and TSSOP (LTC2871)
n Software Selectable RS232/RS485/RS422 Interface
n Point-of-Sale Terminals
n Cable Repeaters
n Protocol Translators
Simultaneous Protocols and
RS485 Termination Switching
RS485 Duplex Switching
28701 TA01
LTC2870
1.7V TO VCC 3V TO 5.5V 1.7V TO VCC 3V TO 5.5V 1.7V TO VCC 3V TO 5.5V
RS485
TERMINATION
RS485
DUPLEX
485/232
DY
Y
Z
A
B
DZ
RA
RB
Y
Z
A
B
DI
TE485
RO
DIN1
ROUT2
DIN2
ROUT1
DOUT1
RIN2
DOUT2
RIN1
LTC2871
120Ω
120Ω
Y
Z
A
B
DI,
DY
RO,
RA
H/F
LTC2870,
LTC2871
HALFFULL
RS485RS232
ON
OFF
VDD VEE
GND 1µF1µF
CAP SW
2.2µF0.1µF
VLVCC
220nF 10µH
VDD VEE
GND 1µF1µF
CAP SW
2.2µF0.1µF
VLVCC
220nF 10µH
VDD VEE
GND 1µF1µF
CAP SW
2.2µF0.1µF
VLVCC
220nF 10µH
LTC2870/LTC2871
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For more information www.linear.com/LTC2870
LTC2870 LTC2870
9 10
TOP VIEW
29
VEE
UFD PACKAGE
28-LEAD (4mm × 5mm) PLASTIC QFN
11 12 13
28 27 26 25 24
14
23
6
5
4
3
2
1
VEE
RA
RB
485/232
RXEN
DXEN
DY
DZ
A
B
VCC
Y
GND
Z
VCC
VDD
TE485
H/F
LB
VL
VCC
GND
FEN
GND
CAP
VEE
GND
SW
7
17
18
19
20
21
22
16
815
TJMAX = 150°C, θJA = 43°C/W
EXPOSED PAD (PIN 29) IS VEE,
MUST BE SOLDERED TO PCB
1
2
3
4
5
6
7
8
9
10
11
12
13
14
TOP VIEW
FE PACKAGE
28-LEAD PLASTIC TSSOP
28
27
26
25
24
23
22
21
20
19
18
17
16
15
LB
H/F
TE485
VEE
RA
RB
485/232
RXEN
DXEN
DY
DZ
FEN
GND
CAP
VL
VCC
GND
A
B
VCC
Y
GND
Z
VCC
VDD
SW
GND
VEE
29
VEE
TJMAX = 150°C, θJA = 30°C/W
EXPOSED PAD (PIN 29) IS VEE,
MUST BE SOLDERED TO PCB
absoluTe MaxiMuM raTings
Input Supplies
VCC, VL ..................................................... 0.3V to 7V
Generated Supplies
VDD ................................................VCC0.3V to 7.5V
VEE .........................................................0.3V to 7.5V
SW ........................................... 0.3V to (VDD + 0.3V)
CAP ............................................. 0.3V to (VEE0.3V)
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
(Notes 1 and 2)
pin conFiguraTion
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
LTC2870/LTC2871
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For more information www.linear.com/LTC2870
orDer inForMaTion
LEAD FREE FINISH TAPE AND REEL PART MARKING PACKAGE DESCRIPTION TEMPERATURE RANGE
LTC2870CFE#PBF
LTC2870IFE#PBF LTC2870CFE#TRPBF
LTC2870IFE#TRPBF LTC 2870FE
L
TC 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/
LTC2871 LTC2871
13 14 15 16
TOP VIEW
39
VEE
UHF PACKAGE
38-LEAD (5mm × 7mm) PLASTIC QFN
17 18 19
38 37 36 35 34 33 32
24
25
26
27
28
29
30
31
8
7
6
5
4
3
2
1VEE
ROUT1
ROUT2
CH2
RX485
DX485
DI
DIN1
DIN2
DX232
RX232
VEE
RIN1
RIN2
A
B
VCC
Y
GND
Z
DOUT1
DOUT2
VCC
VDD
TE485
H/F
LB
VL
RO
VCC
GND
FEN
GND
CAP
VEE
GND
SW
VEE
23
22
21
20
9
10
11
12
TJMAX = 150°C, θJA = 34°C/W
EXPOSED PAD (PIN 39) IS VEE,
MUST BE SOLDERED TO PCB
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
TOP VIEW
FE PACKAGE
38-LEAD PLASTIC SSOP
38
37
36
35
34
33
32
31
30
29
28
27
26
25
24
23
22
21
20
VL
LB
H/F
TE485
VEE
ROUT1
ROUT2
CH2
RX485
DX485
DI
DIN1
DIN2
DX232
RX232
VEE
FEN
GND
CAP
RO
VCC
GND
RIN1
RIN2
A
B
VCC
Y
GND
Z
DOUT1
DOUT2
VCC
VDD
VEE
SW
GND
VEE
39
VEE
TJMAX = 150°C, θJA = 28°C/W
EXPOSED PAD (PIN 39) IS VEE,
MUST BE SOLDERED TO PCB
pin conFiguraTions
PART NUMBER CONFIGURABLE TRANSCEIVER COMBINATIONS (RS485 + RS232) PACKAGES TEMPERATURE GRADES
LTC2870 (0 + 0), (1 + 0), (0 + 2) 28-Lead QFN, 28-Lead TSSOP C, I, H (QFN)
LTC2871 (0 + 0), (1 + 0), (1 + 1), (1 + 2), (0 + 1), (0 + 2) 38-Lead QFN, 38-Lead TSSOP C, I
proDucT selecTion guiDe
http://www.linear.com/product/LTC2870#orderinfo
LTC2870/LTC2871
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For more information www.linear.com/LTC2870
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 3 5.5 V
VLLogic Supply Voltage Operating Range VL ≤ VCC 1.7 VCC V
VCC Supply Current in Shutdown Mode RXEN = VL, DXEN = TE485 = FEN = 0V,
(LTC2870)
DX485 = DX232 = TE485 = FEN = H/F = 0V,
RX485 = RX232 = VL (LTC2871)
l8 60 µA
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)
3.3 mA
VL Supply Current in Transceiver Mode
(Outputs Unloaded)
l0 5 µA
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
6
VCC
VCC
V
V
V
|VOD| Difference in Magnitude of Differential Output
Voltage for Complementary Output States
RL = 27Ω, VCC = 3V (Figure 1)
RL = 50Ω, VCC = 3.13V (Figure 1)
l
l
0.2
0.2
V
V
VOC Common Mode Output Voltage RL = 27Ω or 50Ω (Figure 1) l3 V
|VOC| Difference in Magnitude of Common Mode
Output Voltage for Complementary Output
States
RL = 27Ω or 50Ω (Figure 1) l0.2 V
IOZD485 Three-State (High Impedance) Output Current VOUT = 12V or –7V, VCC = 0V or 3.3V (Figure 2) l–100 125 µA
IOSD485 Maximum Short-Circuit Current –7V ≤ VOUT ≤ 12V (Figure 2) l–250 250 mA
RS485 Receiver
IIN485 Input Current VIN = 12V or –7V, VCC = 0V or 3.3V (Figure 3)
(Note 5)
l–100 125 µA
RIN485 Input Resistance VIN = 12V or –7V, VCC = 0V or 3.3V (Figure 3)
(Note 5)
l96 125
Differential Input Signal Threshold Voltage
(A-B)
–7V ≤ (A or B) ≤ 12V (Note 5) l±200 mV
Input Hysteresis B = 0V (Notes 3, 5) 220 mV
Differential Input Failsafe Threshold Voltage –7V ≤ (A or B) ≤ 12V, (A-B) Rising (Note 5) l–200 –70 0 mV
Input DC Failsafe Hysteresis B = 0V (Note 5) 35 mV
VOL Output Low Voltage Output Low, I(RA, RO) = 3mA (Sinking),
3V ≤ VL ≤ 5.5V
l0.4 V
Output Low, I(RA, RO) = 1mA (Sinking),
1.7V ≤ VL < 3V
l0.4 V
VOH Output High Voltage Output High, I(RA, RO) = –3mA (Sourcing),
3V ≤ VL ≤ 5.5V
lVL – 0.4 V
Output High, I(RA, RO) = –1mA (Sourcing),
1.7V ≤ VL < 3V
lVL – 0.4 V
Three-State (High Impedance) Output Current 0V ≤ (RA, RO) ≤VL, VL = 5.5V l0 ±5 µA
Short-Circuit Output Current 0V ≤ (RA, RO) ≤VL, VL = 5.5V l±125 mA
LTC2870/LTC2871
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For more information www.linear.com/LTC2870
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
RTERM Terminating Resistor TE485 = VL, A – B = 2V, B = –7V, 0V, 10V
(Figure8) (Note 5)
l108 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 l5 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
l
±156
±10
µA
µA
Output Short-Circuit Current Driver Output = 0V l±35 ±90 mA
RS232 Receiver
Input Threshold Voltage l0.6 1.5 2.5 V
Input Hysteresis l0.1 0.4 1.0 V
Output Low Voltage I(RA, RB, ROUT1, ROUT2) = 1mA (Sinking)
1.7V ≤ VL ≤ 5.5V
l0.4 V
Output High Voltage I(RA, RB, ROUT1, ROUT2) = –1mA (Sourcing)
1.7V ≤ VL ≤ 5.5V
lVL – 0.4 V
Input Resistance –15V ≤ (A, B, RIN1, RIN2) ≤ 15V,
RS232 Receiver Enabled
l3 5 7
Three-State (High Impedance) Output Current 0V ≤ (RA, RB, ROUT1, ROUT2) ≤ VLl0 ±5 µA
Output Short-Circuit Current VL = 5.5V
0V ≤ (RA, RB, ROUT1, ROUT2) ≤ VL
l±25 ±50 mA
Logic Inputs
Threshold Voltage l0.4 0.75 VLV
Input Current l0 ±5 µA
Power Supply Generator
VDD Regulated VDD Output Voltage RS232 Drivers Enabled, Outputs Loaded with
RL = 3kΩ to GND, DIN1/DY = VL, DIN2/DZ = 0V
(Note 3)
7 V
VEE Regulated VEE Output Voltage –6.3 V
ESD
LTC2870 Interface Pins (A, B, Y, Z) Human Body Model to GND or VCC, Powered or
Unpowered (Note 7)
±26 kV
LTC2871 Interface Pins (A, B, Y, Z, RIN1,
RIN2, DOUT1, DOUT2)
±16 kV
All Other Pins Human Body Model (Note 7) ±4 kV
LTC2870/LTC2871
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For more information www.linear.com/LTC2870
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
Maximum Data Rate (Note 3) l20 Mbps
tPLHD485
tPHLD485
Driver Propagation Delay RDIFF = 54Ω, CL = 100pF (Figure 4) l20 70 ns
Driver Propagation Delay Difference
|tPLHD485 – tPHLD485|
RDIFF = 54Ω, CL = 100pF (Figure 4) l1 6 ns
tSKEWD485 Driver Skew (Y to Z) RDIFF = 54Ω, CL = 100pF (Figure4) l1 ±6 ns
tRD485, tFD485 Driver Rise or Fall Time RDIFF = 54Ω, CL = 100pF (Figure4) l15 ns
tZLD485, tZHD485,
tLZD485, tHZD485
Driver Output Enable or Disable Time FEN = VL, RL = 500Ω, CL = 50pF (Figure5) l120 ns
tZHSD485, tZLSD485 Driver Enable from Shutdown RL = 500Ω, CL = 50pF (Figure5) l8 µs
tPLHR485, tPHLR485 Receiver Input to Output CL = 15pF, VCM = 1.5V, |A – B| = 1.5V
(Figure6) (Note 5)
l65 85 ns
tSKEWR485 Differential Receiver Skew
|tPLHR485 – tPHLR485|
CL = 15pF (Figure6) l1 6 ns
tRR485, tFR485 Receiver Output Rise or Fall Time CL = 15pF (Figure6) l3 15 ns
tZLR485, tZHR485,
tLZR485, tHZR485
Receiver Output Enable or Disable Time FEN = VL, RL = 1kΩ, CL = 15pF (Figure7) l50 ns
tRTEN485, tRTZ485 Termination Enable or Disable Time FEN = VL, VB = 0V, VAB = 2V (Figure8) (Note 5) l100 µs
RS232 AC Characteristics
Maximum Data Rate RL = 3kΩ, CL = 2500pF
RL = 3kΩ, CL = 500pF
(Note 3)
l
l
100
500
kbps
kbps
Driver Slew Rate (Figure9) RL = 3kΩ, CL = 2500pF
RL = 3kΩ, CL = 50pF
l
l
4
30
V/µs
V/µs
tPHLD232, tPLHD232 Driver Propagation Delay RL = 3kΩ, CL = 50pF (Figure9) l1 2 µs
tSKEWD232 Driver Skew RL = 3kΩ, CL = 50pF (Figure9) 50 ns
tZLD232, tZHD232,
tLZD232, tHZD232
Driver Output Enable or Disable Time FEN = VL, RL = 3kΩ, CL = 50pF (Figure10) l0.4 2 µs
tPHLR232, tPLHR232 Receiver Propagation Delay CL = 150pF (Figure11) l60 200 ns
tSKEWR232 Receiver Skew CL = 150pF (Figure11) 25 ns
tRR232, tFR232 Receiver Rise or Fall Time CL = 150pF (Figure 11) l60 200 ns
tZLR232, tZHR232,
tLZR232, tHZR232
Receiver Output Enable or Disable Time FEN = VL, RL = 1kΩ, CL = 150pF (Figure 12) l0.7 2 µs
Power Supply Generator
VDD/VEE Supply Rise Time FEN = , (Notes 3 and 4) l0.2 2 ms
Note 1: Stresses beyond those listed under Absolute Maximum Ratings
may cause permanent damage to the device. Exposure to any Absolute
Maximum Rating condition for extended periods may affect device
reliability and lifetime.
Note 2: All currents into device pins are positive; all currents out of device
pins are negative. All voltages are referenced to device ground unless
otherwise specified.
Note 3: Guaranteed by other measured parameters and not tested directly.
Note 4: Time from FEN until VDD ≥ 5V and VEE ≤ –5V. External
components as shown in the Typical Application section.
Note 5: Condition applies to A, B for H/F = 0V, and Y, Z for H/F = VL.
Note 6: This IC includes overtemperature protection that is intended
to protect the device during momentary overload conditions.
Overtemperature protection activates at a junction temperature exceeding
150°C. Continuous operation above the specified maximum operating
junction temperature may result in device degradation or failure.
Note 7: Guaranteed by design and not subject to production test.
LTC2870/LTC2871
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For more information www.linear.com/LTC2870
Typical perForMance characTerisTics
VCC Supply Current
vs RS232 Data Rate
VCC Supply Current vs Supply
Voltage, All Transceivers at Max
Rate (LTC2871)
RS485 Driver Differential Output
Voltage vs Temperature
RS485 Driver Propagation Delay
vs Temperature
RS485 Driver Skew
vs Temperature
RS485 Driver Short-Circuit
Current vs Short-Circuit Voltage
VCC Supply Current vs Supply
Voltage in Shutdown Mode
VCC Supply Current vs Supply
Voltage in Fast Enable Mode
VCC Supply Current
vs RS485 Data Rate
INPUT VOLTAGE (V)
3
INPUT CURRENET (µA)
30
25
20
15
10
5
04.5 53.5
28701 G01
5.54
H/F LOW
H/F HIGH
SUPPLY VOLTAGE (V)
3
SUPPLY CURRENT (mA)
4
3
2
14.5 53.5
28701 G02
5.54
–40°C
25°C
85°C
ALL DRIVERS AND RECEIVERS DISABLED
TE485 LOW
DATA RATE (Mbps)
0.1
SUPPLY CURRENT (mA)
100
80
60
40
20
010
28701 G03
1001
TE HIGH
TE LOW
VCC = 5V
VCC = 3.3V
ALL RS485 DRIVERS
AND RECEIVERS
SWITCHING.
CL = 100pF ON EACH
DRIVER OUTPUT.
DATA RATE (kbps)
0
INPUT CURRENT (mA)
35
30
25
15
20
10
5400
28701 G04
500300200100
0.05nF
0.05nF
0.5nF
0.5nF
2.5nF
2.5nF
VCC = 5V
VCC = 3.3V
ALL RS232 DRIVERS
AND RECEIVERS
SWITCHING.
SUPPLY VOLTAGE (V)
3
SUPPLY CURRENT (mA)
120
110
100
90
80
70 4.5 53.5
28701 G05
5.54
85°C
ALL DRIVERS AND
RECEIVERS SWITCHING.
DRIVER OUTPUTS TIED TO
RECEIVER INPUTS.
RS232: 0.5Mbps (CL = 500pF)
RS485: 20Mbps (CL = 100pF)
TE485 HIGH
25°C
–40°C
TEMPERATURE (°C)
–50
VOLTAGE (V)
4.5
3.5
2.5
1.5
4.0
3.0
2.0
1.0
0.5
050 75–25
28701 G06
100250
RL = 100Ω
RL = 54Ω
RL = 100Ω
RL = 54Ω
VCC = 5V
VCC = 3.3V
TEMPERATURE (°C)
–50
DELAY (ns)
50
40
30
20
10
050 75–25
28701 G07
100250
VCC = 3.3V, VL = 1.7V
VCC = 5V, VL = 1.7V
VCC = 3.3V, VL = 3.3V
VCC = 5V, VL = 5V
TEMPERATURE (°C)
–50
SKEW (ns)
3.0
2.5
1.5
2.0
1.0
0.5
050 75–25
28701 G08
100250
OUTPUT LOW
OUTPUT HIGH
SHORT–CIRCUIT VOLTAGE (V)
–15
–10
–5
0
5
10
15
–150
–100
–50
0
50
100
150
SHORT–CIRCUIT CURRENT (mA)
28701 G09
VCC = 5V
VCC = 3.3V
TA = 25°C, VCC = VL = 3.3V, unless otherwise noted.
LTC2870/LTC2871
8
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For more information www.linear.com/LTC2870
Typical perForMance characTerisTics
RS232 Receiver Input Threshold
vs Temperature
RS232 Receiver Output Voltage
vs Load Current
RS485 Termination Resistance
vs Temperature
RS485 Receiver Propagation
Delay vs Temperature
RS485 Receiver Skew
vs Temperature
RS485 Receiver Output Voltage
vs Load Current
TEMPERATURE (°C)
–50
DELAY (ns)
80
70
60
50
40 50 75–25
28701 G10
100250
VCC = 3.3V, VL = 1.7V
VCC = 5V, VL = 1.7V
VCC = 3.3V, VL = 3.3V
VCC = 5V, VL = 5V
TEMPERATURE (°C)
–50
SKEW (ns)
3.0
2.5
2.0
1.0
1.5
0.5
050 75–25
28701 G11
100250
OUTPUT CURRENT (mA)
0
OUTPUT VOLTAGE (V)
6
5
4
2
3
1
082
28701 G12
1064
VL = 5V
VL = 3.3V
VL = 1.7V
TEMPERATURE (°C)
–50
THRESHOLD VOLTAGE (V)
2.0
1.8
1.6
1.4
1.2
1.0 50 75–25
28701 G13
100250
VCC = 5V
VCC = 3.3V
INPUT HIGH
INPUT LOW
OUTPUT CURRENT (mA)
0
OUTPUT VOLTAGE (V)
6
5
4
2
3
1
082
28701 G14
1064
VL = 5V
VL = 3.3V
VL = 1.7V
TEMPERATURE (°C)
–50
RESISTANCE (Ω)
130
118
116
114
112
110
128
126
124
122
120
50 75–25
28701 G15
100250
VCM = –7V
VCM = 2V
VCM = 12V
TA = 25°C, VCC = VL = 3.3V, unless otherwise noted.
LTC2870/LTC2871
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For more information www.linear.com/LTC2870
RS232 Operation at 500kbps RS485 Operation at 20Mbps LTC2870 Drivers Changing Modes
WRAPPING DATA
DOUT LOADS: 5kΩ + 50pF
5V/DIV
28701 G16
1µs/DIV
DOUT1
DOUT2
ROUT1
ROUT2
DIN1
DIN2
H/F HIGH
Y, Z LOADS: 120Ω (DIFF) + 50pF
1V/DIV
5V/DIV
28701 G17
20ns/DIV
RO
Y
DI
Z
5V/DIV
5V/DIV
28701 G18
2µs/DIV
RS232
MODE
RS485
MODE
RS232
MODE
Y
485/232
Z
Typical perForMance characTerisTics
RS232 Driver Outputs Enabling
and Disabling VDD and VEE Powering Up VDD and VEE Ripple
5V/DIV
2V/DIV
28701 G20
40µs/DIV
FEN
VDD
VEE
5V/DIV
28701 G19
40µs/DIV
TOP CURVES: FAST ENABLE DX232
BOTTOM CURVES: SHUTDOWN DX232
FEN = 1
FEN = 0
DOUT1
DOUT2
DOUT1
DX232
DOUT2 10mV/DIV
28701 G21
40µs/DIV
FAST ENABLE MODE,
ALL DRIVERS AND RECEIVERS DISABLED.
VDD RIPPLE
VEE RIPPLE
TA = 25°C, VCC = VL = 3.3V, unless otherwise noted.
LTC2870/LTC2871
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pin FuncTions
PIN NAME
LTC2870
QFN
LTC2870
TSSOP
LTC2871
QFN
LTC2871
TSSOP
DESCRIPTION
VCC 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.
VL25 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 1, 12, 16,
19, 39
5, 16, 20,
23, 39
Generated Negative Supply Voltage for RS232 Driver (–6.3V). Tie all pins together and connect
1μF capacitor between VEE (the VEE pin sequentially after CAP) and GND.
GND 10, 13,
18, 23
13, 16,
21, 26
14, 17,
25, 32
18, 21,
29, 36
Ground. Tie all four pins together
.
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 34 38 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 RS232 Driver Input 2. Do not float.
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).
LTC2870/LTC2871
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For more information www.linear.com/LTC2870
pin FuncTions
PIN NAME
LTC2870
QFN
LTC2870
TSSOP
LTC2871
QFN
LTC2871
TSSOP
DESCRIPTION
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. 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.
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/F27 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 half-
duplex operation where pins Y and Z are both the receiver inputs and driver outputs and pins
A and B do not serve as the receiver inputs. The impedance on A and B and state of differential
termination between A and B is independent of the state of H/F. The H/F pin has no effect on
RS232 operation. 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.
CH2 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.
LTC2870/LTC2871
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For more information www.linear.com/LTC2870
block DiagraM
LTC2870
2870 BD
CONTROL
LOGIC
DRIVERS
RECEIVERS
LOOPBACK
PATH
0.1µF
DXEN
RXEN
TE485
H/F
485/232
FEN
LB
DY
DZ
RA
RB
GND
B
A
Z
Y
VEE
VDD
1.7V TO 5.5V
(≤ VCC)
PULSE-SKIPPING
BOOST
REGULATOR
f = 1.2MHz
RT232
RT485
2.2µF
10µH 220nF
3V TO 5.5V
F
F
CAPSWVCC
VL
232
485
232
232
485
232
5k
RT232
RT485
125k
5k
125k
125k 120Ω
RT485
120Ω
H/F
125k
LTC2870/LTC2871
13
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For more information www.linear.com/LTC2870
block DiagraM
LTC2871
2871 BD
CONTROL
LOGIC
DRIVERS
RECEIVERS
LOOPBACK
PATH
0.1µF
DX232
DX485
RX232
RX485
TE485
H/F
CH2
DIN1
DIN2
RO
ROUT2
GND
RIN2
RIN1
Z
Y
DOUT1
DOUT2
VEE
VDD
1.7V TO 5.5V
(≤ VCC)
FEN
LB
RT232
RT485
2.2µF
10µH 220nF
3V TO 5.5V
DI
ROUT1
A
B
F
F
CAPSWVCC
VL
232
485
232
232
485
232
5k
RT232
RT485
5k
120Ω
RT485
120Ω
H/F
125k
125k
125k
125k
PULSE-SKIPPING
BOOST
REGULATOR
f = 1.2MHz
LTC2870/LTC2871
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TesT circuiTs
Figure1. RS485 Driver DC Characteristics Figure2. RS485 Driver Output Current
Figure3. RS485 Receiver Input Current and Resistance (Note 5)
Figure4. RS485 Driver Timing Measurement
28701 F01
DRIVER
DY/DI
GND
OR
VL
Y
RL
RL
Z
VOD
+
VOC
+
28701 F02
DRIVER
Y OR Z
DY/DI
GND
OR
VL
Z OR Y VOUT
IOZD485, IOSD485
+
28701 F03
RECEIVER
A OR B
B OR A
VIN
IIN485
RIN485 =
VIN
IIN485
+
28701 F04
DRIVER
DY/DI
Y
Z
RDIFF
CL
CL
tPLHD485
tSKEWD485
tPLHD485
tRD485 tFD485
90% 0V
VOD ½VOD
VL
0V
DY/DI
Y, Z
Y - Z 10%
90%
0V 10%
LTC2870/LTC2871
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For more information www.linear.com/LTC2870
TesT circuiTs
Figure5. RS485 Driver Enable and Disable Timing Measurements
Figure6. RS485 Receiver Propagation Delay Measurements (Note 5)
Figure7. RS485 Receiver Enable and Disable Timing Measurements (Note 5)
28701 F05
tZLD485,
tZLSD485 tLZD485
tHZD485
tZHD485,
tZHSD485
½VCC
½VCC
VL
VOL
VCC
VOH
0V
0V
0.5V
DXEN/
DX485
Y OR Z
Z OR Y
DRIVER
DY/DI
DXEN/DX485
VL
OR
GND
Y
RL
Z
GND
OR
VCC
VCC
OR
GND
RL
CL
CL
½VL
0.5V
½VL
28701 F06
RECEIVER
VCM
±VAB/2 A
B
RA/RO
±VAB/2
CL
tPLHR485
tSKEWR485 = tPLHR485 – tPHLR485
tRR485
90%
0V
½VL
A-B
RA/RO 10%
tPHLR485
tFR485
90%
0V
–VAB
V
AB
½VL10%
VL
28701 F07
tZLR485
tLZR485
tHZR485
tZHR485
½VL
½VL
V
L
VOL
VL
VOH
0V
0V
0.5V
RXEN/
RX485
RA/RO
RA/RO
RECEIVER RA/RO
RXEN/RX485
0V TO 3V
3V TO 0V
A
RL
B
VL
OR
GND
CL
½VL
0.5V
½VL
LTC2870/LTC2871
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TesT circuiTs
Figure8. RS485 Termination Resistance and Timing Measurements (Note 5)
Figure9. RS232 Driver Timing and Slew Rate Measurements
Figure10. RS232 Driver Enable and Disable Times
28701 F08
RECEIVER VAB
A
B
V
AB
IA
RTERM =
VB
½VL½VL
TE485
IA
IA
90%
10%
tRTZ485
tRTEN485
0V
VL
TE485
+
+
28701 F09
DRIVER
INPUT
DRIVER
OUTPUT
CL
RL
tPHLD232
tPLHD232
tSKEWD232 = |tPHLD232 – tPLHD232|
tFtR
DRIVER
INPUT
DRIVER
OUTPUT
SLEW RATE = 6V
tF OR tR
3V
–3V
0V
VOLD
V
L
½VL½VL
0V
3V
–3V 0V
VOHD
28701 F10
0V OR VL
DXEN/DX232
DRIVER
OUTPUT
CL
RL
tHZD232
tLZD232
DXEN/
DX232
DRIVER
OUTPUT
DRIVER
OUTPUT
0.5V
tZHD232
tZLD232
5V
5V 0.5V
0V
0V
0V
VOHD
VL
½VL½VL
VOLD
LTC2870/LTC2871
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For more information www.linear.com/LTC2870
TesT circuiTs
Figure11. RS232 Receiver Timing Measurements
Figure12. RS232 Receiver Enable and Disable Times
28701 F11
tPHLR232
tSKEWR232 = |tPLHR232 – tPHLR232|
tPLHR232
tRR232
90%
1.5V 1.5V
½VL10%
tFR232
90%
0V
–3V
VL
+3V
½VL
10% 0V
VL
RECEIVER
OUTPUT
RECEIVER
OUTPUT
RECEIVER
INPUT
RECEIVER
INPUT
CL
28701 F12
–3V OR +3V
RXEN/RX232
RECEIVER
OUTPUT GND
OR VL
CL
RL
tHZR232
tLZR232
RXEN/
RX232
RECEIVER
OUTPUT
RECEIVER
OUTPUT
0.5V
tZHR232
tZLR232
½VL
½VL0.5V
0V
0V
VL
VOHR
VL
½VL½VL
VOLR
LTC2870/LTC2871
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FuncTion Tables
Table 1. LTC2870 Mode Selection Table
FEN 485/232 RXEN DXEN TE485 H/FLB
DC/DC
CONVERTER MODE AND COMMENTS
0 X 1 0 0 X X OFF Low Power Shutdown: All Main Functions Off
0 0 1 0 X X X OFF Low Power Shutdown: All Main Functions Off
1 X 1 0 0 X X ON Fast-Enable: DC/DC Converter On Only
X 0 X 1 X X 0 ON RS232 Drivers On
X 0 0 X X X 0 ON RS232 Receivers On
X 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
Table 2. LTC2871 Mode Selection Table (CH2 = 0)
FEN RX232 DX232 RX485 DX485 TE485 H/FLB
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)
LTC2870 RECEIVER INPUTS
(A, B)
LTC2871 RECEIVER INPUTS
(RIN1, RIN2)
1 X No Fault Hi-Z 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
LTC2870/LTC2871
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FuncTion Tables
Table 5. LTC2871 CH2 CONTROL
CH2 DX232 RX232
RS232 RECEIVER INPUTS RS232 DRIVER OUTPUTS
COMMENTSRIN1 RIN2 DOUT1 DOUT2
X 0 1 Hi-Z Hi-Z Hi-Z Hi-Z 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/FRS485 DRIVER OUTPUTS RS485 RECEIVER INPUTS
0Y, Z A, B
1Y, Z Y, Z
Table 10. LTC2870 Loopback Functions
LB RXEN MODE
0 X Not Loopback
X 1 Not Loopback
1 0 Loopback (RA = DY, RB = DZ)
Table 11. LTC2871 Loopback Functions
LB RX232 RX485 MODE
0 X X Not Loopback
X 1 1 Not Loopback
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)
LTC2870/LTC2871
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For more information www.linear.com/LTC2870
Overview
The LTC2870 and LTC2871 are flexible multiprotocol
transceivers supporting RS485/RS422 and RS232 pro-
tocols. 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 posi-
tive and negative supply rails needed for RS232 operation.
Automatically selected integrated termination resistors
for both RS232 and RS485 protocols are included, elimi-
nating 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 transceiv-
ers into independent I/Os allowing simultaneous opera-
tion of two RS232 transceivers and one RS485 trans-
ceiver. 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.
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 pro-
tection exceeding ±4kV.
DC/DC Converter
The on-chip DC/DC converter operates from the V
CC
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 lev-
els that guarantee greater than ±5V output swing. 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.
applicaTions inForMaTion
Figure13. DC/DC Converter with Required External Components
28701 F13
BOOST
REGULATOR
VCC
3V TO 5.5V
VL
1.7V TO VCC
C1
220nF
L1
10µH
C4
2.2µF
VCC
VDD
VEE
SW
GND
GND
CAP
C5
0.1µF
C2
F
C3
F
VL
Figure14. Demo Circuit 1786A (Bottom) Showing Layout of
External Devices. U.S. Penny Included for Scale
LTC2870/LTC2871
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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
(mA)
MAX
DCR (Ω)
SIZE(mm)
MANUFACTURER
LBC2016T100K
CBC2016T100M
245
380
1.07
1.07
2 × 1.6 × 1.6
2 × 1.6 × 1.6
Taiyo Yuden
www.t-yuden.com
FSLB2520-100K 220 1.1 2.5 × 2 × 1.6 Toko
www.tokoam.com
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.
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 sup-
plies 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 gener-
ated 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 V
CC
and V
DD
generates
inrush current flowing through inductor L1 and capaci-
tors 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.
applicaTions inForMaTion
Figure15. Supply Current Overshoot Protection
for Input Supplies of 4.5V of Higher
28701 F14
0V
5V
≤10µs
C1
220nF
L1
10µH
INRUSH
CURRENT
C4
2.2µF
R1
1/8W
VCC
VDD GND
SW CAP
C2
F
LTC2870/LTC2871
22
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Figure16. RS485 Receiver Input Threshold Characteristics
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 V
L
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 V
L
as their high
supply, they can be driven up to 7V, independent of V
L
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.
RS485 Driver
The RS485 driver provides full RS485/RS422 compat-
ibility. 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.
Driver Overvoltage and Overcurrent Protection
The RS232 and RS485 driver outputs are protected from
short circuits to any voltage within the absolute maximum
range ±15V. The maximum current in this condition is
90mA for the RS232 driver and 250mA for the RS485
driver.
If the RS485 driver output is shorted to a voltage greater
than VCC, when it is active, positive current of up to 150mA
may flow from the driver output back to VCC. If the system
power supply or loading cannot sink this excess current,
clamp V
CC
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 com-
parator 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 direc-
tion, the threshold is typically +110mV. When approach-
ing from the positive direction, the threshold is typically
–110mV. Each of these thresholds has about 35mV 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.
28701 F15
RECEIVER
OUTPUT LOW
–200mV –110mV 0V
RO
110mV 200mV
VAB
RECEIVER
OUTPUT HIGH
LTC2870/LTC2871
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applicaTions inForMaTion
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 ±200mV and is heavily slewed, the output maintains a
nearly perfect signal with almost no duty cycle distortion.
An additional benefit of the window comparator architec-
ture is excellent noise immunity due to the wide effec-
tive differential hysteresis (or AC hysteresis) of about
220mV for normal signals transitioning through the win-
dow region in less than approximately 0.7µs. Increasingly
slower signals will have increasingly less effective hys-
teresis, limited by the DC failsafe value of about 35mV.
The LTC2870 and LTC2871 provide full failsafe opera-
tion that guarantees the receiver output will be a logic
high state when the inputs are shorted, left open, or ter-
minated but not driven, for more than about 0.7µs. The
delay allows normal data signals to transition through the
threshold region without being interpreted as a failsafe
condition.
RS485 Biasing Resistors Not Required
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 ter-
minating 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 dif-
ferential 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 pull-
up needed. When the receivers are disabled the output pin
becomes Hi-Z with leakage of less than ±5μA for voltages
within the VL supply range.
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
(A-B)
200mV/DIV
B
100mV/DIV
A
RO
5V/DIV
28701 F16
200ns/DIV
LTC2870/LTC2871
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applicaTions inForMaTion
RS485 Receiver Input Resistance
The RS485 receiver input resistance from A or B to GND
(Y or Z to GND in half-duplex mode with driver disabled)
is greater than 96kΩ (typically 125kΩ) when the inte-
grated 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 resis-
tance looking into the RS485 receiver pins is shown in
Figure18.
Figure18. Equivalent RS485 Receiver
Input Resistance Into A and B (Note 5)
28701 F17
A
B
TE485
60Ω
60Ω
125k
125k
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.
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 proto-
col devices use the opposite RS485 convention: A is the
negative receiver input, B is the positive receiver input, Y
is the negative driver output, and Z is the positive driver
output.
VOLTAGE (V)
–10
RESISTANCE (Ω)
126
124
122
118
120
116
10–5
28701 F18
1550
VCC = 5.0V
VCC = 3.3V
Figure19. Typical Resistance of the Enabled RS485
Terminator vs Common Mode Voltage on A /B
Selectable RS485 Termination
Proper cable termination is important for good signal
fidelity. When the cable is not terminated with its charac-
teristic 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 eas-
ily change, through logic control, the proper line termina-
tion 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.
LTC2870/LTC2871
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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 man-
ufacturers 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.
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 Cable Length vs Data Rate
For a given data rate, the maximum transmission dis-
tance 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 fac-
tors 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.
DATA RATE (bps)
CABLE LENGTH (FT)
28701 F19
10k
1k
100
10
10k 10M 100M1M100k
LTC2870/LTC2871
MAX DATA RATE
RS485/RS422
MAX DATA RATE
Figure20. Cable Length vs Data Rate (RS485/RS422
Standard Shown in Vertical Solid Line)
LTC2870/LTC2871
26
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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 V
L
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 dif-
ferential 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 feed-
back effects that might cause noise, jitter, or even oscilla-
tions. For example, do not route DI or A/B near the driver
or receiver outputs.
LTC2870/LTC2871
27
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Typical applicaTions
Figure21. LTC2870 in
RS232 Mode
Figure22. LTC2870 in RS232
Mode with Loopback
Figure23. LTC2870 in RS485
Mode, Terminated
Figure24. LTC2870 in RS485
Mode in Loopback
Figure25. LTC2870 in RS485
Mode Half-Duplex
Figure26. LTC2870 in RS485
Mode, Half-Duplex, with
Loopback and Terminated
28701 F20
DXEN
VL
LB
485/232
RXEN
DY
RB
GND
DZ
RA
Y
B
Z
A
LTC2870
28701 F21
485/232
RXEN
DY
RB
GND
DZ
RA
Y
B
Z
A
LTC2870
DXEN
LB
VL
28701 F23
DXEN
VL
RXEN
H/F
TE485
485/232
LB
DY
GND
RA
Y
B
Z
A
LTC2870
28701 F22
DXEN
V
L
RXEN
H/F
LB
485/232
TE485
DY
GND
RA
120Ω
Y
B
Z
A
LTC2870
120Ω
D R
28701 F24
485/232
VL
RXEN
TE485
LB
H/F
DXEN
DY
GND
RA
Y
Z
LTC2870
28701 F25
485/232
H/F
LB
TE485
D R
RXEN
DXEN
VL
DY
120Ω
120Ω
GND
RA
Y
B
Z
A
LTC2870
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
28
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Typical applicaTions
Figure27. LTC2870 Protocol Switching Figure28. LTC2871 in RS485 Mode Figure29. LTC2871 in RS232 Mode
Figure30. LTC2871 Single
RS232 Channel Active
Figure31. LTC2871 in
RS485 and RS232 Mode
Figure32. LTC2871 in RS485 and
RS232 Mode with Loopback and
RS485 Termination
28701 F26
DXEN
485/232
VL
LB
RXEN
H/F
RB
GND
RA
B
A
LTC2870
DZ
DY
Z
120Ω
120Ω
Y
RS
485
RS
232
TE485
28701 F27
VL
CH2
DX232
TE485
H/F
RX485
RX232
DX485
GND
RO
B
A
LTC2871
LB
DI
Z
Y
28701 F28
V
L
CH2
DX485
TE485
H/F
RX232
GND
LTC2871
LB
DIN1
ROUT2
DIN2
ROUT1
DOUT1
RIN2
DOUT2
RIN1
RX485
DX232
28701 F29
VL
DX485
TE485
H/F
RX232
RX485
CH2
DX232
GND
LTC2871
LB
DIN1
ROUT1
DOUT1
RIN1
28701 F30
DX232
DX485
VL
RX485
RX232
TE485
H/F
CH2
GND
LTC2871
LB
DIN1
ROUT2
DIN2
ROUT1
DOUT1
RIN2
DOUT2
RIN1
RO
B
A
DI
Z
Y
28701 F31
DX485
DX232
LB
TE485
VL
RX485
RX232
H/F
CH2
GND
LTC2871
DIN1
ROUT2
DIN2
ROUT1
DOUT1
RIN2
DOUT2
RIN1
RO
B
A
DI
Z
Y
120Ω
120Ω
VCC = 3V to 5.5V, VL = 1.7V to VCC. Logic input pins not shown are tied to a valid logic state. External Components not shown.
LTC2870/LTC2871
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Figure33. LTC2871 in RS485 and
RS232 Mode, Both Half-Duplex
Figure34. LTC2871 in RS485 and RS232
Mode, RS485 Half-Duplex, Loopback
Figure35. LTC2871 in RS485
and RS232 Mode, RS485
Half-Duplex, Terminated
Figure36. RS485 Duplex Switching
Figure37. Microprocessor Interface Figure38. Driving Larger
RS232 Loads
Typical applicaTions
28701 F32
DX485
H/F
V
L
CH2
TE485
RX485
RX232
LB
GND
LTC2871
DX232
D R 485
DIN1
ROUT2
DIN2
ROUT1
DOUT1
RIN2
DOUT2
RIN1
RO
DI
Z
Y
D R 232
28701 F33
DX232
DX485
LB
H/F
V
L
RX485
RX232
TE485
CH2
GND
LTC2871
DIN1
ROUT2
DIN2
ROUT1
DOUT1
RIN2
DOUT2
RIN1
RO
DI
Z
Y
28701 F34
DX232
H/F
DX485
RX485
VL
RX232
LB
CH2
GND
LTC2871
DIN1
ROUT2
DIN2
ROUT1
DOUT1
RIN2
DOUT2
RIN1
RO
DI 120Ω Z
Y
B
A
120Ω
D R 485
28701 F35
485/232
TE485
VL
H/F
LB
RB
GND
RO
B
120Ω
A
LTC2870/
LTC2871
H/F
DI
DY Z
120Ω
Y
HALF
DUPLEX
RS485
FULL
28701 F36
3V TO 5.5V
1.7V TO VCC
VCC
VL
GND
LTC2870/
LTC2871
DY, DIN1
RB, ROUT2
DZ, DIN2
RA, ROUT1
Y
B
Z
A
CONTROL
SIGNALS
µP
28701 F37
LTC2870/
LTC2871
RS232
CL3k
DATA RATE
100kbps
500kbps
CL
5nF
1nF
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
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Logic input pins not shown are tied to a valid logic state. External Components not shown.
Typical applicaTions
Figure39. LTC2870: Making Use of Shared I/O for Various Communication Configurations
Figure40. LTC2870: Using External Connections for Half-Duplex RS232 or RS485 Operation
28701 F38
LTC2870
1.7V TO VCC 3V TO 5.5V
485/232
RXEN
DXEN
TE485
H/F
DY
Y
Z
A
B
DZ
RA
RB
VL
VL
CONTROLLER
CONNECTOR
VCC
GND
RS485
RS485
FULL-DUPLEX
485/232 = 1
H/F = 0
RS485
HALF-DUPLEX
485/232 = 1
H/F = 1
RS232
FULL-DUPLEX
485/232 = 0
H/F = X
RS485
RS485
RS485
RS485
RS485
RS232
RS232
RS232
RS232
28701 F39
LTC2870
1.7V TO V
CC 3V TO 5.5V
485/232
RXEN
DXEN
H/F
DY
Y
Z
A
B
DZ
RA
RB
VL
VL
CONTROLLER CONNECTOR
VCC
TE485
GND
RS485
RS485
HALF-DUPLEX
485/232 = 1
H/F = 0
RS232
HALF-DUPLEX
485/232 = 0
H/F = X
RS485
RS232
RS232
NOTE: THIS CONFIGURATION IS NOT COMPATIBLE
WITH INTERNAL RS485 TERMINATION.
SETTING TE485 HIGH WILL RESULT IN DOUBLE
TERMINATION IN RS485 MODE.
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Figure41. LTC2871: Various Communication Configurations
Figure42. LTC2871: More Communication Configurations Using External Connections
Typical applicaTions
28701 F40
LTC2871
1.7V TO VCC 3V TO 5.5V
DX485
RX232
RX485
DX232
TE485
H/F
DIN1
DIN2
Y
DOUT1
DOUT2
Z
A
B
RIN2
RIN1
DI
ROUT1
RO
ROUT2
VL
VLVCC
GND
RS485
RS232
FULL-DUPLEX
RS485
FULL-DUPLEX
H/F = 0
RS232
FULL-DUPLEX
RS485
HALF-DUPLEX
H/F = 1
RS485
RS485
RS232 RS232
RS232 RS232
CONTROLLER
RS232 RS232
RS485
RS485
CONNECTOR
RS232 RS232
RS485
28701 F41
LTC2871
1.7V TO VCC 3V TO 5.5V
DIN1
DIN2
Y
DOUT1
DOUT2
Z
A
B
RIN2
RIN1
DI
ROUT1
RO
ROUT2
VL
VLVCC
GND
RS485
RS232
HALF-DUPLEX
RS485
FULL-DUPLEX
H/F = 0
RS232
HALF-DUPLEX
RS485
HALF-DUPLEX
H/F = 1
RS485
RS485
RS232 RS232
RS232 RS232
CONTROLLER
RS485
RS485
CONNECTOR
RS485
DX485
RX232
RX485
DX232
TE485
H/F
Logic input pins not shown are tied to a valid logic state. External Components not shown.
LTC2870/LTC2871
32
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For more information www.linear.com/LTC2870
Figure43. LTC2871: Using External Connections for RS485 (Half or Full Duplex) and RS232 (Full Duplex)
Figure44. Schematic Implementation of Figure43, Above, Using a Single Pair of Wires for the Bus I/O
Typical applicaTions
28701 F50
LTC2871
1.7V TO VCC 3V TO 5.5V
DX485
RX232
RX485
DX232
TE485
H/F
DIN1
DIN2
Y
DOUT1
DOUT2
Z
A
B
RIN2
RIN1
DI
ROUT1
RO
ROUT2
VL
VLVCC
GND
RS485
RS485
FULL-DUPLEX
DX232 = 0
RX232 = 1
TE485 = X
H/F = 0
RS485
HALF-DUPLEX
DX232 = 0
RX232 = 1
TE485 = X
H/F = 1
RS485
RS485
CONTROLLER
RS485
RS485
RS232
FULL-DUPLEX
DX232 = 0
RX485 = 1
DX232 = 1
RX232 = 0
TE485 = 0
H/F = X
RS232
RS232
RS232
RS232
RS485
DI
RO
TxAY
RxBZ
VCC
VL
H/
F
CH
DX485
TE485
RX232
DIN1
DI
RX485
DIN2
DX232
ROUT1
RO
FEN
ROUT2
SW
CAP
VDD
VEE
GND
LB
DOUT1
Y
Z
RIN1
A
B
DOUT2
RIN2
L1
10µH
C1
220nF
C4
2.2µF
C2
F
C3
F
LTC2871
RS485 DRIVER
ENABLE
RS485
RS232
28701 F51
Logic input pins not shown are tied to a valid logic state. External Components not shown.
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.
Two logic gates were added (optional) to simplify
the control signals needed from the controller.
LTC2870/LTC2871
33
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For more information www.linear.com/LTC2870
Figure45. RS232 Extension Cord Using RS232 to RS485 Conversion
Figure46. RS485 Full-Duplex Network
Typical applicaTions
28701 F42
DX232
DX485
CH2
TE485
V
L
RX485
RX232
H/F
LB
RX485
RX232
H/F
LB
GND GND
LTC2871 LTC2871
V
L
DX232
DX485
CH2
TE485
RIN1
RO
DIN1
DOUT1
RXIN RS485
UP TO 4000 FT
CAT5e CABLE
RS232
DRIVER
OUT
RXOUT
RS232
DRIVER
IN
B
A
ROUT1
DI
DOUT1
DI
ROUT1
RIN1
DIN1
RO
120Ω
120Ω
Z
Y
120Ω
120Ω
B
A
Z
Y
28701 F43
LTC2870/LTC2871
120Ω
LTC2852
SLAVE
SLAVE MASTER
LTC2855
120Ω
TE485 TE
VL3.3V
120Ω
LTC2852
SLAVE
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
34
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For more information www.linear.com/LTC2870
Typical applicaTions
Figure47. RS232 Triple Transceiver
with Selectable Line Interface
Figure48. RS232 Triple Transceiver
with Selectable Logic Interface
28701 F44
DIN1
ROUT2
DIN2
ROUT1
DOUT1
PORT 1
LOGIC
INTERFACE
PORT 2A/2B
LOGIC
INTERFACE
SELECT LINE 2A
SELECT LINE 2B
PORT 1
LINE
INTERFACE
PORT 2A
LINE
INTERFACE
RIN2
DOUT2
RIN1
LTC2871
DIN2
CH2
CH2
ROUT1
DIN1
ROUT2
DOUT2
PORT 3
LOGIC
INTERFACE
PORT 2B
LINE
INTERFACE
PORT 3
LINE
INTERFACE
RIN1
DOUT1
RIN2
LTC2871
28701 F45
DIN1
ROUT2
DIN2
ROUT1
DOUT1
PORT 1
LOGIC
INTERFACE
PORT 2A
LOGIC
INTERFACE
SELECT LINE 2A
SELECT LINE 2B
PORT 1
LINE
INTERFACE
PORT 2A/2B
LINE
INTERFACE
RIN2
DOUT2
RIN1
LTC2871
PORT 2B
LOGIC
INTERFACE
DIN2
CH2
CH2
ROUT1
DIN1
ROUT2
DOUT2
PORT 3
LOGIC
INTERFACE
PORT 3
LINE
INTERFACE
RIN1
DOUT1
RIN2
LTC2871
28701 F46
H/F
RA,
RO
RS485
INTERFACE
INPUT1
INPUT2
Y
Z
A
B
LTC2870/
LTC2871
SELECT
INPUT2 INPUT1
Logic input pins not shown are tied to a valid logic state. External Components not shown.
Figure49. RS485 Receiver with Multiplexed Inputs
LTC2870/LTC2871
35
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For more information www.linear.com/LTC2870
Typical applicaTions
Figure50. Typical Supply Connections with External Components Shown
Figure51. Running Two LTC2870 or LTC2871 Devices from One Shared Power Source
28701 F48
3V TO 5.5V
VL
VCC SW
22µH 470nF
CAP
GND VDD VEE
LTC2870/
LTC2871 SW
CAP VCC
VEE
VL
VDD GND
LTC2870/
LTC2871
2.2µF
2.2µF 2.2µF
INDUCTOR: TAIYO YUDEN CBC2518T220M,
MURATA LQH32CN220K53
28701 F47
3V TO 5.5V 3V TO 5.5V
1.7V TO VCC
TE485
VL
VDD
VEE
VCC SW
RS485
INTERFACE
10µH 10µH
220nF 220nF
CAP
H/F
GND
LTC2871 LTC2870
GND
VCC
485/232
H/F
VL
VDD
VEE
TE485
DY
RA
SW
RO
DIN1
ROUT1
CAP
B
A
DIN2
ROUT2
F
DOUT1
RIN1
DOUT2
RIN2
RS232
INTERFACE
DI 120Ω Z
Y
B
A
Z
Y
120Ω 120Ω
120Ω
2.2µF
F
2.2µF
0.1µF
1.7V TO VCC
0.1µF
F
F
Logic input pins not shown are tied to a valid logic state.
LTC2870/LTC2871
36
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For more information www.linear.com/LTC2870
Typical applicaTions
Figure52. Quad Transceiver
DI-A
RO-A
TxAY-A
RxBZ-A
DI-B
RO-B
TxAY-B
RxBZ-B
DX485-A
TE485-A
RX232-A
RX485-A
DX232-A
DX485-B
TE485-B
RX232-B
RX485-B
DX232-B
DI-C
RO-C
TxAY-C
RxBZ-C
DX485-C
TE485-C
RX232-C
RX485-C
DX232-C
DI-D
RO-D
TxAY-D
RxBZ-D
DX485-D
TE485-D
RX232-D
RX485-D
DX232-D
VCC
VL
H/
F
CH
DX485
TE485
RX232
DIN1
DI
RX485
DIN2
DX232
ROUT1
RO
FEN
ROUT2
SW
CAP
VDD
VEE
GND
LB
DOUT1
Y
Z
RIN1
A
B
DOUT2
RIN2
L1
22µH
C1
470nF
C4
2.2µF
C2
2.2µF
C3
2.2µF
C5
2.2µF
VCC
VL
H/
F
CH
DX485
TE485
RX232
DIN1
DI
RX485
DIN2
DX232
ROUT1
RO
FEN
ROUT2
SW
CAP
VDD
VEE
GND
LB
DOUT1
Y
Z
RIN1
A
B
DOUT2
RIN2
VCC
VL
H/
F
CH
DX485
TE485
RX232
DIN1
DI
RX485
DIN2
DX232
ROUT1
RO
FEN
ROUT2
SW
CAP
VDD
VEE
GND
LB
DOUT1
Y
Z
RIN1
A
B
DOUT2
RIN2
VCC
VL
H/
F
CH
DX485
TE485
RX232
DIN1
DI
RX485
DIN2
DX232
ROUT1
RO
FEN
ROUT2
SW
CAP
VDD
VEE
GND
LB
DOUT1
Y
Z
RIN1
A
B
DOUT2
RIN2
C2
2.2µF
C3
2.2µF
LTC2871
LTC2871
LTC2871
LTC2871
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
LTC2870/LTC2871
37
28701fb
For more information www.linear.com/LTC2870
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)
(Reference LTC DWG # 05-08-1663 Rev K)
Exposed Pad Variation EB
FE28 (EB) TSSOP REV K 0913
0.09 – 0.20
(.0035 – .0079)
0° 8°
0.25
REF
0.50 – 0.75
(.020 – .030)
4.30 – 4.50*
(.169 – .177)
1 3 4 5678 9 10 11 12 13 14
192022 21 151618 17
9.60 – 9.80*
(.378 – .386)
4.75
(.187)
2.74
(.108)
28 27 26 2524 23
1.20
(.047)
MAX
0.05 – 0.15
(.002 – .006)
0.65
(.0256)
BSC 0.195 – 0.30
(.0077 – .0118)
TYP
2
RECOMMENDED SOLDER PAD LAYOUT
EXPOSED
PAD HEAT SINK
ON BOTTOM OF
PACKAGE
0.45 ±0.05
0.65 BSC
4.50 ±0.10
6.60 ±0.10
1.05 ±0.10
4.75
(.187)
2.74
(.108)
MILLIMETERS
(INCHES) *DIMENSIONS DO NOT INCLUDE MOLD FLASH. MOLD FLASH
SHALL NOT EXCEED 0.150mm (.006") PER SIDE
NOTE:
1. CONTROLLING DIMENSION: MILLIMETERS
2. DIMENSIONS ARE IN
3. DRAWING NOT TO SCALE
SEE NOTE 4
4. RECOMMENDED MINIMUM PCB METAL SIZE
FOR EXPOSED PAD ATTACHMENT
6.40
(.252)
BSC
FE Package
28-Lead Plastic TSSOP (4.4mm)
(Reference LTC DWG # 05-08-1663 Rev K)
Exposed Pad Variation EB
LTC2870/LTC2871
38
28701fb
For more information www.linear.com/LTC2870
package DescripTion
Please refer to http://www.linear.com/product/LTC2870#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
(.187) REF
FE38 (AA) TSSOP REV C 0910
0.09 – 0.20
(.0035 – .0079)
0° 8°
0.25
REF
0.50 – 0.75
(.020 – .030)
4.30 – 4.50*
(.169 – .177)
119
20
REF
9.60 – 9.80*
(.378 – .386)
38
1.20
(.047)
MAX
0.05 – 0.15
(.002 – .006)
0.50
(.0196)
BSC 0.17 – 0.27
(.0067 – .0106)
TYP
RECOMMENDED SOLDER PAD LAYOUT
0.315 ±0.05
0.50 BSC
4.50 REF
6.60 ±0.10
1.05 ±0.10
4.75 REF
2.74 REF
2.74
(.108)
MILLIMETERS
(INCHES) *DIMENSIONS DO NOT INCLUDE MOLD FLASH. MOLD FLASH
SHALL NOT EXCEED 0.150mm (.006") PER SIDE
NOTE:
1. CONTROLLING DIMENSION: MILLIMETERS
2. DIMENSIONS ARE IN
3. DRAWING NOT TO SCALE
SEE NOTE 4
4. RECOMMENDED MINIMUM PCB METAL SIZE
FOR EXPOSED PAD ATTACHMENT
6.40
(.252)
BSC
FE Package
38-Lead Plastic TSSOP (4.4mm)
(Reference LTC DWG # 05-08-1772 Rev C)
Exposed Pad Variation AA
LTC2870/LTC2871
39
28701fb
For more information www.linear.com/LTC2870
package DescripTion
UFD Package
28-Lead Plastic QFN (4mm × 5mm)
(Reference LTC DWG # 05-08-1712 Rev C)
Please refer to http://www.linear.com/product/LTC2870#packaging for the most recent package drawings.
4.00 ±0.10
(2 SIDES)
2.50 REF
5.00 ±0.10
(2 SIDES)
NOTE:
1. DRAWING PROPOSED TO BE MADE A JEDEC PACKAGE OUTLINE MO-220 VARIATION (WGHD-3).
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
PIN 1
TOP MARK
(NOTE 6)
0.40 ±0.10
27 28
1
2
BOTTOM VIEW—EXPOSED PAD
3.50 REF
0.75 ±0.05 R = 0.115
TYP
R = 0.05
TYP
PIN 1 NOTCH
R = 0.20 OR 0.35
× 45° CHAMFER
0.25 ±0.05
0.50 BSC
0.200 REF
0.00 – 0.05
(UFD28) QFN 0816 REV C
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS
APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED
0.70 ±0.05
0.25 ±0.05
0.50 BSC
2.50 REF
3.50 REF
4.10 ±0.05
5.50 ±0.05
2.65 ±0.05
3.10 ±0.05
4.50 ±0.05
PACKAGE OUTLINE
2.65 ±0.10
3.65 ±0.10
3.65 ±0.05
UFD Package
28-Lead Plastic QFN (4mm × 5mm)
(Reference LTC DWG # 05-08-1712 Rev C)
LTC2870/LTC2871
40
28701fb
For more information www.linear.com/LTC2870
package DescripTion
UHF Package
38-Lead Plastic QFN (5mm × 7mm)
(Reference LTC DWG # 05-08-1701 Rev C)
Please refer to http://www.linear.com/product/LTC2870#packaging for the most recent package drawings.
5.00 ±0.10
NOTE:
1. DRAWING CONFORMS TO JEDEC PACKAGE
OUTLINE M0-220 VARIATION WHKD
2. DRAWING NOT TO SCALE
3. ALL DIMENSIONS ARE IN MILLIMETERS
PIN 1
TOP MARK
(SEE NOTE 6)
37
1
2
38
BOTTOM VIEW—EXPOSED PAD
5.50 REF
5.15 ±0.10
7.00 ±0.10
0.75 ±0.05
R = 0.125
TYP
R = 0.10
TYP
0.25 ±0.05
(UH) QFN REF C 1107
0.50 BSC
0.200 REF
0.00 – 0.05
RECOMMENDED SOLDER PAD LAYOUT
APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED
3.00 REF
3.15 ±0.10
0.40 ±0.10
0.70 ±0.05
0.50 BSC
5.5 REF
3.00 REF 3.15 ±0.05
4.10 ±0.05
5.50 ±0.05 5.15 ±0.05
6.10 ±0.05
7.50 ±0.05
0.25 ±0.05
PACKAGE
OUTLINE
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
PIN 1 NOTCH
R = 0.30 TYP OR
0.35 × 45° CHAMFER
UHF Package
38-Lead Plastic QFN (5mm × 7mm)
(Reference LTC DWG # 05-08-1701 Rev C)
LTC2870/LTC2871
41
28701fb
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no representa-
tion that the interconnection of its circuits as described herein will not infringe on existing patent rights.
REV DATE DESCRIPTION PAGE NUMBER
A 11/14 Added H-Grade (–40°C to 125°C)
Removed Absolute Maximum Ratings for VDD – VEE
Increased TJMAX to 150°C
Noted which pins should not float
Expanded DC/DC Converter Applications section
Added Running with External VDD and VEE Supplies Applications section
Expanded RS485 Biasing Resistors Not Required Applications section
Added RS485 Polarity and the LTC1334, LTC1387 Applications section
Added Standards Compatibility Applications section
Revised Figures 25, 26, and 35
Added Figures 43, 44, and 52
1-40
2
3
10-11
20-21
21
23
24-25
26
27, 29
32, 36
B 08/17 Increased Input Hysteresis (TYP), decreased Differential Input Failsafe Threshold Voltage (TYP),
and increased Input DC Failsafe Hysteresis.
Increased maximum VCC back-current in fault condition.
4
22
revision hisTory
LTC2870/LTC2871
42
28701fb
For more information www.linear.com/LTC2870
LINEAR TECHNOLOGY CORPORATION 2010
LT 0817 REV B • PRINTED IN USA
www.linear.com/LTC2870
relaTeD parTs
Typical applicaTion
PART NUMBER DESCRIPTION COMMENTS
LTC2873 RS232/RS485 Single-Bus Multiprotocol Transceiver
with Integrated Termination
One RS232 Transceiver and One RS485 Transceiver on the Same Bus Pins. 3V
to 5.5V Supply with 1.7V to 5.5V Logic Interface
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/
LTC2803/LTC2804
1.8V to 5.5V RS232 Single and Dual Transceivers Up to 1Mbps, 10kV ESD, Logic Supply Pin, Tiny DFN Packages
LTC2854/LTC2855 3.3V 20Mbps RS485 Transceiver with Integrated
Switchable Termination
3.3V Supply, Integrated, Switchable, 120Ω Termination Resistor, 25kV ESD
LTC2859/LTC2861 20Mbps RS485 Transceiver with Integrated
Switchable Termination
5V Supply, Integrated, Switchable, 120Ω Termination Resistor, 15kV ESD
LTM2881 Complete Isolated RS485/RS422 µModule
Transceiver + Power
20Mbps, 2500VRMS Isolation with Integrated DC/DC Converter, Integrated,
Switchable, 120Ω Termination Resistor, 15kV ESD
LTM2885 6500VRMS Isolated RS485/RS232 µModule
Transceiver + Power
20Mbps, 6500VRMS Isolation with Integrated DC/DC Converter, Integrated
Switchable 120Ω Termination Resistor, ±15kV ESD, 5V Supply
LTM2882 Dual Isolated RS232 µModule Transceiver + Power 1Mbps, 2500VRMS Isolation with Integrated DC/DC Converter, ±10kV ESD
Quad RS232 Transceiver with RS485 Communication Over Half-Duplex, Terminated Bus
28701 TA02
3V TO 5.5V
3.3V
VL
VCC SW
22µH 470nF
CAP
TE485
LTC2871
LTC2854
LTC2804
GND
VCC
TE
DI
RO
VCC VL
CAP
RO
DIN1
ROUT1
B
A
DIN2
ROUT2
DOUT1
RIN1
DOUT2
RIN2
DI 120Ω
Z
Y
B
A
120Ω
120Ω
2.2µF
2.2µF 2.2µF
0.1µF
GND VDD VEE
T1IN
ROUT1
T2IN
ROUT2
T1OUT1
SW
RIN1
T2OUT
RIN2
GNDVDD
VEE
LOGIC INPUT PINS NOT SHOWN ARE
TIED TO A VALID LOGIC STATE.

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