LTC2873
Single-Bus RS485/RS232
Multiprotocol Transceiver with
Switchable Termination
DESCRIPTION
FEATURES
One RS485 or One RS232 Transceiver
nn 3V to 5.5V Supply Voltage
nn Up to 20Mbps RS485
nn Slew-Controlled RS232 Operation:
nn Selectable 1Mbps or 250kbps
nn Automatic Selection of Integrated RS485 (120Ω)
and RS232 (5kΩ) Termination Resistors
nn High ESD: ±26kV HBM
nn Logic Loopback Mode
nn 1.7V to 5.5V Logic Interface
nn Supports Up to 256 RS485 Nodes
nn RS485 Receiver Failsafe Eliminates UART Lockup
nn H-Grade Available (–40°C to 125°C)
nn Available in 24-Pin 4mm × 5mm QFN Package
The LTC®2873 is a robust pin-configurable multiprotocol
transceiver that supports RS232, RS485, and RS422
protocols while operating on a single 3V to 5.5V supply.
The LTC2873 can be configured as a half-duplex RS485
transceiver or as an RS232 transceiver using the same
two bus pins.
nn
A pin-controlled integrated termination resistor allows for
easy interface reconfiguration, eliminating external resistors and control relays. Loopback mode steers the driver
inputs to the receiver outputs for diagnostic self-test.
The RS485 receiver supports up to 256 nodes per bus,
and features 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 when driving RS232 levels.
APPLICATIONS
Software Selectable RS232/RS485/RS422 Interface
Industrial Sensors and Actuators
nn Alarm Systems
nn Traffic Control and Monitoring
nn Highway Signs and Jumbo Displays
All registered trademarks and trademarks are the property of their respective owners.
nn
nn
TYPICAL APPLICATION
220nF
1.7V TO VCC
10μH
3V TO 5.5V
VL
SHDN
DATA IN
DATA OUT
485/232
D/R
(485)
RS485/RS232 Mode Switching
2.2μF
1μF
LTC2873
CAP
VCC
SW
RS485
DI
A/DO
120Ω
RO
485/232
RE485
TE485
DE485/
F232
LB
GND
RS232
(485/232
LOW)
DI
485/232
5V/DIV
RS232
RS485
B/RI
VDD
RS232
5k
RS485
(485/232
HIGH)
VEE
B/RI
1μF
A/DO
3V/DIV
1μF
1µs/DIV
2873 TA01b
2873 TA01
REV B
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1
LTC2873
ABSOLUTE MAXIMUM RATINGS
PIN CONFIGURATION
(Notes 1 and 2)
ORDER INFORMATION
GND
VCC
VL
LB
TE485
TOP VIEW
24 23 22 21 20
VEE 1
19 NC
RO 2
18 VCC
485/232 3
17 A/DO
25
VEE
RE485 4
DE485/F232 5
16 GND
15 B/RI
DI 6
14 VCC
13 VDD
SW
GND
9 10 11 12
VEE
8
CAP
SHDN 7
GND
Input Supplies
VCC, VL ..................................................... –0.3V to 7V
Generated Supplies
VDD................................................. VCC – 0.3V to 7.5V
VEE.......................................................... –7.5V to 0.3V
SW................................................ –0.3V to (VDD + 0.3V)
CAP.................................................. (VEE – 0.3V) to 0.3V
A/DO, B/RI....................................................–15V to 15V
DI, 485/232, DE485/F232, RE485,
TE485, LB................................................. –0.3V to 7V
SHDN, RO........................................–0.3V to (VL + 0.3V)
Differential Terminator Voltage (Enabled)
(A/DO to B/RI)...................................................... ±6V
Differential Terminator Voltage (Disabled)
(A/DO to B/RI).................................................... ±30V
Operating Temperature
LTC2873C................................................ 0°C to 70°C
LTC2873I..............................................–40°C to 85°C
LTC2873H........................................... –40°C to 125°C
Storage Temperature Range................... –65°C to 150°C
UFD PACKAGE
24-LEAD (4mm × 5mm) PLASTIC QFN
TJMAX = 150°C, θJA = 43°C/W
EXPOSED PAD (PIN 25) IS VEE, MUST BE SOLDERED TO PCB
http://www.linear.com/product/LTC2873#orderinfo
LEAD FREE FINISH
TAPE AND REEL
PART MARKING
PACKAGE DESCRIPTION
TEMPERATURE RANGE
LTC2873CUFD#PBF
LTC2873CUFD#TRPBF
2873
24-Lead (4mm × 5mm) Plastic QFN
0°C to 70°C
LTC2873IUFD#PBF
LTC2873IUFD#TRPBF
2873
24-Lead (4mm × 5mm) Plastic QFN
–40°C to 85°C
LTC2873HUFD#PBF
LTC2873HUFD#TRPBF
2873
24-Lead (4mm × 5mm) Plastic QFN
–40°C to 125°C
Consult LTC Marketing for parts specified with wider operating temperature ranges.
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/. Some packages are available in 500 unit reels through
designated sales channels with #TRMPBF suffix.
2
REV B
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LTC2873
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 = VL, LB = 0V unless otherwise noted. (Notes 2, 6)
SYMBOL PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
Supplies
VCC
Supply Voltage Operating Range
l
VL
Logic Supply Voltage Operating Range
3
5.5
V
VL ≤ VCC
l
1.7
VCC
V
VCC Supply Current in Shutdown Mode
SHDN = 0V
l
8
30
µA
VCC Supply Current in RS232 Mode or
RS485 Mode, Driver and Receiver Enabled,
Termination Disabled
No Load, SHDN = TE485 = DE485/F232 = VL
RE485 = 0
l
4
9
mA
VCC Supply Current in RS485 Mode with
Receiver and Termination Enabled, Driver
Disabled
No Load, SHDN = 485/232 = VL
DE485/F232 = RE485 = TE485 = 0
l
4
9
mA
VL Supply Current in Any Mode
No Load
l
0
5
µA
Power Supply Generator
VDD
Regulated VDD Output Voltage
SHDN = VL, No Load
7.0
V
VEE
Regulated VEE Output Voltage
SHDN = VL, No Load
–6.3
V
RS485 Driver
|VOD|
Differential Output Voltage
RL = Open, VCC = 3V (Figure 1)
RL = 27Ω, VCC = 4.5V (Figure 1)
RL = 27Ω, VCC = 3V (Figure 1)
RL = 50Ω, VCC = 3.13V (Figure 1)
l
l
l
l
VCC
VCC
VCC
V
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
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
IOSD485
Maximum Short-Circuit Current
–7V ≤ VOUT ≤ 12V (Figure 2)
l
±250
mA
l
125
µA
2.1
1.5
2
RS485 Receiver
IIN485
Input Current (A/DO, B/RI)
(A/DO or B/RI) = 12V or –7V,
VCC = 0V or 3.3V (Figure 3)
RIN485
Input Resistance (A/DO, B/RI)
(A/DO or B/RI) = 12V or –7V,
VCC = 0V or 3.3V (Figure 3)
Differential Input Signal Threshold
Voltage (A/DO to B/RI)
–7V ≤ (A/DO or B/RI) ≤ 12V
Input Hysteresis
B = 0V
Differential Input Failsafe Rising Threshold Voltage –7V ≤ (A/DO or B/RI) ≤ 12V, (A/D0 – B/RI) Rising
125
Receiver Output Low Voltage
kΩ
±200
l
220
l
Input DC Failsafe Hysteresis
VOL
–100
–200
–70
mV
mV
–20
40
mV
mV
Output Low, I(RO) = 3mA (Sinking),
3V ≤ VL ≤ 5.5V
l
0.4
V
Output Low, I(RO) = 1mA (Sinking),
1.7V ≤ VL < 3V
l
0.4
V
REV B
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3
LTC2873
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 = VL, LB = 0V unless otherwise noted. (Notes 2, 6)
SYMBOL PARAMETER
VOH
Receiver Output High Voltage
MIN
TYP
MAX
UNITS
Output High, I(RO) = –3mA (Sourcing),
3V ≤ VL ≤ 5.5V
l
VL – 0.4
V
Output High, I(RO) = –1mA (Sourcing),
1.7V ≤ VL < 3V
l
VL – 0.4
V
0V ≤ RO ≤ VL, VL = 5.5V, RE485 = VL
l
Short-Circuit Current (RO)
0V ≤ RO ≤ VL, VL = 5.5V
l
Terminating Resistor
TE485 = 0V, VAB = 2V,
VB = –7V, 0V, 10V (Figure 8)
l
108
120
Three-State (High Impedance) Output Current (RO)
RTERM
CONDITIONS
0
±5
µA
±135
mA
156
Ω
–5.5
VEE
V
RS232 Driver
VOLD
Output Low Voltage
RL = 3kΩ, VEE ≤ –6V
l
–5
VOHD
Output High Voltage
RL = 3kΩ, VDD ≥ 6.5V
l
5
Output Short-Circuit Current
Driver Output = 0V
l
5.9
VDD
V
±25
±90
mA
RS232 Receiver
Input Threshold Voltage
l
0.6
1.5
2.5
V
Input Hysteresis
l
0.1
0.4
1.0
V
Output Low Voltage
I(RO) = 1mA (Sinking), 1.7V ≤ VL < 5.5V
l
Output High Voltage
I(RO) = –1mA (Sourcing), 1.7V ≤ VL < 5.5V
l
VL – 0.4
0.4
Input Resistance
–15V ≤ B/RI ≤ 15V, 485/232 = 0V
l
3
Output Short-Circuit Current
VL = 5.5V, 0V ≤ RO ≤ VL
l
V
V
5
7
kΩ
±25
±50
mA
Logic Inputs
Threshold Voltage
l
Input Current
l
0.4
0
0.75 • VL
V
±5
µA
ESD
4
Interface Pins (A/DO, B/RI)
Human Body Model to GND or VCC, Powered
or Unpowered (Note 5)
±26
kV
All Other Pins
Human Body Model (Note 5)
±4
kV
REV B
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LTC2873
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 = VL, LB = 0V unless otherwise noted. (Notes 2, 6)
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
RS485 Switching Characteristics
tPLHD485, tPHLD485
20
Mbps
Maximum Data Rate
(Note 3) (Figure 15)
l
Driver Propagation Delay
RDIFF = 54Ω, CL = 100pF (Figure 4)
l
20
70
ns
l
0
±6
ns
Driver Propagation Delay Difference
RDIFF = 54Ω, CL = 100pF (Figure 4)
tPLHD485 – tPHLD485
tSKEWD485
Driver Skew (A/DO to B/RI)
RDIFF = 54Ω, CL = 100pF (Figure 4)
0
±8
ns
tRD485, tFD485
Driver Rise or Fall Time
RDIFF = 54Ω, CL = 100pF (Figure 4)
7.5
12.5
ns
tZLD485, tZHD485,
tLZD485, tHZD485
Driver Output Enable or Disable Time
SHDN = VL, RL = 500Ω, CL = 50pF,
DE485 ↑ and ↓ (Figure 5)
l
120
ns
tZHSD485, tZLSD485
Driver Enable from Shutdown Time
(Note 7)
DE485/F232 = VL, RL = 500Ω,
CL = 50pF, SHDN ↑ (Figure 5)
l
4
12
µs
tHZSD485, tLZSD485
Driver Output Disable Into
Shutdown Time
DE485/F232 = VL, RL = 500Ω,
CL = 50pF, SHDN ↓ (Figure 5)
l
0.5
1
μs
tPLHR485, tPHLR485
Receiver Input to Output Time
CL = 15pF, VCM = 1.5V,
|A/DO to B/RI| = 1.5V, (Figure 6)
l
45
85
ns
tSKEWR485
Differential Receiver Skew
tPLHR485 – tPHLR485
CL = 15pF (Figure 6)
l
0
±9
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
485/232 = SHDN = VL, RL = 1kΩ,
CL = 15pF, RE485 ↓ and ↑ (Figure 7)
l
12
85
ns
tZHSR485, tZLSR485
Receiver Enable from Shutdown Time
(Note 7)
485/232 = VL , RE485 = 0V, RL = 1kΩ,
CL = 15pF , SHDN ↑ (Figure 7)
l
4
12
µs
tHZSR485, tLZSR485
Receiver Output Disable Into
Shutdown Time
485/232 = VL, RE485 = 0V, RL = 1kΩ,
CL = 15pF ,SHDN ↓ (Figure 7)
l
0.5
1
µs
tRTEN485, tRTZ485
Termination Enable or Disable
Time
485/232 = VL, SHDN = VL, B = 0,
(A/DO to B/RI) = 2V (Figure 8)
l
100
µs
Maximum Data Rate (Figure 15)
RL = 3kΩ, CL = 2.5nF, (Fast, Slow Modes)
RL = 3kΩ, CL = 1nF, (Fast, Slow Modes)
RL = 3kΩ, CL = 0.25nF, (Fast Mode)
l
l
l
100
250
1000
Driver Slew Rate (Figure 9)
RL = 3kΩ, CL = 2.5nF, (Fast, Slow Modes)
RL = 3kΩ, CL = 50pF, (Slow Mode)
RL = 3kΩ, CL = 50pF, (Fast Mode)
l
l
l
2
tPHLD232, tPLHD232
Driver Propagation Delay (Figure 9)
RL = 3kΩ, CL = 50pF, (Slow Mode)
RL = 3kΩ, CL = 50pF, (Fast Mode)
l
l
tSKEWD232
Driver Skew (Figure 9)
RL = 3kΩ, CL = 50pF, (Slow Mode)
RL = 3kΩ, CL = 50pF, (Fast Mode)
tZLSD232, tZHSD232
Driver Enable from Shutdown
Time (Figure 7)
VDD = 7.0V, VEE = –6.3V, 485/232 = 0V,
RL = 3kΩ, CL = 50pF, SHDN ↑ (Figure 10)
tLZSD232, tHZSD232
Driver Output Disable into
Shutdown Time
tPHLR232, tPLHR232
tSKEWR232
tRR232, tFR232
RS232 Switching Characteristics
kbps
kbps
kbps
30
150
V/µs
V/µs
V/µs
1.5
0.4
3
1
µs
µs
0
0
±400
±100
ns
ns
l
5
12
µs
485/232 = 0V, RL = 3kΩ, CL = 50pF,
SHDN ↓ (Figure 10)
l
0.6
2
µs
Receiver Propagation Delay
CL = 150pF (Figure 11)
l
60
200
Receiver Skew
CL = 150pF (Figure 11)
Receiver Output Rise or Fall Time
CL = 150pF (Figure 11)
25
l
70
ns
ns
200
ns
REV B
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5
LTC2873
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 = VL, LB = 0V unless otherwise noted. (Notes 2, 6)
SYMBOL
PARAMETER
CONDITIONS
TYP
MAX
tZLSR232, tZHSR232
Receiver Enable from Shutdown
Time (Note 7)
VDD = 7.0V, VEE = –6.3V, 485/232 = 0V,
RL = 1kΩ, CL = 150pF, SHDN ↑ (Figure 12)
l
MIN
5
12
UNITS
µs
tLZSR232, tHZSR232
Receiver Disable Into Shutdown Time
485/232 = 0V, RL = 1kΩ, CL = 150pF
SHDN ↓ (Figure 12)
l
0.4
2
μs
Mode Change Characteristics
tRDY
VDD and VEE Supply Rise Time
(Time from Shutdown to RS485 Ready)
(Figure 13)
l
0.2
1
ms
tDR232
Time from RS485 Mode to RS232 Mode
RS232 Driver Ready
(Figure 14)
l
0.2
1
µs
tR232
Time from RS485 Mode to RS232 Mode
RS232 Receiver Ready
(Figure 14)
l
0.8
3
µs
tDR485
Time from RS232 Mode to RS485
(Figure 14)
Mode RS485 Driver and Receiver Ready
l
70
250
ns
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 SHDN ↑ until VDD ≥ 5V and VEE ≤ –5V. External
components as shown in the Typical Application section.
6
Note 5: Guaranteed by design and not subject to production test.
Note 6:Testing was done with VDD and VEE back driven to valid supply
levels for functions that require these supplies, unless otherwise noted.
Note 7: If enabling from shutdown, where VDD and VEE supplies are
collapsed, allow the extra time it takes to generate valid VDD and VEE
supplies (tRDY).
REV B
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LTC2873
TYPICAL PERFORMANCE CHARACTERISTICS
VCC Supply Current vs Supply
Voltage in Shutdown Mode
VCC Supply Current vs Supply
Voltage in RDY Mode
5
SUPPLY CURRENT (mA)
12
11
10
9
8
7
100
DRIVER, RECEIVER AND TERMINATOR
DISABLED
14
13
VCC Supply Current
vs RS485 Data Rate
80
4
SUPPLY CURRENT (mA)
15
SUPPLY CURRENT (μA)
TA = 25°C, VCC = VL = 3.3V, unless otherwise noted.
85°C
3
25°C
–40°C
2
60
TE485 LOW
40
20
TE485 HIGH
6
3
3.5
4
4.5
SUPPLY VOLTAGE (V)
5
1
5.5
3.5
3
4.5
4
SUPPLY VOLTAGE (V)
5
5.5
VCC Supply Current vs RS232
Data Rate in Slow Mode
2.5nF
12
1nF
2.5nF
8
24
VCC = 5V
VCC = 3.3V
1nF
75pF
0
50
100
150
DATA RATE (kbps)
200
DRIVER AND
RECEIVER ENABLED
A/DO TIED TO B/RI
20
VARIOUS LOADS
16
250
250pF
0
200
400
600
DATA RATE (kbps)
800
DELAY (ns)
SKEW (ns)
0
25
50
TEMPERATURE (°C)
75
100
150
1.5
100
0.5
2873 G07
–25
0
25
50
TEMPERATURE (°C)
–25
0
25
50
TEMPERATURE (°C)
100
75
2873 G06
2.0
–1.0
–50
VCC = 5V
VCC = 3.3V
RS485 Driver Short-Circuit
Current vs Short-Circuit Voltage
–0.5
–25
1.5
0
–50
1000
0
10
RL = 54Ω
0.5
2.5nF
1.0
0
–50
2.0
1.0
SHORT–CIRCUIT CURRENT (mA)
VCC = 3.3V, VL = 1.7V
VCC = 5V, VL = 1.7V
VCC = 3.3V, VL = 3.3V
VCC = 5V, VL = 5V
20
RL = 100Ω
2.5
RS485 Driver Skew
vs Temperature
30
RL = 54Ω
3.0
2873 G05
RS485 Driver Propagation Delay
vs Temperature
40
3.5
75pF
4
RL = 100Ω
4.0
250pF
1nF
12
4.5
VCC = 5V
VCC = 3.3V
75pF
2873 G04
50
1nF
2.5nF
RS485 Driver Differential Output
Voltage vs Temperature
8
75pF
4
100
2873 G03
VCC Supply Current vs RS232
Data Rate in Fast Mode
SUPPLY CURRENT (mA)
SUPPLY CURRENT (mA)
DRIVER AND
RECEIVER ENABLED
A/DO TIED TO B/RI
16 VARIOUS LOADS
1
10
DATA RATE (Mbps)
2873 G02
2873 G01
20
0
0.1
VOLTAGE (V)
5
VCC = 5V
VCC = 3.3V
RS485 DRIVER
AND RECEIVER
SWITCHING.
CL = 100pF ON EACH
DRIVER OUTPUT.
75
100
2873 G08
VCC = 5V
VCC = 3.3V
50
OUTPUT LOW
0
–50
–100
–150
–15
OUTPUT HIGH
–10
–5
0
5
10
SHORT–CIRCUIT VOLTAGE (V)
15
2873 G09
REV B
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7
LTC2873
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
2.0
6
1.5
5
OUTPUT VOLTAGE (V)
80
TA = 25°C, VCC = VL = 3.3V, unless otherwise noted.
SKEW (ns)
DELAY (ns)
1.0
60
0.5
0
50
–0.5
40
–50
–25
50
0
25
TEMPERATURE (°C)
75
100
–1.0
–50
–25
0
25
50
TEMPERATURE (°C)
75
INPUT LOW
1.2
VCC = 5V
VCC = 3.3V
75
100
3
2
0
0
2
8
4
6
OUTPUT CURRENT (mA)
2µs/DIV
8
10
VCM = –7V
VCM = 2V
VCM = 12V
128
126
124
122
120
118
116
110
–50
–25
50
0
25
TEMPERATURE (°C)
75
100
2873 G15
RS232 Operation at 1Mbps
FAST Mode (DE485/F232 High)
LTC2873 Drivers Changing Modes
50pF FROM A/DO to B/RI,
TERMINATION ENABLED
DI
5V/DIV
A/DO
1V/DIV
B/RI
1V/DIV
RO
3V/DIV
2873 G16
130
2873 G14
A=B
3V/DIV
SLOW MODE
10
112
DI
3V/DIV
FAST MODE
8
4
6
OUTPUT CURRENT (mA)
114
1
RS232 Driver Switching at 250kbps
A
3V/DIV
2
RS485 Termination Resistance
vs Temperature
4
2873 G13
DI
3V/DIV
0
2873 G12
RESISTANCE (Ω)
1.6
0
25
50
TEMPERATURE (°C)
0
100
VL = 5V
VL = 3.3V
VL = 1.7V
5
1.8
OUTPUT VOLTAGE (V)
THRESHOLD VOLTAGE (V)
6
–25
2
RS232 Receiver Output Voltage
vs Load Current
2.0
1.0
–50
3
2873 G11
RS232 Receiver Input Threshold
vs Temperature
1.4
4
1
2873 G10
INPUT HIGH
VL = 5V
VL = 3.3V
VL = 1.7V
400ns/DIV
2873 G17
RO
5V/DIV
20ns/DIV
2873 G18
REV B
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LTC2873
TYPICAL PERFORMANCE CHARACTERISTICS
TA = 25°C, VCC = VL = 3.3V, unless otherwise noted.
RS232/RS485 Mode Switching
DI
485/232
5V/DIV
RS232
RS485
B/RI
A/DO
3V/DIV
2873 G19
2µs/DIV
Transition from Shutdown to RS232
Driver Output Going High and Low
SHDN
VDD
A HIGH
3V/DIV
A LOW
VEE
2873 G20
40µs/DIV
VDD and VEE Ripple
VDD RIPPLE
10mV/DIV
VEE RIPPLE
2873 G21
40µs/DIV
RS485 READY MODE,
ALL DRIVERS AND RECEIVERS DISABLED
REV B
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9
LTC2873
PIN FUNCTIONS
VEE (Pins 1, 10, 25): Generated Negative Supply Voltage
for RS232 Driver (–6.3V). Tie all pins together and connect
1µF capacitor between VEE (Pin 10) and GND. Exposed
pad (Pin 25) must be soldered to PCB to maintain low
thermal resistance.
RO (Pin 2): RS485 Differential Receiver Output and RS232
Receiver Output. Logic level referenced to GND and VL.
485/232 (Pin3): 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 LTC2873 pins. Logic level referenced to
GND and VL. Do not float.
RE485 (Pin4): RS485 Receiver Enable. In RS485 mode, a
logic high disables the RS485 receiver, leaving its output
Hi-Z and a logic low enables the RS485 receiver. This input
has no function in RS232 mode (485/232 low). Logic level
referenced to GND and VL. Do not float.
DE485/F232 (Pin 5): RS485 Driver Enable and RS232 Fast
Mode Enable. In RS485 mode (485/232 high), a logic low
disables the RS485 driver leaving the driver outputs in a
Hi-Z state and a logic high enables the RS485 driver. In
RS232 mode (485/232 low), a logic high enables Fast mode
with maximum data rate of 1Mbps. A logic low enables
Slow mode with a maximum data rate of 250kbps. Logic
level referenced to GND and VL. Do not float.
DI (Pin 6): RS485 and RS232 Driver Input. Logic level
referenced to GND and VL. Do not float.
SHDN (Pin7): Shutdown Control. A logic low disables the
LTC2873 into low power shutdown state, independent
of the other inputs. Driver and receiver outputs become
Hi-Z. Logic level referenced to GND and VL. Do not float.
GND (Pin 8, 11, 16, 20): Ground. Tie all four pins together.
CAP (Pin 9): Charge Pump Capacitor for Generated Negative
Supply Voltage VEE. Connect a 220nF capacitor between
CAP and SW.
SW (Pin 12): Switch Pin. Connect 10µH inductor between
SW and VCC. See Inductor Selection section for further
details.
10
VDD (Pin 13): Generated Positive Supply Voltage for
RS232 Driver (+7.0V). Connect 1µF capacitor between
VDD and GND.
VCC (Pin 14, 18, 21): 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.
B/RI (Pin 15): RS485 Negative Receiver Input and Driver
Output. In RS232 mode, this is the RS232 receiver input.
A/DO (Pin 17): RS485 Positive Receiver Input and Driver
Output. In RS232 mode, this is the RS232 driver output.
NC (Pin 19): Not connected internally.
VL (Pin 22): 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 to VCC. Keep
VL ≤ VCC for operation guaranteed to meet specifications.
However, VL > VCC will not damage the device, provided
that absolute maximum limits are respected. See “VL Logic
Supply and Logic Pins” in Applications section for more
information.
LB (Pin 23): Loopback Enable. A logic high enables logic
loopback diagnostic mode, internally routing the driver
input logic signals to the receiver output pins. This applies
to RS232 and RS485 operation. 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. Logic level referenced to GND
and VL. Do not float.
TE485 (Pin24): RS485 Termination Enable. In RS485 mode,
a logic low enables a 120Ω resistor between pins A/DO
and B/RI. A logic high opens the resistor between A/DO
and B/RI, leaving the pins unterminated. In RS485 mode,
the 5k resistor between B/RI and GND is never engaged.
In RS232 mode, the 120Ω resistor between A/DO and B/
RI is never engaged, regardless of the state of TE485, and
the 5k resistor between B/RI and GND is always engaged.
Logic level referenced to GND and VL. Do not float.
REV B
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LTC2873
BLOCK DIAGRAM
1.7V TO 5.5V
(≤ VCC)
3V TO 5.5V
10µH
0.1µF
220nF
2.2µF
VL
VCC
SW
CAP
SHDN
VDD
485/232
DE485/F232
RT232
CONTROL
LOGIC
RE485
RT485
TE485
VEE
PULSE-SKIPPING
BOOST
REGULATOR
f = 1.2MHz
1µF
1µF
DRIVERS
LB
232
DI
A/DO
RT485
485
120Ω
B/RI
LOOPBACK
PATH
125k
RECEIVERS
232
125k
RT232
5k
RO
485
GND
2873 BD
REV B
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11
LTC2873
TEST CIRCUITS
A/DO
GND
OR
VL
DI
+
RL
VOD
DRIVER
B/RI
–
RL
GND
OR
VL
+
VOC
–
DI
A/DO OR
B/RI
IOSD485
B/RI
OR A/DO
+
–
DRIVER
2873 F01
VOUT
2873 F02
Figure 1. RS485 Driver DC Characteristics
Figure 2. RS485 Driver Output Current
IIN485
+
–
A/DO OR
B/RI
B/RI
RECEIVER
OR A/DO
VIN
RIN485 =
VIN
IIN485
2873 F03
Figure 3. RS485 Receiver Input Current and Resistance
tPLHD485
DI
A/DO
DI
RDIFF
DRIVER
B/RI
CL
CL
A/RO – B/DI
0V
tSKEWD485
B/RI
A/DO
VL
tPHLD485
½VOD
VOD
90%
10%
0V
0V
tRD485
90%
10%
tFD485
2873 F04
Figure 4. RS485 Driver Timing Measurement
12
REV B
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LTC2873
TEST CIRCUITS
RL
A/DO
VL
OR
GND
DI
DE485/F232
OR SHDN
GND
OR
VCC
CL
B/RI
RL
CL
VCC
OR
GND
VL
½VL
tZLD485,
tZLSD485
0V
tLZD485,
tLZSD485
½VCC
A/DO OR B/RI
DRIVER
DE485/F232
OR SHDN
½VL
VCC
0.5V
VOH
0.5V
½VCC
B/RI OR A/DO
VOL
0V
tHZD485,
tHZSD485
tZHD485,
tZHSD485
2873 F05
Figure 5. RS485 Driver Enable and Disable Timing Measurements
VAB
±VAB/2
A/DO TO B/RI
A/DO
VCM
RECEIVER
B/RI
0V
tPLHR485
RO
CL
±VAB/2
90%
RO
10%
tPHLR485
½VL
½VL
tRR485
90%
10%
tFR485
tSKEWR485 = tPLHR485 – tPHLR485
–VAB
VL
0V
2873 F06
Figure 6. RS485 Receiver Propagation Delay Measurements
RE485
OR SHDN
0V TO 3V
3V TO 0V
VL
½VL
tZLR485,
tZLRSR485
A/DO
B/RI
½VL
RO
RECEIVER
RL
VL
OR
GND
0V
tLZR485,
tLZSR485
VL
½VL
RO
0.5V
CL
RE485
OR SHDN
VOH
0.5V
½VL
RO
VOL
0V
tHZR485,
tHZSR485
tZHR485,
tZHRSR485
2873 F07
Figure 7. RS485 Receiver Enable and Disable Timing Measurements
IA
RECEIVER
TE485
A/DO
RTERM =
+
–
VAB
IA
VL
TE485
VAB
B/RI
½VL
tRTEN485
IA
+
–
½VL
tRTZ485
0V
90%
VB
10%
2873 F08
Figure 8. RS485 Termination Resistance and Timing Measurements
REV B
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13
LTC2873
TEST CIRCUITS
DI
A/DO
DI
RL
VL
tPHLD232
½VL
½VL
tPLHD232
tF
CL
3V
A/DO
–3V
SLEW RATE =
0V
tR
0V
0V
6V
VOHD
3V
–3V
VOLD
tSKEWD232 = |tPHLD232 – tPLHD232|
tF OR tR
2873 F09
Figure 9. RS232 Driver Timing and Slew Rate Measurements
VL
A/DO
0V OR VL
SHDN
RL
½VL
SHDN
½VL
tZHSD232
CL
0V
tHZSD232
VOHD
0.5V
5V
A/DO
tZLSD232
0V
tLZSD232
–5V
A/DO
0V
0.5V
VOLD
2873 F10
Figure 10. RS232 Driver Enable and Disable Times
B/RI
B/RI
RO
1.5V
tPHLR232
CL
90%
RO
+3V
1.5V
10%
–3V
tPLHR232
½VL
½VL
VL
90%
10%
tRR232
tFR232
tSKEWR232 = |tPLHR232 – tPHLR232|
0V
2873 F11
Figure 11. RS232 Receiver Timing Measurements
VL
RL
RO
–3V OR +3V
SHDN
GND
OR VL
SHDN
½VL
½VL
CL
RO
RO
0.5V
½VL
tZLR232,
tZLSR232
0V
tHZR232,
tHZSR232
tZHR232,
tZHSR232
½VL
VOHR
0V
tLZR232,
tLZSR232
VL
0.5V
VOLR
2873 F12
Figure 12. RS232 Receiver Enable and Disable Times
14
REV B
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LTC2873
TEST CIRCUITS
C1
220nF
L1
10µH
VCC
C4
2.2µF
STEP
+
–
V
VCC
SW CAP
VDD
VL
DI
LTC2873
RO
VEE
SHDN
485/232
DE485/F232
A/DO
RE485
B/RI
TE485
LB
GND
½VL
SHDN
C2
1µF
+6.3V
0V
VDD
VEE
C3
1µF
0V
–5.9V
tVEE
tVDD
tRDY = MAX (tVDD, tVEE)
2873 F13
Figure 13. Timing Coming Out of Shutdown Mode
L1
10µH
VCC
C4
2.2µF
STEP
+
–
V
C1
220nF
½VL
½VL
485/232
VCC
SW CAP
VDD
VL
DI
LTC2873
RO
VEE
SHDN
485/232
DE485/F232
A/DO
RE485
B/RI
TE485
LB
GND
A
C4
1µF
B
C3
1µF
CB
50pF
3V
CA
50pF
0V
tR232
½VL
½VL
RO
RA
3k
tDR485
tR232
2873 F14
Figure 14. Mode Change Timing
VCC
C1
220nF
L1
10µH
C4
2.2µF
VCC
VCC
SW CAP
VDD
VL
DI
LTC2873
RO
VEE
SHDN
485/232
DE485/F232
A/DO
RE485
B/RI
TE485
LB
GND
C4
2.2µF
C2
1µF
C3
1µF
C1
220nF
L1
10µH
HIGH FOR FAST MODE
LOW FOR SLOW MODE
VCC
SW CAP
VDD
VL
DI
LTC2873
RO
VEE
SHDN
485/232
DE485/F232
A/DO
RE485
B/RI
TE485
LB
GND
C2
1µF
C3
1µF
2873 F15
(a)
(b)
Figure 15. Testing Max Data Rate for (a) RS485 and (b) RS232. Observe that Data In Matches Data Out
REV B
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15
LTC2873
FUNCTION TABLE
KEY: 0 = Logic Low; 1 = Logic High; RX = Receiver; TX = Driver; l = Enabled; LB = Receiver Output is the
Data Input Signal (Looped Back)
INPUTS
RESULT
SHDN
485/232
RE485
DE485/
F232
TE485
LB
MODE
0
X
X
X
X
X
SHUTDOWN
DC/DC
CONV.
RS232
RS485
RX
TX
l
l
l
l
LB
l
RS232
FAST
l
l
l
l
LB
l
RS485
READY
l
RX
TX
TERM
1
0
X
0
X
0
1
0
X
0
X
1
1
0
X
1
X
0
1
0
X
1
X
1
1
1
1
0
1
X
1
1
1
0
0
X
1
1
0
0
0
0
l
l
l
1
1
0
0
0
1
l
LB
l
1
1
0
0
1
0
l
l
1
1
0
0
1
1
l
LB
1
1
0
1
0
0
l
l
l
l
1
1
0
1
0
1
l
LB
l
l
1
1
0
1
1
0
l
l
l
1
1
0
1
1
1
l
LB
l
1
1
1
1
0
X
l
l
1
1
1
1
1
X
l
l
16
RS232 SLOW
RS485
l
l
l
REV B
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LTC2873
APPLICATIONS INFORMATION
The LTC2873 is a flexible multiprotocol transceiver supporting RS485/RS422 and RS232 protocols.
The LTC2873 features rugged operation with ESD ratings
of ±26kV HBM on the RS232 and RS485 receiver inputs
and driver outputs, both unpowered and powered. All
other pins offer protection exceeding ±4kV.
This device 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 termination components and switching relays. A
logic loopback control is included for self-test and debug.
DC/DC Converter
The on-chip DC/DC converter operates from the VCC
input, generating a 7.0V VDD supply and a charge
pumped –6.3V VEE supply, as shown in Figure 16. VDD
and VEE power the output stage of the RS232 drivers
and are regulated to levels that guarantee greater than
±5V output swing.
The LTC2873 bus interface is a single two-pin port that can
be configured as either an RS232 driver/receiver pair or a
differential RS485 (and RS422) transceiver depending on
the state of the 485/232 pin. In RS485 mode, the driver
and receiver can be enabled independently with the DE485/
F232 and RE485 pins, or by tying these signals together, a
single control selects transmit or receive modes. A 120Ω
termination resistor is automatically engaged between pins
A/DO and B/RI in RS485 mode if TE485 is low.
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. Locate C1 – C4 close to their associated
pins shown in Figure 16. Refer to Layout Considerations
section for guidance on circuit board layout.
Bypass capacitor C5 on the logic supply pin can be omitted
if VL is connected to VCC. See the VL Logic Supply section
for more details about the VL logic supply.
When the LTC2873 is in RS232 mode, the RS232 driver
and receivers are both active and a 5k resistor is engaged
at the receiver input to ground. The slew rate in RS232
mode can be set to support 1Mbps or 250kbps operation
using the DE485/F232 pin.
VCC
3V TO 5.5V
C1
220nF
L1
10µH
C4
2.2µF
14
VL
1.7V TO VCC
12
VCC
SW
9
CAP
VDD
22 VL
C5
0.1µF
BOOST
REGULATOR
20 GND
11
VEE
13
C2
1µF
10
GND
2873 F16
C3
1µF
NOTE: NOT ALL PINS SHOWN. IN THE CASE OF DUPLICATE PINS FOR VCC,
GND, AND VEE, EXTERNAL COMPONENTS SHOULD BE POSITIONED
CLOSEST TO THE NUMBERED PIN SHOWN ABOVE.
Figure 16. Simplified DC/DC Converter with Required External Components
REV B
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17
LTC2873
APPLICATIONS INFORMATION
Powering Multiple Devices
Multiple LTC2873 devices can be powered using the boost
regulator from only one of the devices, requiring only
one inductor (L1) and charge pump cap (C1). 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 charge pump 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.
Figure 32 shows an example of how to connect four devices.
Inductor Selection
A 10µH or 22μH (±20%) 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 1.
Capacitor Selection
The small size of ceramic capacitors makes them ideal
for the LTC2873. 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 charge pump 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 absolute maximum levels of ±7.5V.
Ideal supply levels are 7.2V and –6.5V as these are each
just wider than the regulation points of 7.0V 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
RS232 mode and in RS485 mode when the termination
is enabled. It is okay to run the LTC2873 in RS485 mode
with internal termination disabled (TE485 high), when VDD
and VEE are not present or fully settled.
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 17.
This precaution is not relevant for supply voltages below
4.5V or rise times longer than 10µs.
Table 1. Recommended Inductors
PART NUMBER
74479888310
L (μH)
ISAT (mA)
MAX DCR (Ω)
SIZE (mm)
10
250
0.5
2.5 × 2 × 1
CBC2016T100K (or M)
10
380
1.07
2 × 1.6 × 1.6
CBC2518T220K (or M)
22
320
1.0
2.5 × 1.8 × 1.8
BRC2016T220K (or M)
22
310
1.3
2 × 1.6 × 1.6
LQH32CN220K53
22
250
0.92
3.2 × 2.5 × 1.6
18
MANUFACTURER
Wurth Elektronik
Taiyo Yuden
www.t-yuden.com
Murata
www.murata.com
REV B
For more information www.analog.com
LTC2873
APPLICATIONS INFORMATION
greater than 96kΩ (typically 125kΩ) to ground over the
entire common mode range of –7V to +12V. This resistance
is actually the RS485 receiver input resistance, which is
connected to the same pins.
5V
0V
≤10µs
R1
1Ω
1/8W
C4
2.2µF
C1
220nF
L1
10µH
RS232 Driver with Speed Selection
INRUSH
CURRENT
12
SW
9
CAP
VCC
14
13
VDD
11
GND
2873 F17
C2
1µF
Figure 17. Supply Current Overshoot Protection for
Input Supplies of 4.5V or Higher and Rise Times Faster
Than 10μs
VL Logic Supply and Logic Pins
A separate logic supply pin VL allows the LTC2873 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. In particular,
supply currents can be somewhat higher than specified.
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 SHDN, which must not
exceed VL by more than 0.3V. 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 compatibility.
When enabled, if DI is high, (A/DO to B/RI) is positive.
With the driver disabled, the A/DO and B/RI resistance is
The RS232 driver provides full compatibility with the TIA/
EIA-232-F (RS232) specification. When in RS232 mode,
the driver is automatically enabled. Like all RS232 drivers, it is inverting, so that when the input, DI, is low, the
output, A/DO, is high, and vice-versa.
The RS232 driver slew rate can be selected to support
data rates of up to 250kbps or 1Mbps with the DE485/
F232 pin. Since RS232 signals are single ended and large
amplitude, compared with RS485, radiated emissions may
be a concern. To minimize emissions, the speed selection
should be set to Slow mode by setting DE485/F232 low
for data rates of 250kbps or less. For higher data rates, up
to 1Mbps, Fast mode must be engaged by setting DE485/
F232 high. Even in Fast mode the driver transitions are slew
controlled to minimize emissions. See "Typical Performance
Characteristics" section for examples of the waveforms.
Driver Overvoltage and Overcurrent Protection
The RS232 and RS485 driver outputs are protected from
short circuits to any voltage within the absolute maximum
range of ±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 about
100mA can 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 during momentary
overload conditions. Overtemperature protection activates
at a junction temperature exceeding about 165°C (not
tested in production). NOTE: Continuous operation above
the specified maximum operating junction temperature
may result in device degradation or failure.
REV B
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19
LTC2873
TYPICAL APPLICATIONS
RS485 Balanced Receiver with Full Failsafe Operation
The LTC2873 RS485 receiver has a differential threshold
voltage that is about +110mV for signals that are rising
and –110mV for signals that are falling, as illustrated in
Figure 18. If a differential input signal lingers in the window
between these thresholds for more than about 0.7µs, the
rising threshold changes from +110mV to –70mV, while
the falling threshold remains at –110mV. Thus, differential
inputs that are shorted, open, or terminated but not driven
for more than 0.7µs produce a high on the receiver output,
indicating a failsafe condition.
RA
RISING THRESHOLD
SHIFTS IF SIGNAL IS
IN WINDOW > ~0.7µs
TO SUPPORT
FAILSAFE
–110mV –70mV
0V
+110mV
(A-B)
200mV/DIV
RO
5V/DIV
200ns/DIV
2873 F19
Figure 19. 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
RS485 Biasing Network Not Required
A/DO TO B/RI
2873 F18
Figure 18. RS485 Receiver Input Threshold Characteristics
with Typical Values Shown
The benefit of this dual threshold architecture is that
it supports full failsafe operation yet offers a balanced
threshold, centered on 0V, for normal data signals. This
balance preserves 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 19, where a signal is driven through 4000ft of
CAT-5e cable at 3Mbps. Even though the differential signal peaks are at only 200mV and is heavily slewed, the
output maintains a nearly perfect signal with almost no
duty cycle distortion.
An additional benefit of the balanced architecture is excellent noise immunity due to the wide effective differential
input signal hysteresis of 220mV for signals transitioning
through the window region in less than 0.7µs. Increasingly
slower signals will have increasingly less effective hysteresis, limited by the DC failsafe hysteresis of about 40mV.
20
B
200mV/DIV
A
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 LTC2873 eliminates the
need for external network biasing resistors provided they
are used in a network of transceivers with similar internal
failsafe features. This also allows the network to support
a high number of nodes, up to 256, by eliminating the
bias resistor loading. The LTC2873 transceiver operates
correctly on biased, unbiased, or under-biased 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
REV B
For more information www.analog.com
LTC2873
APPLICATIONS INFORMATION
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.
RS485 Receiver Input Resistance
In RS485 mode, the RS485 receiver input resistance from
A/DO or B/RI to GND is 125kΩ (typical) 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 loopback mode, or unpowered.
The equivalent input resistance looking into the RS485
receiver pins is shown in Figure 20.
125k
Proper cable termination is important for good signal fidelity. When the cable is not terminated with its characteristic
impedance, reflections cause waveform distortion.
The LTC2873 offers an integrated switchable 120Ω termination resistor between pins A/DO and B/RI.
This termination supports communication over a twisted
pair cable with characteristic impedance of 120Ω or 100Ω,
including CAT-5 cables. It has 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
only. However, the driving end of a line does not need to
be terminated. By turning off termination at the driver, the
reduced load results in less power dissipation and a larger
signal swing on the bus. TE485 can be tied to DE485/F232
to logically switch the termination on only when the driver
is inactive if the termination enable/disable delays can be
tolerated in the overall system level timing. If the delays are
not acceptable, tie TE485 low to enable termination for all
modes of RS485 operation, whether driving or receiving.
The termination resistance is maintained over the entire
RS485 common mode range of –7V to 12V as shown in
Figure 21. The voltage across pins with the terminating
resistor enabled should not exceed 6V as indicated in the
Absolute Maximum Ratings table.
A/DO
60Ω
TE485
125k
Selectable RS485 Termination
60Ω
126
B/RI
VCC = 5.0V
VCC = 3.3V
124
Figure 20. Equivalent RS485 Receiver Input Resistance
Into A/DO and B/RI
RS232 Receiver Input Resistance
In RS232 mode, the receiver input resistance on the
B/RI pin is always 5k to GND. In any other mode, this
resistor is switched out. The 120Ω RS485 termination
resistor between pins A/DO and B/RI is never engaged
in RS232 mode, regardless of the state of TE485 pin.
RESISTANCE (Ω)
2873 F20
122
120
118
116
–10
–5
0
5
VOLTAGE (V)
10
15
2873 F21
Figure 21. Typical Resistance of the Enabled RS485
Terminator vs Common Mode Voltage of A/DO and B/RI
REV B
For more information www.analog.com
21
LTC2873
APPLICATIONS INFORMATION
Logic Loopback
Robust ESD Protection
A loopback mode connects the driver inputs to the receiver
outputs (non-inverting) providing an echo for self-test. This
applies to both RS232 and RS485 transceivers. Loopback
mode is entered when the LB pin is set to a logic-high and
the relevant receiver is enabled. The RS485 driver output
can be disabled in loopback mode if DE485/F232 is held
low, or functions normally with DE485/F232 high. The
RS232 driver output cannot be disabled when loopback
is engaged in RS232 mode, and functions normally. The
loopback signal traverses a path from the logic input circuit at DI to the logic output at RO and does not exercise
the entire driver or receiver circuit. Thus loopback, alone,
is not a sufficient test to ensure full functionality of the
LTC2873. Loopback does not affect the operation of the
termination resistors.
The LTC2873 features exceptionally robust ESD protection.
The transceiver interface pins (A and B) are protected to
±26kV human body model with respect to GND, VCC, or VL
without latchup or damage. This protection holds whether
the device is unpowered or powered in any mode of operation. To note, ±26kV is an upper limit of the tester—the
actual device protection level is higher. Every other pin on
the device is protected to ±4kV ESD (HBM) for all-around
robustness. Figure 22 shows the LTC2873 being struck
repeatedly with 26kV of ESD energy (air gap discharge)
during operation with no damage or circuit latchup.
RS485 Cable Length vs Data Rate
Many factors contribute to the maximum cable length
that can be used for RS485 or RS422 communication,
including driver transition times, receiver threshold, duty
Figure 22. LTC2873 Struck Repeatedly with 26kV of ESD Energy While Operating. No Damage or Circuit Latchup Occurs
22
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REV B
LTC2873
APPLICATIONS INFORMATION
cycle distortion, cable properties and data rate. A typical
curve of cable length versus maximum data rate is shown
in Figure 23. Various regions of this curve reflect different
performance limiting factors in data transmission.
At frequencies below 100kbps, the maximum cable length
is determined by DC resistance in the cable. In this example, a cable longer than 4000ft will attenuate the signal
at the far end to less than what can be reliably detected
by the receiver.
For data rates above 100kbps, the capacitive and inductive
properties of the cable begin to dominate this relationship. The attenuation of the cable is frequency and length
dependent, resulting in increased rise and fall times at
the far end of the cable. At high data rates or long cable
lengths, these transition times become a significant part
of the signal bit time. Jitter and inter symbol interference
aggravate this so that the time window for capturing valid
data at the receiver becomes impossibly small.
The boundary at 20Mbps in Figure 23 represents the
guaranteed maximum operating rate of the LTC2873. The
dashed vertical line at 10Mbps represents the specified
maximum data rate in the RS485 standard. This boundary
is not a limit, but reflects the maximum data rate that the
specification was written for. It should be emphasized
that the plot in Figure 23 shows a typical relation between
maximum data rate and cable length. Results with the
LTC2873 will vary, depending on cable properties such
as conductor gauge, characteristic impedance, insulation
material, and solid versus stranded conductors.
CABLE LENGTH (FT)
10k
1k
LTC2873
MAX DATA RATE
100
RS485/RS422
MAX DATA RATE
10
10k
100k
1M
10M
DATA RATE (bps)
100M
2873 F23
Figure 23. Cable Length vs Data Rate
(RS485/RS422 Standard Shown in Vertical Dashed Line)
Layout Considerations
All VCC pins must be connected together and all ground
pins must be connected together on the PC board with
very low impedance traces or dedicated planes. A 2.2µF,
or larger, bypass capacitor should be placed less than
7mm away from VCC Pin 14. This VCC pin, as well as GND
Pin 11, mainly service the DC/DC converter. Additional
bypass capacitors of 0.1µF or larger, can be added from
VCC pin 18 to ground pin 16 if the traces back to the 2.2µF
capacitor are indirect or narrow. These VCC and ground
pins mainly service the RS485 driver. Table 2 summarizes
the bypass capacitor requirements. The capacitors listed
in the table should be placed closest to their respective
supply and ground pin.
Table 2. Bypass Capacitor Requirements
CAPACITOR (µF)
SUPPLY (PIN)
RETURN (PIN)
COMMENT
2.2
VCC (14)
GND (11)
Required
1.0
VDD (13)
GND (11)
Required
1.0
VEE (10)
GND (11)
Required
0.1
VL (22)
GND (20)
Required*
0.1
VCC (18)
GND (16)
Optional
0.1
VCC (21)
GND (20)
Optional
*If VL is not connected to VCC.
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.
The PC board traces connected to high speed bus signals
A/DO and B/RI should be symmetrical and as short as
possible to minimize capacitive imbalance and to maintain
good differential signal integrity. To minimize capacitive
loading effects, the differential signals should be separated
by more than the width of a trace and should not be routed
on top of each other if they are on different signal planes.
Care should be taken to route outputs away from any
sensitive inputs to reduce feedback effects that might
cause noise, jitter, or even oscillations. For example, DI,
A/DO, and B/RI should not be routed near RO.
REV B
For more information www.analog.com
23
LTC2873
TYPICAL APPLICATIONS
Supply Connections and External Components Necessary for Operation Are Not Shown.
H = Logic High; L = Logic Low, X = Don’t Care (Logic High or Logic Low)
RS232 SLOW
RS232 FAST
RS232 + LOOPBACK
LTC2873
LTC2873
LTC2873
DI
A/DO
DI
A/DO
DI
A/DO
RO
B/RI
RO
B/RI
RO
B/RI
L
H
L X
X
LB
X
TE485
X
RE485
L H
5k
DE485/F232
H
485/232
SHDN
L
SHDN
LB
X
LB
TE485
X
TE485
RE485
L
RE485
DE485/F232
L
485/232
485/232
H
5k
DE485/F232
SHDN
5k
X H
2873 F24
Figure 24. RS232 Configurations
RS485 RX
RS485 TX
RS485 TX + RX
LTC2873
LTC2873
LTC2873
TE485
LB
H H H
RE485
LB
L
B/RI
RO
DE485/F232
TE485
H H H H H
A/DO
485/232
B/RI
SHDN
RO
DI
L H
L
RS485 RX + TERM
RS485 TX + TERM
RS485 TX + RX + TERM
LTC2873
LTC2873
LTC2873
RE485
TE485
LB
L
120Ω
B/RI
RO
DE485/F232
L
A/DO
H H H
L
L
L
485/232
H H H H
DI
SHDN
LB
L
LB
TE485
L
TE485
RE485
L
RE485
DE485/F232
L
120Ω
B/RI
RO
DE485/F232
485/232
H H
A/DO
485/232
120Ω
B/RI
RO
DI
SHDN
A/DO
DI
SHDN
B/RI
RE485
L
A/DO
DE485/F232
LB
L H
DI
SHDN
TE485
L
RE485
DE485/F232
H H
SHDN
485/232
RO
A/DO
485/232
DI
2873 F25
Figure 25. RS485 Configurations
24
REV B
For more information www.analog.com
LTC2873
TYPICAL APPLICATIONS
Supply Connections and External Components Necessary for Operation Are Not Shown.
H = Logic High; L = Logic Low, X = Don’t Care (Logic High or Logic Low)
RS485 LOOPBACK
RS485 LOOPBACK + TX
LTC2873
LTC2873
A/DO
DI
B/RI
H H H
LB
TE485
RE485
DE485/F232
SHDN
L H H
L H H
RS485 LOOPBACK + TERM
RS485 LOOPBACK + TX + TERM
LTC2873
LTC2873
DI
A/DO
120Ω
B/RI
L H
LB
L
TE485
RE485
H H H
SHDN
DE485/F232
RO
LB
L
TE485
RE485
H H H
485/232
DE485/F232
RO
A/DO
120Ω
B/RI
485/232
DI
SHDN
120Ω
B/RI
RO
LB
TE485
L
RE485
DE485/F232
H H
SHDN
485/232
RO
A/DO
485/232
DI
L H
2873 F26
Figure 26. RS485 + Loopback Configurations
SHUTDOWN
RS485 READY
RS485 READY + TERM
LTC2873
LTC2873
LTC2873
LB
120Ω
B/RI
TE485
H H
DE485/F232
L
RO
485/232
L H H
DC/DC
CONV
ON
RE485
H H
A/DO
DI
SHDN
X
LB
X
B/RI
TE485
LB
X
RO
RE485
TE485
X X
DC/DC
CONV
ON
DE485/F232
RE485
B/RI
DE485/F232
L
485/232
SHDN
RO
A/DO
DI
485/232
DC/DC
CONV
OFF
SHDN
A/DO
DI
L H
L
L
2873 F27
Figure 27. Shutdown, RS485 Ready and RS485 Ready + Term Configurations
REV B
For more information www.analog.com
25
LTC2873
TYPICAL APPLICATIONS
Supply Connections and External Components Necessary for Operation Are Not Shown.
H = Logic High; L = Logic Low, X = Don’t Care (Logic High or Logic Low)
LTC2873
A/DO
DI
H
L
LTC2873
DR
A/DO
A/DO
120Ω
120Ω
RO
TE485
LB
RE485
DE485/F232
485/232
SHDN
H H
H
L
LB
H H
B/RI
RO
TE485
L
A/DO
485/232
LB
L
DR
SHDN
TE485
H H
DI
RE485
B/RI
LTC2873
RE485
DE485/F232
B/RI
485/232
SHDN
RO
DI
DE485/F232
DI
LB
RE485
DE485/F232
H H
TE485
LTC2873
RO
485/232
SHDN
B/RI
L
L
DR
2873 F28
DR
Figure 28. Typical RS485 Half Duplex Network
LTC2873
LTC2873
DI
A/DO
A/DO
DI
RO
B/RI
B/RI
RO
H
L
FAST SLOW
LB
485/232
L
TE485
SHDN
X
RE485
LB
X
DE485/F232
TE485
L
5k
RE485
485/232
H
DE485/F232
SHDN
5k
X
X
L
FAST SLOW
2873 F29
Figure 29. Typical RS232 Communications Link
26
REV B
For more information www.analog.com
LTC2873
TYPICAL APPLICATIONS
Supply Connections and External Components Necessary for Operation Are Not Shown.
H = Logic High; L = Logic Low, X = Don’t Care (Logic High or Logic Low)
3V TO 5.5V
1.7V TO VCC
LTC2873
VCC
VL
µP
LOGIC
LEVEL
SIGNALS
LINE
LEVEL
SIGNALS
RS232
AND/OR
RS485
GND
2873 F30
Figure 30. Low Voltage Microprocessor Interface
LTC2873
A/DO
DI
120Ω
RO
L
RECEIVE
SLOW
H
TRANSMIT
FAST
Y
X
485/232
LB
X MODE
H RS485
L RS232
B/RI
DE485/F232
RE485
TE485
SHDN
Y
2873 F31
H H
Figure 31. Receiver-Only RS485 Termination for Power Savings
REV B
For more information www.analog.com
27
LTC2873
TYPICAL APPLICATIONS
NUMBER OF
LTC2873 DEVICES
L1
C1
C2
C3
MINIMUM
C4-X
SINGLE TRANSCEIVER
1
10µH
220nF
1µF
1µF
2.2µF
DUAL TRANSCEIVER
2
10µH
220nF
1µF
1µF
2.2µF
APPLICATION
TRIPLE TRANSCEIVER
3
22µH
470nF
2.2µF
2.2µF
1µF
QUAD TRANSCEIVER
4
22µH
470nF
2.2µF
2.2µF
1µF
L1
22µH
3V TO 5.5V
C4-A
1µF
SHDN
485/232-A
DX485-A
RX485-A
TE485-A
LB
DI-A
RO-A
C4-B
1µF
485/232-B
DX485-B
RX485-B
TE485-B
DI-B
RO-B
C4-C
1µF
485/232-C
DX485-C
RX485-C
TE485-C
DI-C
RO-C
C4-D
1µF
485/232-D
DX485-D
RX485-D
TE485-D
DI-D
RO-D
VCC
VL
LTC2873
SW
CAP
VDD
VEE
GND
SHDN
485/232
DE485/F232
RE485
TE485
LB
A
A/DO
DI
B/RI
RO
VCC
VL
LTC2873
LTC2873
LTC2873
C2
2.2µF
A/DO-A
B/RI-A
A/DO-B
B/RI-B
SW
CAP
VDD
VEE
GND
SHDN
485/232
DE485/F232
RE485
TE485
LB
C
A/DO
DI
B/RI
RO
VCC
VL
C3
2.2µF
SW
CAP
VDD
VEE
GND
SHDN
485/232
DE485/F232
RE485
TE485
LB
B
A/DO
DI
B/RI
RO
VCC
VL
C1
470nF
A/DO-C
B/RI-C
SW
CAP
VDD
VEE
GND
SHDN
485/232
DE485/F232
RE485
TE485
LB
D
A/DO
DI
B/RI
RO
A/DO-D
B/RI-D
2873 F32
Figure 32. Quad Transceiver
28
REV B
For more information www.analog.com
LTC2873
TYPICAL APPLICATIONS
3500VRMS Isolated RS485/RS232 Transceiver
220nF
RO
485/232
RS485 D R
DI
VCC1
VL1
ON1
OUTD
OUTE
OUTF
INC
INB
INA
EOUTD
GND1
GND1
ISOLATION BARRIER
LTM2892-S
3V TO
5.5V
VCC2
VL2
ON2
IND
INE
INF
OUTC
OUTB
OUTA
EOUTA
GND2
GND2
ISOLATED
5V IN
10μH
10k
CAP
VCC
SW
VL
VDD
SHDN
485/232
DE485/F232
VEE
LTC2873
RE485
TE485
RS485
A/DO
RO
GND
1μF
ISO
1μF
ISO
DI
LB
2.2μF
ISO
120Ω
RS485
(485/232
HIGH)
RS232
(485/232
LOW)
B/RI
RS232
5k
2873 TA02
ISO
ISOLATED
GND IN
REV B
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29
LTC2873
PACKAGE DESCRIPTION
Please refer to http://www.linear.com/product/LTC2873#packaging/ for the most recent package drawings.
UFD Package
24-Lead Plastic QFN (4mm × 5mm)
(Reference LTC DWG # 05-08-1696 Rev A)
0.70 ±0.05
4.50 ±0.05
3.10 ±0.05
2.00 REF
2.65 ±0.05
3.65 ±0.05
PACKAGE OUTLINE
0.25 ±0.05
0.50 BSC
3.00 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)
R = 0.05 TYP
2.00 REF
R = 0.115
TYP
23
0.75 ±0.05
PIN 1 NOTCH
R = 0.20 OR C = 0.35
24
0.40 ±0.10
PIN 1
TOP MARK
(NOTE 6)
1
2
5.00 ±0.10
(2 SIDES)
3.00 REF
3.65 ±0.10
2.65 ±0.10
(UFD24) QFN 0506 REV A
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
30
REV B
For more information www.analog.com
LTC2873
REVISION HISTORY
REV
DATE
DESCRIPTION
A
05/16
Applied Note 7 to tZLSR232, tZHSR232.
B
04/18
PAGE NUMBER
6
Added Exposed Pad soldering requirement to VEE pin description.
9
Corrected recommended Wurth inductor part number.
17
Updated plot RS485 Driver Short-Circuit Current vs Short-Circuit Voltage.
7
Changed the approximate failsafe timeout from 1.3µs to 0.7µs.
20
Corrected a pin state in Figure 25 for the RS485 RX configuration.
24
Added a new Typical Application circuit.
32
Added LTM2885 to the Related Parts Table.
32
REV B
Information furnished by Analog Devices is believed to be accurate and reliable. However, no responsibility is assumed by Analog
Devices for its use, nor for any infringements of patents or other rights of third parties that may result from its use. Specifications
subject to change without notice. No license For
is granted
implication or
otherwise under any patent or patent rights of Analog Devices.
more by
information
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31
LTC2873
TYPICAL APPLICATION
2500VRMS Isolated RS485/RS232 Transceiver with Isolated Power
220nF
CAP
LTM2887-3S
ON
485/232
DR
DI
PWR_MON
RO
VCC
VL
VCC
ON
GND1
SDOE
GND1
CS
GND1
SDI
GND1
SCK
GND1
DO2
DO1
SDO
VCC2
I1
ISOLATION BARRIER
3.3V
CS2
AVCC2
SDI2
IVCC2
VL2
AVL2
GND2
SCK2
GND2
IVL2
GND2
SDO2
GND2
I2
SHDN
10μH
VCC
SW
VDD
LTC2873
485/232
DE485/F232
VL
BOOST
REGULATOR
VEE
RE485
TE485
1μF
ISO
1μF
ISO
RS485
DI
A/DO
RO
LB
2.2μF
ISO
120Ω
RS485
(485/232
HIGH)
RS232
(485/232
LOW)
B/RI
RS232
GND
5k
2873 TA03
RELATED PARTS
PART NUMBER
DESCRIPTION
COMMENTS
LTC2870, LTC2871
RS232/RS485 Multiprotocol Transceiver with
Integrated Termination
Two RS232 and One RS485 Transceivers. 3V to 5.5V Supply,
Automatic Selection of Termination Resistors, Duplex Control,
Logic Supply Pin, Up to ±26kV ESD
LTC2872
RS232/RS485 Dual Multiprotocol Transceiver
with Integrated Termination
Four RS232 and Two RS485 Transceivers. 3V to 5.5V Supply,
Automatic Selection of Termination Resistors, Duplex Control,
Logic Supply Pin, ±15kV ESD
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
LTM®2881
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 120ohm Termination Resistor, ±15kV ESD, 5V
Supply
LTM2882
Dual Isolated RS232 µModule Transceiver + Power
1Mbps, 2500VRMS Isolation with Integrated DC/DC Converter,
±10kV ESD
32
REV B
D16844-0-4/18(B)
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