LTC2873CUFD#PBF [Linear]
LTC2873 - Single-Bus RS485/RS232 Multiprotocol Transceiver with Switchable Termination; Package: QFN; Pins: 24; Temperature Range: 0°C to 70°C;
| 型号： | LTC2873CUFD#PBF |
| 厂家： | 
Linear |
| 描述： | LTC2873 - Single-Bus RS485/RS232 Multiprotocol Transceiver with Switchable Termination; Package: QFN; Pins: 24; Temperature Range: 0°C to 70°C |
| 文件： | 总30页 (文件大小：1126K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
LTC2873
Single-Bus RS485/RS232
Multiprotocol Transceiver with
Switchable Termination
FeaTures
DescripTion
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.
n
One RS485 or One RS232 Transceiver
n
3V to 5.5V Supply Voltage
n
Up to 20Mbps RS485
n
Slew-Controlled RS232 Operation:
n
Selectable 1Mbps or 250kbps
n
Automatic Selection of Integrated RS485 (120Ω)
and RS232 (5kΩ) Termination Resistors
High ESD: 26kV HBM
Logic Loopback Mode
1.7V to 5.5V Logic Interface
Supports Up to 256 RS485 Nodes
A pin-controlled integrated termination resistor allows for
easy interface reconfiguration, eliminating external resis-
tors 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.
n
n
n
n
n
RS485 Receiver Failsafe Eliminates UART Lockup
n
H-Grade Available (–40°C to 125°C)
n
Available in 24-Pin 4mm × 5mm QFN Package
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
n
L, LT, LTC, LTM, Linear Technology, the Linear logo and µModule are registered trademarks of
Linear Technology Corporation. All other trademarks are the property of their respective owners.
Software Selectable RS232/RS485/RS422 Interface
n
Industrial Sensors and Actuators
n
Alarm Systems
Traffic Control and Monitoring
Highway Signs and Jumbo Displays
n
n
Typical applicaTion
220nF
10μH
1.7V TO V
3V TO 5.5V
CC
2.2μF
1μF
RS485/RS232 Mode Switching
LTC2873
RS485
V
V
CAP
SW
CC
L
RS485
(485/232 (485/232
HIGH) LOW)
RS232
SHDN
DI
DI
DATA IN
A/DO
B/RI
485/232
5V/DIV
RS232
RS485
120Ω
RO
DATA OUT
B/RI
485/232
485/232
V
DD
1μF
A/DO
3V/DIV
RE485
TE485
RS232
D/R
(485)
5k
DE485/
V
EE
F232
1μF
LB
2873 TA01b
GND
1µs/DIV
2873 TA01
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For more information www.linear.com/LTC2873
LTC2873
absoluTe MaxiMuM raTings
pin conFiguraTion
(Notes 1 and 2)
TOP VIEW
Input Supplies
V , V .................................................... –0.3V to 7V
CC
L
Generated Supplies
................................................V – 0.3V to 7.5V
24 23 22 21 20
V
V
1
2
3
4
5
6
7
19
18
17
16
15
14
13
NC
V
DD
CC
EE
RO
485/232
RE485
V .........................................................–7.5V to 0.3V
CC
EE
A/DO
GND
B/RI
SW............................................... –0.3V to (V + 0.3V)
DD
25
CAP................................................. (V – 0.3V) to 0.3V
V
EE
EE
DE485/F232
DI
A/DO, B/RI ...................................................–15V to 15V
V
V
CC
DD
DI, 485/232, DE485/F232, RE485,
SHDN
8
9
10 11 12
TE485, LB ................................................ –0.3V to 7V
SHDN, RO .......................................–0.3V to (V + 0.3V)
L
Differential Terminator Voltage (Enabled)
UFD PACKAGE
24-LEAD (4mm × 5mm) PLASTIC QFN
(A/DO to B/RI) ..................................................... 6V
Differential Terminator Voltage (Disabled)
(A/DO to B/RI) ................................................... 30V
Operating Temperature
T
= 150°C, θ = 43°C/W
JA
JMAX
EXPOSED PAD (PIN 25) IS V , MUST BE SOLDERED TO PCB
EE
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
orDer inForMaTion http://www.linear.com/product/LTC2873#orderinfo
LEAD FREE FINISH
LTC2873CUFD#PBF
LTC2873IUFD#PBF
LTC2873HUFD#PBF
TAPE AND REEL
PART MARKING
PACKAGE DESCRIPTION
TEMPERATURE RANGE
0°C to 70°C
LTC2873CUFD#TRPBF
LTC2873IUFD#TRPBF
LTC2873HUFD#TRPBF
2873
24-Lead (4mm × 5mm) Plastic QFN
24-Lead (4mm × 5mm) Plastic QFN
24-Lead (4mm × 5mm) Plastic QFN
2873
–40°C to 85°C
2873
–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.
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For more information www.linear.com/LTC2873
LTC2873
elecTrical characTerisTics The ldenotes 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
Supplies
CONDITIONS
MIN
TYP
MAX
UNITS
l
l
l
l
V
CC
V
L
Supply Voltage Operating Range
3
5.5
V
V
Logic Supply Voltage Operating Range
V ≤ V
1.7
V
CC
L
CC
V
V
Supply Current in Shutdown Mode
Supply Current in RS232 Mode or
SHDN = 0V
8
4
30
µA
mA
CC
No Load, SHDN = TE485 = DE485/F232 = V
RE485 = 0
9
9
5
CC
L
RS485 Mode, Driver and Receiver Enabled,
Termination Disabled
l
l
V
Supply Current in RS485 Mode with
No Load, SHDN = 485/232 = V
DE485/F232 = RE485 = TE485 = 0
4
0
mA
µA
CC
L
Receiver and Termination Enabled, Driver
Disabled
V Supply Current in Any Mode
L
No Load
Power Supply Generator
V
DD
V
EE
Regulated V Output Voltage
SHDN = V , No Load
7.0
V
V
DD
L
Regulated V Output Voltage
SHDN = V , No Load
–6.3
EE
L
RS485 Driver
l
l
l
l
|V
OD
|
Differential Output Voltage
R = Open, V = 3V (Figure 1)
V
V
V
V
V
L
CC
CC
CC
CC
R = 27Ω, V = 4.5V (Figure 1)
2.1
1.5
2
V
V
L
CC
R = 27Ω, V = 3V (Figure 1)
L
CC
R = 50Ω, V = 3.13V (Figure 1)
L
CC
l
l
∆|V |
OD
Difference in Magnitude of Differential Output
Voltage for Complementary Output States
R = 27Ω, V = 3V (Figure 1)
0.2
0.2
V
L
CC
R = 50Ω, V = 3.13V (Figure 1)
L
CC
l
l
V
Common Mode Output Voltage
R = 27Ω or 50Ω (Figure 1)
3
V
V
OC
L
∆|V
|
Difference in Magnitude of Common Mode Output R = 27Ω or 50Ω (Figure 1)
Voltage for Complementary Output States
0.2
OC
L
l
l
I
Maximum Short-Circuit Current
–7V ≤ V
≤ 12V (Figure 2)
OUT
250
125
mA
OSD485
RS485 Receiver
I
Input Current (A/DO, B/RI)
(A/DO or B/RI) = 12V or –7V,
= 0V or 3.3V (Figure 3)
–100
–200
µA
kΩ
mV
IN485
V
CC
R
Input Resistance (A/DO, B/RI)
(A/DO or B/RI) = 12V or –7V,
= 0V or 3.3V (Figure 3)
125
IN485
V
CC
l
l
Differential Input Signal Threshold
Voltage (A/DO to B/RI)
–7V ≤ (A/DO or B/RI) ≤ 12V
200
–20
Input Hysteresis
B = 0V
220
–70
40
mV
mV
mV
V
Differential Input Failsafe Rising Threshold Voltage –7V ≤ (A/DO or B/RI) ≤ 12V, (A/D0 – B/RI) Rising
Input DC Failsafe Hysteresis
l
l
V
Receiver Output Low Voltage
Output Low, I(RO) = 3mA (Sinking),
0.4
0.4
OL
3V ≤ V ≤ 5.5V
L
Output Low, I(RO) = 1mA (Sinking),
1.7V ≤ V < 3V
V
L
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For more information www.linear.com/LTC2873
LTC2873
elecTrical characTerisTics The ldenotes 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
Receiver Output High Voltage
CONDITIONS
MIN
V – 0.4
TYP
MAX
UNITS
l
l
V
Output High, I(RO) = –3mA (Sourcing),
V
OH
L
3V ≤ V ≤ 5.5V
L
Output High, I(RO) = –1mA (Sourcing),
V – 0.4
L
V
1.7V ≤ V < 3V
L
l
l
l
Three-State (High Impedance) Output Current (RO) 0V ≤ RO ≤ V , V = 5.5V, RE485 = V
L
0
5
µA
mA
Ω
L
L
Short-Circuit Current (RO)
Terminating Resistor
0V ≤ RO ≤ V , V = 5.5V
135
156
L
L
R
TERM
TE485 = 0V, V = 2V,
108
120
AB
V = –7V, 0V, 10V (Figure 8)
B
RS232 Driver
l
l
l
V
OLD
V
OHD
Output Low Voltage
R = 3kΩ, V ≤ –6V
–5
5
–5.5
5.9
25
V
V
V
L
EE
EE
Output High Voltage
R = 3kΩ, V ≥ 6.5V
V
DD
L
DD
Output Short-Circuit Current
Driver Output = 0V
90
mA
RS232 Receiver
Input Threshold Voltage
l
l
l
l
l
l
0.6
0.1
1.5
0.4
2.5
1.0
0.4
V
V
Input Hysteresis
Output Low Voltage
Output High Voltage
Input Resistance
I(RO) = 1mA (Sinking), 1.7V ≤ V < 5.5V
V
L
I(RO) = –1mA (Sourcing), 1.7V ≤ V < 5.5V
V – 0.4
L
V
L
–15V ≤ B/RI ≤ 15V, 485/232 = 0V
3
5
7
kΩ
mA
Output Short-Circuit Current
V = 5.5V, 0V ≤ RO ≤ V
25
50
L
L
Logic Inputs
l
l
Threshold Voltage
Input Current
0.4
0.75 • V
V
L
0
5
µA
ESD
Interface Pins (A/DO, B/RI)
All Other Pins
Human Body Model to GND or V , Powered
26
4
kV
kV
CC
or Unpowered (Note 5)
Human Body Model (Note 5)
2873fa
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For more information www.linear.com/LTC2873
LTC2873
swiTching characTerisTics The ldenotes 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
RS485 Switching Characteristics
Maximum Data Rate
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
l
l
(Note 3) (Figure 15)
20
Mbps
ns
t
, t
Driver Propagation Delay
R
DIFF
= 54Ω, C = 100pF (Figure 4)
20
0
70
6
PLHD485 PHLD485
L
Driver Propagation Delay Difference
l
R
= 54Ω, C = 100pF (Figure 4)
ns
DIFF
L
t
– t
PHLD485
PLHD485
t
t
Driver Skew (A/DO to B/RI)
Driver Rise or Fall Time
R
R
= 54Ω, C = 100pF (Figure 4)
0
8
ns
ns
ns
SKEWD485
DIFF
L
, t
= 54Ω, C = 100pF (Figure 4)
7.5
12.5
120
RD485 FD485
DIFF
L
l
l
l
l
l
t
t
, t
, t
,
Driver Output Enable or Disable Time SHDN = V , R = 500Ω, C = 50pF,
L L L
ZLD485 ZHD485
LZD485 HZD485
DE485 ↑ and ↓ (Figure 5)
t
t
t
t
t
, t
Driver Enable from Shutdown Time
(Note 7)
DE485/F232 = V , R = 500Ω,
4
0.5
45
0
12
1
µs
μs
ns
ns
ZHSD485 ZLSD485
L
L
C = 50pF, SHDN ↑ (Figure 5)
L
, t
Driver Output Disable Into
Shutdown Time
DE485/F232 = V , R = 500Ω,
L L
HZSD485 LZSD485
C = 50pF, SHDN ↓ (Figure 5)
L
, t
Receiver Input to Output Time
C = 15pF, V = 1.5V,
85
9
PLHR485 PHLR485
L
CM
|A/DO to B/RI| = 1.5V, (Figure 6)
Differential Receiver Skew
C = 15pF (Figure 6)
L
SKEWR485
t
– t
PHLR485
PLHR485
l
l
, t
Receiver Output Rise or Fall Time
C = 15pF (Figure 6)
L
3
15
85
ns
ns
RR485 FR485
t
t
, t
, t
,
Receiver Output Enable or
Disable Time
485/232 = SHDN = V , R = 1kΩ,
12
ZLR485 ZHR485
LZR485 HZR485
L
L
C = 15pF, RE485 ↓ and ↑ (Figure 7)
L
l
l
l
t
t
t
, t
Receiver Enable from Shutdown Time 485/232 = V , RE485 = 0V, R = 1kΩ,
4
12
1
µs
µs
µs
ZHSR485 ZLSR485
L
L
(Note 7)
C = 15pF , SHDN ↑ (Figure 7)
L
, t
Receiver Output Disable Into
Shutdown Time
485/232 = V , RE485 = 0V, R = 1kΩ,
L
0.5
HZSR485 LZSR485
L
L
C = 15pF ,SHDN ↓ (Figure 7)
, t
Termination Enable or Disable
Time
485/232 = V , SHDN = V , B = 0,
100
RTEN485 RTZ485
L
L
(A/DO to B/RI) = 2V (Figure 8)
RS232 Switching Characteristics
Maximum Data Rate (Figure 15)
l
l
l
R = 3kΩ, C = 2.5nF, (Fast, Slow Modes)
100
250
1000
kbps
kbps
kbps
L
L
R = 3kΩ, C = 1nF, (Fast, Slow Modes)
L
L
R = 3kΩ, C = 0.25nF, (Fast Mode)
L
L
l
l
l
Driver Slew Rate (Figure 9)
R = 3kΩ, C = 2.5nF, (Fast, Slow Modes)
2
V/µs
V/µs
V/µs
L
L
R = 3kΩ, C = 50pF, (Slow Mode)
30
150
L
L
R = 3kΩ, C = 50pF, (Fast Mode)
L
L
l
l
t
t
t
t
, t
Driver Propagation Delay (Figure 9)
Driver Skew (Figure 9)
R = 3kΩ, C = 50pF, (Slow Mode)
1.5
0.4
3
1
µs
µs
PHLD232 PLHD232
L
L
R = 3kΩ, C = 50pF, (Fast Mode)
L
L
R = 3kΩ, C = 50pF, (Slow Mode)
0
0
400
100
ns
ns
SKEWD232
L
L
R = 3kΩ, C = 50pF, (Fast Mode)
L
L
l
l
l
, t
Driver Enable from Shutdown
Time (Note 7)
V
= 7.0V, V = –6.3V, 485/232 = 0V,
5
12
µs
ZLSD232 ZHSD232
DD
L
EE
R = 3kΩ, C = 50pF, SHDN ↑ (Figure 10)
L
, t
Driver Output Disable into
Shutdown Time
485/232 = 0V, R = 3kΩ, C = 50pF,
0.6
2
µs
LZSD232 HZSD232
L
L
SHDN ↓ (Figure 10)
t
t
t
, t
Receiver Propagation Delay
Receiver Skew
C = 150pF (Figure 11)
L
60
25
70
200
ns
ns
ns
PHLR232 PLHR232
C = 150pF (Figure 11)
L
SKEWR232
l
, t
Receiver Output Rise or Fall Time
C = 150pF (Figure 11)
L
200
RR232 FR232
2873fa
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For more information www.linear.com/LTC2873
LTC2873
swiTching characTerisTics The ldenotes 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
= 7.0V, V = –6.3V, 485/232 = 0V,
MIN
TYP
MAX
UNITS
l
l
t
, t
Receiver Enable from Shutdown
Time (Note 7)
V
5
12
µs
ZLSR232 ZHSR232
DD
EE
R = 1kΩ, C = 150pF, SHDN ↑ (Figure 12)
L
L
t
, t
Receiver Disable Into Shutdown Time 485/232 = 0V, R = 1kΩ, C = 150pF
0.4
2
μs
LZSR232 HZSR232
L
L
SHDN ↓ (Figure 12)
Mode Change Characteristics
l
l
l
l
t
t
t
t
V
and V Supply Rise Time
(Figure 13)
0.2
0.2
0.8
70
1
1
ms
µs
µs
ns
RDY
DD
EE
(Time from Shutdown to RS485 Ready)
Time from RS485 Mode to RS232 Mode (Figure 14)
RS232 Driver Ready
DR232
R232
Time from RS485 Mode to RS232 Mode (Figure 14)
RS232 Receiver Ready
3
Time from RS232 Mode to RS485
Mode RS485 Driver and Receiver Ready
(Figure 14)
250
DR485
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 5: Guaranteed by design and not subject to production test.
Note 6:Testing was done with V and V back driven to valid supply
levels for functions that require these supplies, unless otherwise noted.
DD
EE
Note 7: If enabling from shutdown, where V and V supplies are
DD
EE
collapsed, allow the extra time it takes to generate valid V and V
DD
EE
supplies (t ).
RDY
Note 3: Guaranteed by other measured parameters and not tested directly.
Note 4: Time from SHDN ↑ until V ≥ 5V and V ≤ –5V. External
DD
EE
components as shown in the Typical Application section.
TA = 25°C, VCC = VL = 3.3V, unless otherwise noted.
Typical perForMance characTerisTics
VCC Supply Current vs Supply
Voltage in Shutdown Mode
VCC Supply Current vs Supply
Voltage in RDY Mode
VCC Supply Current
vs RS485 Data Rate
100
5
4
3
2
1
15
14
13
12
11
10
9
V
CC
V
CC
= 5V
= 3.3V
RS485 DRIVER
AND RECEIVER
SWITCHING.
DRIVER, RECEIVER AND TERMINATOR
DISABLED
80
60
40
20
0
CL = 100pF ON EACH
DRIVER OUTPUT.
85°C
25°C
TE485 LOW
–40°C
8
7
TE485 HIGH
6
5
3
3.5
4
4.5
5
5.5
0.1
1
10
100
3
3.5
4
4.5
5
5.5
SUPPLY VOLTAGE (V)
DATA RATE (Mbps)
SUPPLY VOLTAGE (V)
2873 G02
2873 G03
2873 G01
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For more information www.linear.com/LTC2873
LTC2873
TA = 25°C, VCC = VL = 3.3V, unless otherwise noted.
Typical perForMance characTerisTics
VCC Supply Current vs RS232
Data Rate in Slow Mode
VCC Supply Current vs RS232
Data Rate in Fast Mode
RS485 Driver Differential Output
Voltage vs Temperature
20
16
12
8
24
20
16
12
8
4.5
DRIVER AND
DRIVER AND
V
V
= 5V
= 3.3V
V
V
= 5V
= 3.3V
CC
CC
CC
CC
R
= 100Ω
= 54Ω
L
RECEIVER ENABLED
A/DO TIED TO B/RI
VARIOUS LOADS
RECEIVER ENABLED
A/DO TIED TO B/RI
VARIOUS LOADS
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
R
L
250pF
1nF
2.5nF
2.5nF
1nF
R
= 100Ω
= 54Ω
L
75pF
R
1nF
1nF
L
250pF
75pF
2.5nF
75pF
75pF
V
CC
V
CC
= 5V
= 3.3V
2.5nF
4
4
0
50
100
150
200
250
0
200
400
600
800
1000
–50
–25
0
25
50
75
100
DATA RATE (kbps)
DATA RATE (kbps)
TEMPERATURE (°C)
2873 G04
2873 G05
2873 G06
RS485 Driver Propagation Delay
vs Temperature
RS485 Driver Skew
vs Temperature
RS485 Driver Short-Circuit
Current vs Short-Circuit Voltage
2.0
1.5
1.0
0.5
0
150
100
50
50
40
30
20
10
0
V
CC
V
CC
= 5V
= 3.3V
V
V
V
V
= 3.3V, V = 1.7V
L
CC
CC
CC
CC
= 5V, V = 1.7V
L
= 3.3V, V = 3.3V
L
= 5V, V = 5V
L
OUTPUT LOW
0
–50
–100
–150
OUTPUT HIGH
10
–0.5
–1.0
–50
–25
0
25
50
75
100
–10
–5
0
5
15
–50
–25
0
25
50
75
100
TEMPERATURE (°C)
SHORT-CIRCUIT VOLTAGE (V)
TEMPERATURE (°C)
2873 G08
2873 G09
2873 G07
RS485 Receiver Propagation
Delay vs Temperature
RS485 Receiver Skew
vs Temperature
RS485 Receiver Output Voltage
vs Load Current
2.0
1.5
1.0
0.5
0
6
5
4
3
2
1
0
80
70
60
50
40
V
V
V
= 5V
= 3.3V
= 1.7V
L
L
L
V
V
V
V
= 3.3V, V = 1.7V
L
CC
CC
CC
CC
= 5V, V = 1.7V
L
= 3.3V, V = 3.3V
L
= 5V, V = 5V
L
–0.5
–1.0
–50
–25
0
25
50
75
100
0
2
4
6
8
10
–50
–25
0
25
50
75
100
TEMPERATURE (°C)
OUTPUT CURRENT (mA)
TEMPERATURE (°C)
2873 G11
2873 G12
2873 G10
2873fa
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LTC2873
TA = 25°C, VCC = VL = 3.3V, unless otherwise noted.
Typical perForMance characTerisTics
RS232 Receiver Input Threshold
vs Temperature
RS232 Receiver Output Voltage
vs Load Current
RS485 Termination Resistance
vs Temperature
130
128
126
124
122
120
118
116
114
112
110
2.0
1.8
1.6
1.4
1.2
1.0
6
5
4
3
2
1
0
V
V
V
= 5V
= 3.3V
= 1.7V
L
L
L
V
CM
V
CM
V
CM
= –7V
= 2V
= 12V
INPUT HIGH
INPUT LOW
V
CC
V
CC
= 5V
= 3.3V
–50
–25
0
25
50
75
100
–50
–25
0
25
50
75
100
0
2
4
6
8
10
TEMPERATURE (°C)
TEMPERATURE (°C)
OUTPUT CURRENT (mA)
2873 G15
2873 G13
2873 G14
RS232 Operation at 1Mbps
FAST Mode (DE485/F232 High)
RS232 Driver Switching at 250kbps
LTC2873 Drivers Changing Modes
50pF FROM A/DO to B/RI,
TERMINATION ENABLED
DI
3V/DIV
DI
3V/DIV
DI
5V/DIV
A/DO
1V/DIV
A = B
3V/DIV
A
3V/DIV
B/RI
1V/DIV
RO
3V/DIV
RO
5V/DIV
FAST MODE
SLOW MODE
2µs/DIV
2873 G16
2873 G17
2873 G18
400ns/DIV
20ns/DIV
Transition from Shutdown to RS232
Driver Output Going High and Low
RS232/RS485 Mode Switching
VDD and VEE Ripple
DI
V
DD
RIPPLE
SHDN
485/232
5V/DIV
RS232
V
DD
RS485
10mV/DIV
A HIGH
V
RIPPLE
EE
3V/DIV
A LOW
B/RI
V
EE
A/DO
3V/DIV
2873 G20
2873 G19
2873 G21
40µs/DIV
40µs/DIV
2µs/DIV
RS485 READY MODE,
ALL DRIVERS AND RECEIVERS DISABLED
2873fa
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LTC2873
pin FuncTions
V
(Pins 1, 10, 25): Generated Negative Supply Voltage
V
(Pin 13): Generated Positive Supply Voltage for
EE
DD
forRS232Driver(–6.3V).Tie allpinstogetherandconnect
RS232 Driver (+7.0V). Connect 1µF capacitor between
1µF capacitor between V (Pin 10) and GND. Exposed
V
DD
and GND.
EE
pad (Pin 25) must be soldered to PCB to maintain low
V
(Pin 14, 18, 21): Input Supply (3V to 5.5V). Tie all
CC
thermal resistance.
threepinstogetherandconnecta2.2µForlargercapacitor
RO(Pin2):RS485DifferentialReceiverOutputandRS232
Receiver Output. Logic level referenced to GND and V .
between V (adjacent to V ) and GND.
CC DD
L
B/RI (Pin 15): RS485 Negative Receiver Input and Driver
Output. In RS232 mode, this is the RS232 receiver input.
485/232(Pin3):InterfaceSelectInput.Alogiclowenables
RS232 mode and a high enables RS485 mode. The mode
determines which transceiver inputs and outputs are ac-
cessible at the LTC2873 pins. Logic level referenced to
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.
GND and V . Do not float.
L
V (Pin 22): Logic Supply (1.7V to 5.5V) for the Receiver
RE485 (Pin4): RS485 Receiver Enable. In RS485 mode, a
logic high disables the RS485 receiver, leaving its output
Hi-ZandalogiclowenablestheRS485receiver.Thisinput
has no function in RS232 mode (485/232 low). Logic level
L
Outputs, Driver Inputs, and Control Inputs. Bypass this
pin to GND with a 0.1µF capacitor if not tied to V . Keep
CC
V ≤ V for operation guaranteed to meet specifications.
L
CC
However, V > V will not damage the device, provided
referenced to GND and V . Do not float.
L
CC
L
thatabsolutemaximumlimitsarerespected.See“V Logic
L
DE485/F232(Pin5):RS485DriverEnableandRS232Fast
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
RS232mode(485/232low),alogichighenablesFastmode
with maximum data rate of 1Mbps. A logic low enables
Slow mode with a maximum data rate of 250kbps. 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
level referenced to GND and V . Do not float.
L
DI (Pin 6): RS485 and RS232 Driver Input. Logic level
referenced to GND and V . Do not float.
L
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
and V . Do not float.
L
TE485(Pin24):RS485TerminationEnable.InRS485mode,
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.
Hi-Z. Logic level referenced to GND and V . Do not float.
L
GND(Pin8,11,16,20):Ground.Tie allfourpinstogether.
CAP(Pin9):ChargePumpCapacitorforGeneratedNegative
Supply Voltage V . Connect a 220nF capacitor between
EE
CAP and SW.
SW (Pin 12): Switch Pin. Connect 10µH inductor between
Logic level referenced to GND and V . Do not float.
L
SW and V . See Inductor Selection section for further
CC
details.
2873fa
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For more information www.linear.com/LTC2873
LTC2873
block DiagraM
1.7V TO 5.5V
(≤ V
)
3V TO 5.5V
10µH
CC
220nF
SW
0.1µF
2.2µF
V
L
V
CAP
CC
SHDN
485/232
DE485/F232
RE485
V
V
DD
EE
1µF
PULSE-SKIPPING
BOOST
REGULATOR
f = 1.2MHz
RT232
RT485
CONTROL
LOGIC
1µF
TE485
DRIVERS
LB
232
485
DI
A/DO
B/RI
RT485
125k
120Ω
LOOPBACK
PATH
125k
RECEIVERS
232
RT232
5k
RO
485
2873 BD
GND
2873fa
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For more information www.linear.com/LTC2873
LTC2873
TesT circuiTs
A/DO
A/DO OR
B/RI
I
OSD485
R
R
L
GND
DI
+
OR
DRIVER
V
OD
GND
OR
V
L
–
DI
DRIVER
B/RI
+
L
V
L
V
B/RI
OR A/DO
+
OC
V
OUT
–
–
2873 F01
2873 F02
Figure 1. RS485 Driver DC Characteristics
Figure 2. RS485 Driver Output Current
I
A/DO OR
B/RI
IN485
B/RI
OR A/DO
RECEIVER
+
V
IN
–
V
IN
R
=
IN485
I
2873 F03
IN485
Figure 3. RS485 Receiver Input Current and Resistance
V
L
t
t
PHLD485
DI
PLHD485
A/DO
0V
t
SKEWD485
B/RI
C
C
L
L
DI
V
½V
OD
OD
R
DIFF
DRIVER
A/DO
B/RI
90%
10%
90%
10%
0V
0V
A/RO – B/DI
t
t
FD485
RD485
2873 F04
Figure 4. RS485 Driver Timing Measurement
2873fa
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LTC2873
TesT circuiTs
V
L
DE485/F232
OR SHDN
½V
½V
GND
OR
L
L
R
L
L
t
,
ZLD485
0V
t
,
t
A/DO
V
CC
LZD485
ZLSD485
V
CC
t
LZSD485
C
C
L
V
OR
GND
L
½V
½V
A/DO OR B/RI
B/RI OR A/DO
CC
CC
DI
0.5V
0.5V
DRIVER
V
V
OL
OH
B/RI
R
V
OR
CC
DE485/F232
OR SHDN
0V
GND
t
,
t
,
HZD485
ZHD485
t
t
L
HZSD485
ZHSD485
2873 F05
Figure 5. RS485 Driver Enable and Disable Timing Measurements
V
AB
0V
A/DO TO B/RI
RO
V
V
/2
/2
A/DO
B/RI
AB
AB
–V
AB
t
t
PLHR485
PHLR485
RO
V
V
RECEIVER
CM
L
90%
10%
90%
10%
½V
½V
C
L
L
L
0V
t
t
RR485
FR485
t
= t
– t
SKEWR485 PLHR485 PHLR485
2873 F06
Figure 6. RS485 Receiver Propagation Delay Measurements
V
L
RE485
OR SHDN
½V
½V
L
L
0V
t
,
ZLR485
t
,
LZR485
t
ZLRSR485
t
LZSR485
A/DO
B/RI
V
L
0V TO 3V
3V TO 0V
V
OR
GND
R
L
L
RO
½V
½V
RO
L
L
RECEIVER
0.5V
0.5V
V
V
OL
OH
C
L
RE485
OR SHDN
RO
0V
t
t
,
t
,
HZR485
ZHR485
t
HZSR485
ZHRSR485
2873 F07
Figure 7. RS485 Receiver Enable and Disable Timing Measurements
V
I
AB
A
R
TERM
=
V
L
I
A
TE485
½V
½V
L
L
A/DO
B/RI
+
–
0V
RECEIVER
V
V
AB
t
t
RTZ485
RTEN485
90%
I
TE485
A
10%
+
–
B
2873 F08
Figure 8. RS485 Termination Resistance and Timing Measurements
2873fa
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LTC2873
TesT circuiTs
V
L
t
PHLD232
½V
L
½V
L
DI
A/DO
DI
t
PLHD232
0V
t
t
R
F
V
OHD
R
L
C
L
3V
3V
–3V
0V
0V
A/DO
–3V
V
OLD
6V
t OR t
t
= |t
– t
|
SLEW RATE =
SKEWD232
PHLD232 PLHD232
F
R
2873 F09
Figure 9. RS232 Driver Timing and Slew Rate Measurements
V
L
½V
½V
L
L
A/DO
SHDN
0V OR V
L
0V
t
t
HZSD232
ZHSD232
SHDN
R
C
L
L
V
OHD
0.5V
5V
A/DO
A/DO
0V
0V
t
t
LZSD232
ZLSD232
–5V
0.5V
V
OLD
2873 F10
Figure 10. RS232 Driver Enable and Disable Times
+3V
–3V
1.5V
B/RI
1.5V
B/RI
RO
t
t
PLHR232
PHLR232
V
C
L
L
90%
10%
90%
10%
RO
½V
½V
L
L
0V
t
t
RR232
FR232
t
= |t
– t
|
SKEWR232
PLHR232 PHLR232
2873 F11
Figure 11. RS232 Receiver Timing Measurements
V
L
R
L
½V
½V
RO
SHDN
L
L
GND
OR V
–3V OR +3V
0V
L
t
t
t
t
HZR232,
HZSR232
ZHR232,
ZHSR232
SHDN
C
L
V
OHR
0.5V
0.5V
RO
RO
½V
½V
L
L
0V
t
t
t
LZR232,
LZSR232
ZLR232,
ZLSR232
V
t
L
V
OLR
2873 F12
Figure 12. RS232 Receiver Enable and Disable Times
2873fa
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LTC2873
TesT circuiTs
C1
220nF
L1
10µH
V
CC
C4
2.2µF
½V
L
V
SW CAP
CC
SHDN
+6.3V
V
DI
RO
SHDN
485/232
DE485/F232
RE485
V
DD
L
C2
0V
0V
V
DD
LTC2873
1µF
STEP
V
V
EE
EE
+
C3
1µF
–
V
–5.9V
t
VEE
A/DO
B/RI
t
VDD
TE485
LB
GND
t
= MAX (t
, t
)
RDY
VDD VEE
2873 F13
Figure 13. Timing Coming Out of Shutdown Mode
C1
220nF
L1
10µH
V
CC
½V
L
½V
485/232
L
C4
2.2µF
V
SW CAP
CC
A
V
DI
RO
SHDN
485/232
DE485/F232
RE485
TE485
LB
V
DD
L
3V
C4
LTC2873
1µF
B
0V
V
EE
STEP
t
R232
C3
1µF
+
–
V
½V
½V
L
L
RO
A/DO
B/RI
CA
50pF
RA
3k
t
DR485
t
CB
50pF
R232
GND
2873 F14
Figure 14. Mode Change Timing
C1
C1
L1
10µH
L1
220nF
220nF
10µH
V
V
CC
CC
C4
C4
2.2µF
2.2µF
V
SW CAP
V
SW CAP
CC
CC
V
V
V
V
DD
L
DD
L
C2
C2
DI
RO
SHDN
485/232
DE485/F232
RE485
DI
RO
SHDN
485/232
DE485/F232
RE485
LTC2873
LTC2873
1µF
1µF
V
V
EE
EE
C3
1µF
C3
1µF
HIGH FOR FAST MODE
LOW FOR SLOW MODE
A/DO
B/RI
A/DO
B/RI
TE485
TE485
LB
GND
LB
GND
2873 F15
(a)
(b)
Figure 15. Testing Max Data Rate for (a) RS485 and (b) RS232. Observe that Data In Matches Data Out
2873fa
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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
RS232
RS485
DE485/
DC/DC
CONV.
SHDN
485/232
RE485
TE485
LB
MODE
F232
RX
TX
RX
TX
TERM
0
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
X
0
0
0
0
1
1
1
1
1
1
1
1
1
1
1
1
X
X
X
X
X
1
1
0
0
0
0
0
0
0
0
1
1
X
0
0
1
1
0
0
0
0
0
0
1
1
1
1
1
1
X
X
X
X
X
1
0
0
0
1
1
0
0
1
1
0
1
X
0
1
0
1
X
X
0
1
0
1
0
1
0
1
X
X
SHUTDOWN
RS232 SLOW
l
l
l
l
l
l
l
l
l
l
l
l
l
l
l
l
l
LB
l
l
l
l
l
RS232
FAST
LB
RS485
READY
l
l
l
l
LB
l
LB
l
l
l
l
l
l
l
l
l
RS485
LB
l
LB
l
2873fa
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For more information www.linear.com/LTC2873
LTC2873
applicaTions inForMaTion
The LTC2873 is a flexible multiprotocol transceiver sup-
porting 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.
Thisdevicecanbepoweredfromasingle3Vto5.5Vsupply
withoptionallogicinterfacesupplyaslowas1.7V.Aninte-
gratedDC/DCconverterprovidesthepositiveandnegative
supply rails needed for RS232 operation. Automatically
selected integrated termination resistors for both RS232
andRS485protocolsareincluded,eliminatingtheneedfor
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 V
CC
input, generating a 7.0V V
supply and a charge
DD
pumped –6.3V V supply, as shown in Figure 16. V
EE
DD
and V power the output stage of the RS232 drivers
EE
and are regulated to levels that guarantee greater than
5V output swing.
TheLTC2873businterfaceisasingletwo-pinportthatcan
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
andreceivercanbeenabledindependentlywiththeDE485/
F232 and RE485 pins, or by tying these signals together, a
single control selects transmit or receive modes. A 120Ω
terminationresistorisautomaticallyengagedbetweenpins
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.
BypasscapacitorC5onthelogicsupplypincanbeomitted
if V is connected to V . See the V Logic Supply section
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.
L
CC
L
for more details about the V logic supply.
L
V
CC
C1
220nF
L1
10µH
3V TO 5.5V
C4
2.2µF
22
14
12
9
V
L
V
SW
CAP
CC
1.7V TO V
CC
V
DD
V
13
10
L
C2
1µF
C5
0.1µF
BOOST
REGULATOR
V
EE
GND
20
GND
C3
1µF
11
2873 F16
NOTE: NOT ALL PINS SHOWN. IN THE CASE OF DUPLICATE PINS FOR V
,
CC
GND, AND V , EXTERNAL COMPONENTS SHOULD BE POSITIONED
EE
CLOSEST TO THE NUMBERED PIN SHOWN ABOVE.
Figure 16. Simplified DC/DC Converter with Required External Components
2873fa
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LTC2873
applicaTions inForMaTion
Powering Multiple Devices
ESR is low and they retain their capacitance over relatively
wide voltage and temperature ranges. Use a voltage rating
of at least 10V.
MultipleLTC2873devicescanbepoweredusingtheboost
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:
Running with External V and V Supplies
DD
EE
The inductor and charge pump cap, C1, can be omitted
onlyifV andV areexternallysupplied.Bypasscapson
DD
EE
1. No more than four RS232 drivers can be supplied from
a single device.
V
and V must remain in place. In this circumstance,
DD
EE
groundtheSWpinandfloattheCAPpin. Externalsupplies
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
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 V and V bypass caps
DD
EE
must be increased to 2.2µF.
3. Ground the SW pin on devices with inactive boost
converters.
some switching noise but will not harm the part. V
DD
and V supplies must be present for proper operation in
EE
4. Connect CAP pins together for all devices.
RS232 mode and in RS485 mode when the termination
is enabled. It is okay to run the LTC2873 in RS485 mode
5. Connect V pins together for all devices.
EE
withinternalterminationdisabled(TE485high),whenV
DD
6. Connect V pins together for all devices.
DD
and V are not present or fully settled.
EE
Figures 32 shows an example of how to connect four
devices.
Inrush Current and Supply Overshoot Precaution
Incertainapplications,fastsupplyslewratesaregenerated
Inductor Selection
when power is connected. If the V voltage is greater
CC
than 4.5V and its rise time is faster than 10µs, the pins
A10µHor22μH( 20ꢀ)inductorwithasaturationcurrent
V
and SW can exceed their absolute maximum values
(I ) rating of at least 220mA and a DCR (copper wire
DD
SAT
duringstart-up. WhensupplyvoltageisappliedtoV , the
resistance) of less than 1.3Ω is required. Some very small
CC
voltage difference between V and V generates inrush
inductorsmeetingtheserequirementsarelistedinTable1.
CC
DD
currentflowingthroughinductorL1andcapacitorsC1and
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.
Capacitor Selection
The small size of ceramic capacitors makes them ideal
for the LTC2873. Use X5R or X7R dielectric types; their
Table 1. Recommended Inductors
PART NUMBER
74479888310
L (μH)
10
I
(mA)
MAX DCR (Ω)
SIZE (mm)
2.5 × 2 × 1
MANUFACTURER
Wurth Elektronik
SAT
250
0.5
1.07
1.0
CBC2016T100K (or M)
CBC2518T220K (or M)
BRC2016T220K (or M)
10
380
320
310
2 × 1.6 × 1.6
2.5 × 1.8 × 1.8
2 × 1.6 × 1.6
Taiyo Yuden
www.t-yuden.com
22
22
1.3
Murata
www.murata.com
LQH32CN220K53
22
250
0.92
3.2 × 2.5 × 1.6
2873fa
17
For more information www.linear.com/LTC2873
LTC2873
applicaTions inForMaTion
5V
greater than 96kΩ (typically 125kΩ) to ground over the
entirecommonmoderangeof–7Vto+12V.Thisresistance
is actually the RS485 receiver input resistance, which is
connected to the same pins.
0V
≤10µs
R1
C1
220nF
L1
10µH
1Ω
1/8W
INRUSH
RS232 Driver with Speed Selection
C4
2.2µF
CURRENT
SW
CAP
GND
12
13
9
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 driv-
ers, it is inverting, so that when the input, DI, is low, the
output, A/DO, is high, and vice-versa.
V
CC
14
2873 F17
11
V
DD
C2
1µF
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,comparedwithRS485,radiatedemissionsmay
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/
F232high.EveninFastmodethedrivertransitionsareslew
controlledtominimizeemissions.SeeTypicalPerformance
Characteristics section for examples of the waveforms.
Figure 17. Supply Current Overshoot Protection for
Input Supplies of 4.5V or Higher and Rise Times Faster
Than 10μs
V Logic Supply and Logic Pins
L
A separate logic supply pin V allows the LTC2873 to
L
interface with any logic signal from 1.7V to 5.5V. All logic
I/Os use V as their high supply. For proper operation, V
L
L
L
should not be greater than V . During power-up, if V
CC
is higher than V , the device will not be damaged, but
CC
Driver Overvoltage and Overcurrent Protection
behavior of the device is not guaranteed. In particular,
supply currents can be somewhat higher than specified.
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
If V is not connected to V , bypass V with a 0.1µF
L
CC
L
capacitor to GND.
RS232 and RS485 driver outputs are undriven and the
RS485 termination resistors are disabled when V or V
L
CC
greaterthanV ,whenitisactive,positivecurrentofabout
is grounded or V is disconnected.
CC
CC
100mA can flow from the driver output back to V . If the
CC
Although all logic input pins reference V as their high
L
system power supply or loading cannot sink this excess
supply, they can be driven up to 7V, independent of V
L
current, clamp V to GND with a Zener diode (e.g., 5.6V,
CC
and V , with the exception of SHDN, which must not
CC
1W, 1N4734) to prevent an overvoltage condition on V .
CC
exceed V by more than 0.3V. Logic input pins do not
L
All devices also feature thermal shutdown protection that
disables the drivers, receivers, and RS485 terminators in
case of excessive power dissipation during momentary
overloadconditions.Overtemperatureprotectionactivates
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.
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
TheRS485driverprovidesfullRS485/RS422compatibility.
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
2873fa
18
For more information www.linear.com/LTC2873
LTC2873
Typical applicaTions
RS485 Balanced Receiver with Full Failsafe Operation
B
200mV/DIV
A
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
Figure18.Ifadifferentialinputsignallingersinthewindow
between these thresholds for more than about 1.3µs, the
rising threshold changes from +110mV to –70mV, while
thefallingthresholdremainsat–110mV.Thus,differential
inputs that are shorted, open, or terminated but not driven
for morethan1.3µs produce a high on thereceiveroutput,
indicating a failsafe condition.
(A-B)
200mV/DIV
RO
5V/DIV
2873 F19
200ns/DIV
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
RA
RISING THRESHOLD
SHIFTS IF SIGNAL IS
IN WINDOW > ~1.3µs
TO SUPPORT
FAILSAFE
RS485 Biasing Network Not Required
A/DO TO B/RI
–110mV –70mV
0V
+110mV
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.
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 sig-
nal peaks are at only 200mV and is heavily slewed, the
output maintains a nearly perfect signal with almost no
duty cycle distortion.
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.
An additional benefit of the balanced architecture is excel-
lent noise immunity due to the wide effective differential
input signal hysteresis of 220mV for signals transitioning
throughthewindowregioninlessthan1.3µs.Increasingly
slower signals will have increasingly less effective hyster-
esis, limited by the DC failsafe hysteresis of about 40mV.
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
2873fa
19
For more information www.linear.com/LTC2873
LTC2873
applicaTions inForMaTion
Selectable RS485 Termination
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.
Propercableterminationisimportantforgoodsignalfidel-
ity. Whenthecableisnotterminatedwithitscharacteristic
impedance, reflections cause waveform distortion.
The LTC2873 offers an integrated switchable 120Ω ter-
mination resistor between pins A/DO and B/RI.
Receiver Outputs
This termination supports communication over a twisted
paircablewithcharacteristicimpedanceof120Ωor100Ω,
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
toleratedintheoverallsystemleveltiming.Ifthedelaysare
not acceptable, tie TE485 low to enable termination for all
modes of RS485 operation, whether driving or receiving.
The RS232 and RS485 receiver outputs are internally
driven high (to V ) or low (to GND) with no external pull
L
up needed. When the receivers are disabled the output pin
becomes Hi-Z with leakage of less than 5µA for voltages
within the V supply range.
L
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 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 loopback mode, or unpowered.
The equivalent input resistance looking into the RS485
receiver pins is shown in Figure 20.
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.
125k
A/DO
60Ω
TE485
60Ω
125k
126
V
CC
V
CC
= 5.0V
= 3.3V
B/RI
2873 F20
124
122
120
118
116
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.
–10
–5
0
5
10
15
VOLTAGE (V)
2873 F21
Figure 21. Typical Resistance of the Enabled RS485
Terminator vs Common Mode Voltage of A/DO and B/RI
2873fa
20
For more information www.linear.com/LTC2873
LTC2873
applicaTions inForMaTion
Logic Loopback
Robust ESD Protection
TheLTC2873featuresexceptionallyrobustESDprotection.
The transceiver interface pins (A and B) are protected to
26kV human body model with respect to GND, V , or V
Aloopbackmodeconnectsthedriverinputstothereceiver
outputs(non-inverting)providinganechoforself-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 cir-
cuit 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.
CC
L
without latchup or damage. This protection holds whether
thedeviceisunpoweredorpoweredinanymodeofopera-
tion. 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
2873fa
21
For more information www.linear.com/LTC2873
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.
Layout Considerations
All V pins must be connected together and all ground
CC
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
At frequencies below 100kbps, the maximum cable length
is determined by DC resistance in the cable. In this ex-
ample, a cable longer than 4000ft will attenuate the signal
at the far end to less than what can be reliably detected
by the receiver.
7mm away from V Pin 14. This V pin, as well as GND
CC
CC
Pin 11, mainly service the DC/DC converter. Additional
bypass capacitors of 0.1µF or larger, can be added from
V
pin 18 to ground pin 16 if the traces back to the 2.2µF
CC
capacitor are indirect or narrow. These V and ground
CC
Fordataratesabove100kbps, thecapacitiveandinductive
properties of the cable begin to dominate this relation-
ship. 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.
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)
GND (11)
GND (11)
GND (11)
GND (20)
GND (16)
GND (20)
COMMENT
Required
Required
Required
Required*
Optional
2.2
1.0
1.0
0.1
0.1
0.1
V
(14)
(13)
(10)
CC
DD
V
V
EE
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.
V (22)
L
V
(18)
(21)
.
CC
CC
V
Optional
*If V is not connected to V
L
CC
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.
10k
The PC board traces connected to high speed bus signals
A/DO and B/RI should be symmetrical and as short as
possibletominimizecapacitiveimbalanceandtomaintain
good differential signal integrity. To minimize capacitive
loadingeffects,thedifferentialsignalsshouldbeseparated
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.
1k
LTC2873
MAX DATA RATE
100
RS485/RS422
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.
MAX DATA RATE
10
10k
100k
1M
10M
100M
DATA RATE (bps)
2873 F23
Figure 23. Cable Length vs Data Rate (RS485/RS422
Standard Shown in Vertical Solid Line)
2873fa
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For more information www.linear.com/LTC2873
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
5k
RO
B/RI
5k
RO
B/RI
5k
H
L
L
X
X
L
H
L
H
X
X
L
H
L
X
X
X H
2873 F24
Figure 24. RS232 Configurations
RS485 RX
RS485 TX
RS485 TX + RX
LTC2873
LTC2873
LTC2873
DI
DI
A/DO
B/RI
DI
A/DO
B/RI
A/DO
B/RI
RO
RO
RO
H
H
H
L
H
L
H
H
H
H
H
L
H
H
H
L
H
L
RS485 RX + TERM
RS485 TX + TERM
RS485 TX + RX + TERM
LTC2873
LTC2873
LTC2873
A/DO
DI
A/DO
DI
A/DO
DI
120Ω
B/RI
120Ω
B/RI
120Ω
B/RI
RO
RO
RO
H
H
L
L
L
L
H
H
H
H
L
L
H
H
H
L
L
L
2873 F25
Figure 25. RS485 Configurations
2873fa
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For more information www.linear.com/LTC2873
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
DI
A/DO
B/RI
DI
A/DO
120Ω
B/RI
RO
RO
H
H
L
L
H
H
H
H
H
L H H
RS485 LOOPBACK + TERM
RS485 LOOPBACK + TX + TERM
LTC2873
LTC2873
DI
A/DO
120Ω
DI
A/DO
120Ω
B/RI
B/RI
RO
RO
H
H
H
L
L
H
H
H
H
L
L H
2873 F26
Figure 26. RS485 + Loopback Configurations
SHUTDOWN
RS485 READY
RS485 READY + TERM
LTC2873
LTC2873
LTC2873
DI
A/DO
B/RI
DI
A/DO
B/RI
DI
A/DO
DC/DC
CONV
OFF
DC/DC
CONV
ON
DC/DC
CONV
ON
120Ω
B/RI
RO
RO
RO
L
X
X
X
X
X
H
H
L
H
H
L
H
H
L
H
L
L
2873 F27
Figure 27. Shutdown, RS485 Ready and RS485 Ready + Term Configurations
2873fa
24
For more information www.linear.com/LTC2873
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
B/RI
DI
RO
LTC2873
LTC2873
H
H
H
L
D R
DI
A/DO
A/DO
DI
120Ω
B/RI
120Ω
B/RI
RO
RO
LTC2873
DI
A/DO
H
H
L
L
H
H
L
L
B/RI
RO
D R
D R
H
H
H
L
2873 F28
D R
Figure 28. Typical RS485 Half Duplex Network
LTC2873
LTC2873
A/DO
B/RI
DI
DI
A/DO
RO
RO
B/RI
5k
5k
H
L
X
X
L
H
L
X
X
L
FAST SLOW
FAST SLOW
2873 F29
Figure 29. Typical RS232 Communications Link
2873fa
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For more information www.linear.com/LTC2873
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 V
CC
LTC2873
V
V
CC
L
µP
LOGIC
LEVEL
SIGNALS
LINE
LEVEL
SIGNALS
RS232
AND/OR
RS485
GND
2873 F30
Figure 30. Low Voltage Microprocessor Interface
LTC2873
A/DO
120Ω
DI
RO
B/RI
DE485/F232
Y
RE485
TE485
X
H
L
MODE
RS485 RECEIVE TRANSMIT
RS232 SLOW FAST
L
H
Y
X
485/232
2873 F31
H
H
Figure 31. Receiver-Only RS485 Termination for Power Savings
2873fa
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LTC2873
Typical applicaTions
NUMBER OF
MINIMUM
C4-X
APPLICATION
LTC2873 DEVICES
L1
C1
C2
1µF
C3
1µF
SINGLE TRANSCEIVER
DUAL TRANSCEIVER
TRIPLE TRANSCEIVER
QUAD TRANSCEIVER
1
2
3
4
10µH
10µH
22µH
22µH
220nF
220nF
470nF
470nF
2.2µF
2.2µF
1µF
1µF
1µF
2.2µF
2.2µF
2.2µF
2.2µF
1µF
L1
22µH
3V TO 5.5V
C4-A
C1
LTC2873
1µF
470nF
V
SW
CAP
CC
L
V
V
DD
C2
2.2µF
SHDN
SHDN
V
EE
C3
2.2µF
485/232-A
DX485-A
RX485-A
TE485-A
LB
485/232
DE485/F232
RE485
TE485
GND
LB
A
DI-A
RO-A
DI
RO
A/DO
B/RI
A/DO-A
B/RI-A
LTC2873
C4-B
V
SW
CC
1µF
V
L
CAP
V
DD
SHDN
V
EE
485/232-B
DX485-B
RX485-B
TE485-B
485/232
DE485/F232
RE485
TE485
GND
LB
B
DI-B
RO-B
DI
RO
A/DO
B/RI
A/DO-B
B/RI-B
LTC2873
C4-C
V
SW
CC
1µF
V
L
CAP
V
DD
SHDN
V
EE
485/232-C
DX485-C
RX485-C
TE485-C
485/232
DE485/F232
RE485
TE485
GND
LB
C
DI-C
RO-C
DI
RO
A/DO
B/RI
A/DO-C
B/RI-C
LTC2873
C4-D
V
SW
CC
1µF
V
L
CAP
V
DD
SHDN
V
EE
485/232-D
DX485-D
RX485-D
TE485-D
485/232
DE485/F232
RE485
TE485
GND
LB
D
DI-D
RO-D
DI
RO
A/DO
B/RI
A/DO-D
B/RI-D
2873 F32
Figure 32. Quad Transceiver
2873fa
27
For more information www.linear.com/LTC2873
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-ꢀ696 Rev A)
0.70 0.05
4.50 0.05
3.ꢀ0 0.05
2.65 0.05
2.00 REF
3.65 0.05
PACKAGE OUTLINE
0.25 0.05
0.50 BSC
3.00 REF
4.ꢀ0 0.05
5.50 0.05
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS
APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED
R = 0.05 TYP
PIN ꢀ NOTCH
2.00 REF
R = 0.20 OR C = 0.35
R = 0.ꢀꢀ5
TYP
0.75 0.05
4.00 0.ꢀ0
(2 SIDES)
23
24
0.40 0.ꢀ0
PIN ꢀ
TOP MARK
(NOTE 6)
ꢀ
2
5.00 0.ꢀ0
(2 SIDES)
3.00 REF
3.65 0.ꢀ0
2.65 0.ꢀ0
(UFD24) QFN 0506 REV A
0.25 0.05
0.200 REF
0.50 BSC
0.00 – 0.05
BOTTOM VIEW—EXPOSED PAD
NOTE:
ꢀ. 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.ꢀ5mm ON ANY SIDE
5. EXPOSED PAD SHALL BE SOLDER PLATED
6. SHADED AREA IS ONLY A REFERENCE FOR PIN ꢀ LOCATION
ON THE TOP AND BOTTOM OF PACKAGE
2873fa
28
For more information www.linear.com/LTC2873
LTC2873
revision hisTory
REV
DATE
DESCRIPTION
PAGE NUMBER
A
05/16 Applied Note 7 to t
, t
.
6
9
ZLSR232 ZHSR232
Added Exposed Pad soldering requirement to V pin description.
EE
Corrected recommended Wurth inductor part number.
17
2873fa
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.
29
LTC2873
Typical applicaTion
3500VRMS Isolated RS485/RS232 Transceiver
ISOLATED
5V IN
LTM2892-S
3V TO
220nF
10μH
V
V
V
CC2
CC1
5.5V
10k
2.2μF
ISO
V
L1
ON1
L2
V
CAP
SW
V
L
CC
ON2
IND
INE
RO
OUTD
OUTE
OUTF
INC
V
DD
1μF
ISO
1μF
ISO
RS485
SHDN
485/232
INF
V
LTC2873
EE
485/232
DE485/F232
OUTC
OUTB
OUTA
EOUTA
GND2
GND2
RS485 D R
INB
INA
EOUTD
GND1
GND1
RE485
TE485
DI
RS232
RS485
(485/232 (485/232
HIGH) LOW)
DI
A/DO
B/RI
RO
LB
120Ω
5k
RS232
GND
2873 TA02
ISOLATED
GND IN
ISO
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
LTC1387
Single 5V RS232/RS485 Multiprotocol Transceiver
Single 5V RS232/RS485 Multiprotocol Transceiver
Dual Port, Single 5V Supply, Configurable, 10kV ESD
Single Port, Configurable
LTC2801/LTC2802/ 1.8V to 5.5V RS232 Single and Dual Transceivers
LTC2803/LTC2804
Up to 1Mbps, 10kV ESD, Logic Supply Pin, Tiny DFN Packages
LTC2854/LTC2855
LTC2859/LTC2861
LTM®2881
3.3V 20Mbps RS485 Transceiver with Integrated
Switchable Termination
3.3V Supply, Integrated, Switchable, 120Ω Termination Resistor,
25kV ESD
20Mbps RS485 Transceiver with Integrated
Switchable Termination
Complete Isolated RS485/RS422 µModule®
Transceiver + Power
5V Supply, Integrated, Switchable, 120Ω Termination Resistor,
15kV ESD
20Mbps, 2500V
Isolation with Integrated DC/DC Converter,
RMS
Integrated Switchable 120Ω Termination Resistor, 15kV ESD
LTM2882
Dual Isolated RS232 µModule Transceiver + Power
1Mbps, 2500V
10kV ESD
Isolation with Integrated DC/DC Converter,
RMS
2873fa
LT 0516 REV A • PRINTED IN USA
LinearTechnology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
30
(408)432-1900 FAX: (408) 434-0507 www.linear.com/LTC2873
●
●
LINEAR TECHNOLOGY CORPORATION 2016
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