LTM2881IV-3PBF [Linear]
Complete Isolated RS485/RS422 μModule Transceiver + Power; 完整的隔离型RS485 / RS422微型模块收发器+电源
| 型号： | LTM2881IV-3PBF |
| 厂家： | 
Linear |
| 描述： | Complete Isolated RS485/RS422 μModule Transceiver + Power |
| 文件： | 总20页 (文件大小：313K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
LTM2881
Complete Isolated
RS485/RS422 µModule
Transceiver + Power
FEATURES
DESCRIPTION
n
Isolator ꢀModule Technology
The LTM®2881 is a complete galvanically isolated full-
duplex RS485/RS422 μModule® transceiver. No external
components are required. A single supply powers both
sides of the interface through an integrated, isolated, low
noise, efficient 5V output DC/DC converter.
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Isolated RS485/RS422 Transceiver: 2500V
RMS
n
Integrated Isolated DC/DC Converter: 1W, 62% Efficiency
n
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n
n
n
n
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No External Components Required
20Mbps or Low EMI 250kbps Data Rate
High ESD: 15kV HꢁM on Transceiver Interface
High Common Mode Transient Immunity: 30kV/μs
Integrated Selectable 120Ω Termination
Coupledinductorsandanisolationpowertransformerprovide
2500V
of isolation between the line transceiver and the
RMS
logic interface. This device is ideal for systems where the
ground loop is broken allowing for large common mode
voltagevariation.Uninterruptedcommunicationisguaranteed
for common mode transients greater than 30kV/ꢀs.
3.3V ꢁLTM2881-3ꢂ or 5.0V ꢁLTM2881-5ꢂ Operation
1.62V to 5.5V Logic Supply Pin for Flexible Digital Interface
Common Mode Working Voltage: 560V
PEAK
High Input Impedance Failsafe RS485 Receiver
Current Limited Drivers and Thermal Shutdown
Compatible with TIA/EIA-485-A Specification
High Impedance Output During Internal Fault Condition
Low Current Shutdown Mode ꢁ< 10μAꢂ
Maximum data rates are 20Mbps or 250kbps in slew
limited mode. Transmit data, DI and receive data, RO, are
implemented with event driven low jitter processing. The
receiver has a one-eighth unit load supporting up to 256
nodes per bus. A logic supply pin allows easy interfacing
with different logic levels from 1.62V to 5.5V, independent
of the main supply.
General Purpose CMOS Isolated Channel
Small, Low Profile ꢁ15mm × 11.25mm × 2.8mmꢂ
Surface Mount LGA Package
Enhanced ESD protection allows this part to withstand up
to 15kVꢁhumanbodymodelꢂonthetransceiverinterface
pins to isolated supplies and 10kV through the isolation
barrier to logic supplies without latch-up or damage.
APPLICATIONS
n
Isolated RS485/RS422 Interface
Industrial Networks
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Breaking RS485 Ground Loops
L, LT, LTC, LTM, Linear Technology, μModule and the Linear logo are registered trademarks of
Linear Technology Corporation. All other trademarks are the property of their respective owners.
TYPICAL APPLICATION
Isolated Half-Duplex RS485 μModule Transceiver
LTM2881 Operating Through 35kV/μs CM Transient
3.3V
MULTIPLE SWEEPS
OF COMMON MODE
TRANSIENTS
V
CC
LTM2881
PWR
500V/DIV
V
L
A
B
RO
DI
RE
RO
TWISTED-PAIR
CABLE
1V/DIV
1V/DIV
TE
DE
Y
Z
DI
2881 TA01a
50ns/DIV
GND
GND2
2881 TA01
2881f
1
LTM2881
ABSOLUTE MAXIMUM RATINGS
PIN CONFIGURATION
(Note 1)
TOP VIEW
V
V
to GND ..................................................–0.3V to 6V
CC2
V to GND ....................................................–0.3V to 6V
CC
1
2
3
4
5
6
7
8
to GND2...............................................–0.3V to 6V
D
TE DI DE RE RO
V
L
ON
OUT
L
A
B
C
D
E
F
Interface Voltages
ꢁA, B, Y, Zꢂ to GND2........................V
–15V to 15V
V
CC2
GND
CC
Signal Voltages ON, RO, DI, DE,
RE, TE, D
to GND......................... –0.3V to V +0.3V
L
OUT
Signal Voltages SLO,
D to GND2....................................–0.3V to V +0.3V
G
H
J
IN
CC2
Operating Temperature Range
LTM2881C................................................ 0°C to 70°C
LTM2881I.............................................–40°C to 85°C
Storage Temperature Range...................–55°C to 125°C
Peak Reflow Temperature ꢁSoldering, 10 secꢂ ...... 245°C
GND2
K
L
D
IN
SLO
Y
Z
B
A
V
CC2
LGA PACKAGE
32-PIN ꢁ15mm s 11.25mm s 2.8mmꢂ
= 125°C, θ = 32°C/W WEIGHT = 1
g
T
JMAX
JA
ORDER INFORMATION
LEAD FREE FINISH
LTM2881CV-3#PBF
LTM2881IV-3#PBF
LTM2881CV-5#PBF
LTM2881IV-5#PBF
TRAY
PART MARKING*
LTM2881V-3
LTM2881V-3
LTM2881V-5
LTM2881V-5
PACKAGE DESCRIPTION
TEMPERATURE RANGE
0°C to 70°C
LTM2881CV-3#PBF
LTM2881IV-3#PBF
LTM2881CV-5#PBF
LTM2881IV-5#PBF
32-Pin ꢁ15mm × 11.25mm × 2.8mmꢂ LGA
32-Pin ꢁ15mm × 11.25mm × 2.8mmꢂ LGA
32-Pin ꢁ15mm × 11.25mm × 2.8mmꢂ LGA
32-Pin ꢁ15mm × 11.25mm × 2.8mmꢂ LGA
–40°C to 85°C
0°C to 70°C
–40°C to 85°C
Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container.
For more information on lead free part marking, go to: http://www.linear.com/leadfree/
This product is only offered in trays. For more information go to: http://www.linear.com/packaging/
2881f
2
LTM2881
ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. LTM2881-3 VCC = 3.3V, LTM2881-5 VCC = 5.0V, VL = 3.3V, GND = GND2 =
0V, ON = VL unless otherwise noted.
SYMꢁOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
Power Supply
l
l
V
V
CC
Supply Voltage
LTM2881-3
LTM2881-5
3.0
4.5
3.3
5.0
3.6
5.5
V
V
CC
l
l
V
V Supply Voltage
1.62
5.5
10
V
L
L
I
I
V
CC
V
CC
Supply Current in Off Mode
Supply Current in On Mode
ON = 0V
0
μA
CCPOFF
l
l
LTM2881-3 DE = 0V, RE = V , No Load
LTM2881-5 DE = 0V, RE = V , No Load
20
15
25
19
mA
mA
CCS
L
L
l
V
V
Regulated V
Output Voltage, Loaded
Output Voltage, No Load
LTM2881-3 DE = 0V, RE = V , I
= 100mA
= 180mA
4.7
4.7
5.0
5.0
V
V
CC2
CC2
L
LOAD
LOAD
LTM2881-5 DE = 0V, RE = V , I
L
Regulated V
Efficiency
DE = 0V, RE = V , No Load
4.8
5.0
50
5.35
250
V
%
CC2NOLOAD
CC2
L
I
= 100mA ꢁNote 2ꢂ
CC2
l
I
V
Short-Circuit Current
DE = 0V, RE = V , V
= 0V
mA
CC2S
CC2
L
CC2
Driver
l
l
l
|V
OD
|
Differential Driver Output Voltage
R = ∞ ꢁFigure 1ꢂ
R = 27Ω ꢁRS485ꢂ ꢁFigure 1ꢂ
R = 50Ω ꢁRS422ꢂ ꢁFigure 1ꢂ
V
CC2
V
CC2
V
CC2
V
V
V
1.5
2
l
Δ|V
|
OD
Difference in Magnitude of Driver Differential R = 27Ω or R = 50Ω ꢁFigure 1ꢂ
Output Voltage for Complementary Output
States
0.2
V
l
l
V
Driver Common Mode Output Voltage
R = 27Ω or R = 50Ω ꢁFigure 1ꢂ
R = 27Ω or R = 50Ω ꢁFigure 1ꢂ
3
V
V
OC
Δ|V
|
Difference in Magnitude of Driver
Common Mode Output Voltage for
Complementary Output States
0.2
OC
l
l
I
I
Driver Three-State ꢁHigh Impedanceꢂ Output DE = 0V, ꢁY or Zꢂ = –7V, +12V
Current on Y and Z
10
μA
OZD
Maximum Driver Short-Circuit Current
–7V ≤ ꢁY or Zꢂ ≤ 12V ꢁFigure 2ꢂ
–250
250
mA
OSD
Receiver
l
l
l
l
l
R
Receiver Input Resistance
RE = 0V or V , V = –7V, –3V, 3V, 7V,
96
125
120
kΩ
Ω
IN
L
IN
12V ꢁFigure 3ꢂ
R
TE
Receiver Termination Resistance Enabled
Receiver Input Current ꢁA, Bꢂ
TE = V , V = 2V, V = –7V, 0V, 10V
108
156
125
L
AB
B
ꢁFigure 8ꢂ
I
ON = 0V V = 0V or 5V, V = 12V
μA
μA
V
IN
CC2
IN
ꢁFigure 3ꢂ
ON = 0V V = 0V or 5V, V = –7V
–100
–0.2
CC2
IN
ꢁFigure 3ꢂ
V
TH
Receiver Differential Input Threshold Voltage –7V ≤ B ≤ 12V
ꢁA-Bꢂ
0.2
ΔV
TH
Receiver Input Failsafe Hysteresis
Receiver Input Failsafe Threshold
B = 0V
B = 0V
25
mV
V
–0.2
–0.05
0
Logic
l
V
V
Logic Input Low Voltage
Logic Input High Voltage
1.62V ≤ V ≤ 5.5V
0.4
V
IL
L
l
l
D
IN
0.67•V
2
V
V
IH
CC2
SLO
DI, TE, DE, ON, RE:
l
l
V ≥ 2.35V
0.67•V
0.75•V
V
V
L
L
L
1.62V ≤ V < 2.35V
L
2881f
3
LTM2881
ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. LTM2881-3 VCC = 3.3V, LTM2881-5 VCC = 5.0V, VL = 3.3V, GND = GND2 =
0V, ON = VL unless otherwise noted.
SYMꢁOL
PARAMETER
CONDITIONS
MIN
TYP
0
MAX
UNITS
μA
l
I
Logic Input Current
Logic Input Hysteresis
Output High Voltage
1
INL
V
V
ꢁNote 2ꢂ
150
mV
V
HYS
l
l
Output High, I
= –4mA
V –0.4
L
OH
LOAD
ꢁSourcingꢂ, 5.5V ≥ V ≥ 3V
L
Output High, I
= –1mA
V –0.4
L
V
LOAD
ꢁSourcingꢂ, 1.62V ≤ V < 3V
L
l
l
V
OL
Output Low Voltage
Output Low, I
= 4mA
0.4
0.4
V
V
LO AD
ꢁSinkingꢂ, 5.5V ≥ V ≥ 3V
L
Output High, I
ꢁSinkingꢂ, 1.62V ≤ V < 3V
= 1mA
LOAD
L
l
l
I
I
Three-State ꢁHigh Impedanceꢂ Output Current RE = V , 0V ≤ RO ≤ V
1
μA
OZR
L
L
on RO
Short-Circuit Current
0V ≤ ꢁRO or D ꢂ ≤ V
85
mA
OSR
OUT
L
SWITCHING CHARACTERISTICS The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. LTM2881-3 VCC = 3.3V, LTM2881-5 VCC = 5.0V, VL = 3.3V, GND = GND2 =
0V, ON = VL unless otherwise noted.
SYMꢁOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
Driver SLO = V
CC2
f
Maximum Data Rate
Driver Input to Output
ꢁNote 3ꢂ
20
Mbps
ns
MAX
l
l
l
l
l
t
t
R
= 54Ω, C = 100pF
60
1
85
8
PLHD
PHLD
DIFF
L
ꢁFigure 4ꢂ
Δt
Driver Input to Output Difference
R
= 54Ω, C = 100pF
ns
ns
ns
ns
PD
DIFF
L
|t
PLHD
– t
|
PHLD
ꢁFigure 4ꢂ
t
Driver Output Y to Output Z
R
= 54Ω, C = 100pF
1
8
SKEWD
DIFF
L
ꢁFigure 4ꢂ
t
t
Driver Rise or Fall Time
R
= 54Ω, C = 100pF
4
12.5
170
RD
FD
DIFF
L
ꢁFigure 4ꢂ
t
t
, t
,
Driver Output Enable or Disable Time
R = 500Ω, C = 50pF
ZLD ZHD
L
L
, t
ꢁFigure 5ꢂ
LZD HZD
Driver SLO = GND2
f
Maximum Data Rate
Driver Input to Output
ꢁNote 3ꢂ
250
kbps
μs
MAX
t
t
R
= 54Ω, C = 100pF
1
1.55
500
500
1.5
PLHD
PHLD
DIFF
L
ꢁFigure 4ꢂ
Δt
Driver Input to Output Difference
R
= 54Ω, C = 100pF
50
ns
ns
μs
ns
PD
DIFF
L
|t
PLHD
– t
|
PHLD
ꢁFigure 4ꢂ
t
Driver Output Y to Output Z
R
= 54Ω, C = 100pF
200
0.9
SKEWD
DIFF
L
ꢁFigure 4ꢂ
l
l
t
t
Driver Rise or Fall Time
R
DIFF
= 54Ω, C = 100pF
RD
FD
L
ꢁFigure 4ꢂ
t
t
, t
,
Driver Output Enable or Disable Time
R = 500Ω, C = 50pF
400
ZLD ZHD
L
L
, t
ꢁFigure 5ꢂ
LZD HZD
2881f
4
LTM2881
SWITCHING CHARACTERISTICS The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. LTM2881-3 VCC = 3.3V, LTM2881-5 VCC = 5.0V, VL = 3.3V, GND = GND2 =
0V, ON = VL unless otherwise noted.
SYMꢁOL
Receiver
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
l
l
l
l
l
t
t
Receiver Input to Output
Differential Receiver Skew
C = 15pF, V = 2.5V, |V | = 1.4V,
100
1
140
8
ns
ns
ns
ns
μs
PLHR
PHLR
L
CM
AB
t and t < 4ns, ꢁFigure 6ꢂ
R
F
t
C = 15pF
SKEWR
L
|t
- t
|
ꢁFigure 6ꢂ
PLHR PHLR
t
RR
t
FR
Receiver Output Rise or Fall Time
C = 15pF
3
12.5
50
L
ꢁFigure 6ꢂ
t
t
, t
, t
,
Receiver Output Enable Time
R =1kΩ, C = 15pF
ZLR ZHR
LZR HZR
L
L
ꢁFigure 7ꢂ
t
, t
Termination Enable or Disable Time
RE = 0V, DE = 0V, V = 2V, V = 0V
100
RTEN RTZ
AB
B
ꢁFigure 8ꢂ
Generic Logic Input
l
l
t
t
D
to D
Input to Output
C = 15pF,
60
100
800
ns
μs
PLHL1
PHLL1
IN
OUT
L
t and t < 4ns
R
F
Power Supply Generator
–GND2 Supply Start-Up Time
V
CC2
325
ON
V , No Load
L
ꢁ0V to 4.5Vꢂ
ISOLATION CHARACTERISTICS TA = 25°C, LTM2881-3 VCC = 3.3V, LTM2881-5 VCC = 5.0V, VL = 3.3V unless
otherwise noted.
SYMꢁOL
PARAMETER
CONDITIONS
MIN
2500
4400
30
TYP
MAX
UNITS
V
Rated Dielectric Insulation Voltage
1 Minute ꢁDerived from 1 Second Testꢂ
V
RMS
ISO
1 Second
ꢁNote 2ꢂ
ꢁNote 2ꢂ
V
DC
Common Mode Transient Immunity
Maximum Working Insulation Voltage
Partial Discharge
kV/μs
V
IORM
560
V
PEAK
V
= 1050 V
ꢁNote 2ꢂ
PEAK
<5
pC
PR
9
Input to Output Resistance
Input to Output Capacitance
Creepage Distance
ꢁNote 2ꢂ
ꢁNote 2ꢂ
ꢁNote 2ꢂ
>10
Ω
pF
6
9.48
mm
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: Guaranteed by design and not subject to production test.
Note 3: Maximum Data rate is guaranteed by other measured parameters
Note 4: This μModule transceiver includes over temperature protection
that is intended to protect the device during momentary overload
conditions. Junction temperature will exceed 125°C when over
temperature protection is active. Continuous operation above specified
maximum operating junction temperature may result in device degradation
or failure.
and is not tested directly.
2881f
5
LTM2881
TYPICAL PERFORMANCE CHARACTERISTICS TA = 25°C, LTM2881-3 VCC = 3.3V, LTM2881-5
VCC = 5.0V, VL = 3.3V unless otherwise noted.
Driver Propagation Delay
vs Temperature
Receiver Skew vs Temperature
Driver Skew vs Temperature
2.0
2.0
1.5
80
75
70
65
60
55
50
1.5
1.0
1.0
0.5
0.5
0
0
–0.5
–0.5
–1.0
–1.0
–50
–25
0
25
50
75
100
–50
–25
0
25
50
75
100
–50
–25
0
25
50
75
100
TEMPERATURE ꢁ°Cꢂ
TEMPERATURE ꢁ°Cꢂ
TEMPERATURE ꢁ°Cꢂ
2881 G01
2881 G02
2881 G03
Driver Output Low/High Voltage
vs Output Current
Driver Differential Output Voltage
vs Temperature
RTERM vs Temperature
130
128
126
124
122
120
118
116
114
112
110
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
6
5
4
OUTPUT HIGH
R = ∞
R = 100Ω
R = 54Ω
3
2
OUTPUT LOW
1
0
–50
–25
0
25
50
75
100
0
10
20
30
40
50
60
70
–50
–25
0
25
50
75
100
TEMPERATURE ꢁ°Cꢂ
OUTPUT CURRENT ꢁmAꢂ
TEMPERATURE ꢁ°Cꢂ
2881 G04
2881 G05
2881 G06
Receiver Output Voltage vs
Output Current (Source and Sink)
Receiver Propagation Delay
vs Temperature
Supply Current vs Data Rate
4
3
2
1
0
120
115
110
105
100
95
200
180
160
140
120
100
80
SOURCE
R = 54 ꢁ–3ꢂ
R = 100 ꢁ–3ꢂ
R = 54 ꢁ–5ꢂ
R = 100 ꢁ–5ꢂ
60
40
R = ∞ ꢁ–3ꢂ
R = ∞ ꢁ–5ꢂ
20
SINK
90
–50
0
0.1
0
1
2
3
4
5
–25
0
25
50
75
100
1
10
OUTPUT CURRENT ꢁmAꢂ
TEMPERATURE ꢁ°Cꢂ
DATA RATE ꢁMbpsꢂ
2881 G07
2881 G08
2881 G09
2881f
6
LTM2881
TYPICAL PERFORMANCE CHARACTERISTICS TA = 25°C, LTM2881-3 VCC = 3.3V, LTM2881-5
VCC = 5.0V, VL = 3.3V unless otherwise noted.
VCC2 Surplus Current
vs Temperature
VCC Supply Current vs Temperature
VCC2 vs Load Current
at ILOAD = 100mA on VCC2
350
250
200
150
100
50
6
LTM2881-3
300
LTM2881-5
LTM2881-5 ꢁRS485 60mAꢂ
5
4
250
LTM2881-3
200
LTM2881-5
LTM2881-5 ꢁRS485 90mAꢂ
LTM2881-3 ꢁRS485 60mAꢂ
150
100
50
3
2
LTM2881-3 ꢁRS485 90mAꢂ
0
0
–50
–25
0
25
50
75
100
–50
–25
0
25
50
75
100
10 20 40 60 80 100 120 140 160 180
LOAD CURRENT ꢁmAꢂ
TEMPERATURE ꢁ°Cꢂ
TEMPERATURE ꢁ°Cꢂ
V
CC2
2881 G10
2881 G11
2881 G12
VCC2 Power Efficiency
VCC2 Load Step (100mA)
VCC2 Noise
70
60
50
40
30
20
10
LTM2881-5
V
CC2
100mV/DIV
LTM2881-3
10mV/DIV
I
LOAD
50mA/DIV
2881 G14
2881 G15
1000μs/DIV
200μs/DIV
0
50
100
150
200
I
CC
2 OUTPUT CURRENT ꢁmAꢂ
2881 G13
2881f
7
LTM2881
PIN FUNCTIONS
LOGIC SIDE (V , V , GND)
ISOLATED SIDE (V , GND2)
CC2
CC
L
D
(Pin A1): General Purpose Logic Output. Logic
D (Pin L1): General Purpose Isolated Logic Input. Logic
IN
OUT
output connected through isolation path to D . Under
input on the isolated side relative to V
and GND2. A
IN
CC2
the condition of an isolation communication failure D
is in a high impedance state.
logic high on D will generate a logic high on D . A
OUT
IN OUT
logic low on D will generate a logic low on D
.
IN
OUT
TE (Pin A2): Terminator Enable. A logic high enables a
termination resistor ꢁtypically 120Ωꢂ between pins A
and B.
SLO (Pin L2): Driver Slew Rate Control. A low input,
relative to GND2, will force the driver into a reduced slew
rate mode for reduced EMI. A high input, relative to GND2,
puts the driver into full speed mode to support maximum
data rates.
DI (Pin A3): Driver Input. If the driver outputs are enabled
ꢁDE highꢂ, then a low on DI forces the driver noninverting
output ꢁYꢂ low and the inverting output ꢁZꢂ high. A high
on DI, with the driver outputs enabled, forces the driver
noninverting output ꢁYꢂ high and inverting output ꢁZꢂ low.
Y (Pin L3): Non Inverting Driver Output. High impedance
when the driver is disabled.
Z (Pin L4): Inverting Driver Output. High impedance when
the driver is disabled.
DE (Pin A4): Driver Enable. A logic low disables the driver
leaving the outputs Y and Z in a high impedance state. A
logic high enables the driver.
ꢁ (Pin L5): Inverting Receiver Input. Impedance is > 96kΩ
in receive mode with TE low or unpowered.
RE (Pin A5): Receiver Enable. A logic low enables the
receiver output. A logic high disables RO to a high
impedance state.
A (Pin L6): Non Inverting Receiver Input. Impedance is
> 96kΩ in receive mode with TE low or unpowered.
V
(Pins L7-L8): Isolated Supply Voltage. Internally
CC2
RO (Pin A6): Receiver Output. If the receiver output is
enabled ꢁRE lowꢂ and if A – B is > 200mV, RO is a logic
high, if A – B is < 200mV RO is a logic low. If the receiver
inputs are open, shorted, or terminated without a valid
signal, RO will be high. Under the condition of an isolation
communication failure RO is in a high impedance state.
generated from V by an isolated DC/DC converter and
CC
regulated to 5V.
GND2 (Pins K1-K8): Isolated Side Circuit Ground. The
pads should be connected to the isolated ground and/or
cable shield.
V (Pin A7): Logic Supply. Interface supply voltage for
L
pins RO, RE, TE, DI, DE, D , and ON. Recommended
OUT
operating voltage is 1.62V to 5.5V.
ON (Pin A8): Enable. Enables power and data
communication through the isolation barrier. If ON is high
the part is enabled and power and communications are
functional to the isolated side. If ON is low the logic side
is held in reset and the isolated side is unpowered.
GND (Pins ꢁ1-ꢁ5): Circuit Ground.
V
(Pins ꢁ6-ꢁ8): Supply Voltage. Recommended
CC
operating voltage is 3V to 3.6V for LTM2881-3 and 4.5V
to 5.5V for LTM2881-5.
2881f
8
LTM2881
BLOCK DIAGRAM
V
2.2μF
CC
V
CC2
5V
REG
ISOLATED
2.2μF
DC/DC
CONVERTER
V
L
2.2μF
A
B
RO
RX
RE
DE
DI
ISOLATED
ISOLATED
COMM
INTERFACE
120Ω
COMM
INTERFACE
Y
Z
DX
ON
TE
SLO
D
IN
D
OUT
GND
GND2
2881 B
D
= LOGIC SIDE COMMON
= ISOLATED SIDE COMMON
TEST CIRCUITS
Y
Z
Y
Z
R
I
OSD
GND
DI
GND
DI
+
OR
DRIVER
OR
DRIVER
V
OD
V
V
L
L
–
R
+
–
+
–7V TO 12V
V
OC
–
2881 F01
2881 F02
Figure 1. Driver DC Characteristics
Figure 2. Driver Output Short-Circuit Current
I
IN
A OR B
B OR A
RECEIVER
+
V
IN
–
2881 F03
V
I
IN
IN
R
=
IN
Figure 3. Receiver Input Current and Input Resistance
2881f
9
LTM2881
TEST CIRCUITS
V
L
t
t
DI
Y, Z
PLHD
PHLD
Y
Z
0V
t
C
C
SKEWD
L
L
DI
DRIVER
R
DIFF
V
1/2 V
OD
OD
2881 F04a
90%
90%
0
0
ꢁY-Zꢂ
10%
10%
2881 F04b
t
t
FD
RD
Figure 4. Driver Timing Measurement
V
L
GND
OR
CC2
R
R
L
DE
Y OR Z
Z OR Y
1/2 V
L
Y
Z
0V
V
C
C
t
L
L
ZLD
t
V
LZD
L
DI
V
CC2
OR
DRIVER
DE
1/2 V
1/2 V
GND
CC2
0.5V
V
CC2
L
OR
GND
0.5V
2881 F05a
CC2
0V
2881 F05b
t
t
HZD
ZHD
Figure 5. Driver Enable and Disable Timing Measurements
t
t
F
R
V
90%
10%
AB
90%
A-B
0
A
B
10%
V
/2
/2
AB
–V
AB
RO
t
t
PHLR
PLHR
V
RECEIVER
CM
V
L
90%
10%
C
90%
10%
L
V
AB
1/2 V
1/2 V
RO
L
L
2881 F06a
0
2881 F06b
t
t
RR
FR
Figure 6. Receiver Propagation Delay Measurements
2881f
10
LTM2881
TEST CIRCUITS
V
L
RE
RO
RO
1/2 V
L
0V
A
0V OR V
CC2
t
t
t
ZLR
LZR
R
V
L
L
RO
V
L
OR
RECEIVER
RE
1/2 V
1/2 V
L
L
GND
B
0.5V
0.5V
C
V
OR 0V
L
V
CC2
OL
V
OH
2881 F07a
0V
2881 F07b
t
ZHR
HZR
Figure 7. Receiver Enable/Disable Time Measurements
V
AB
I
R
=
A
TE
I
A
V
L
A
B
TE
1/2 V
L
RO
+
–
RECEIVER
V
V
0V
AB
t
RTEN
t
RTZ
90%
I
A
10%
+
–
TE
B
2881 F08
Figure 8. Termination Resistance and Timing Measurements
FUNCTIONAL TABLE
DC/DC
CONVERTER
LOGIC INPUTS
MODE
A, ꢁ
Y, Z
RO
TERMINATOR
ON
1
RE
0
TE
0
DE
0
Receive
Transceiver
Transmit
R
R
R
Hi-Z
Driven
Driven
Hi-Z
Enabled
Enabled
Hi-Z
On
On
On
On
Off
Off
Off
Off
On
Off
IN
IN
IN
1
0
0
1
1
1
0
1
1
0
1
0
Receive + Term On
Off
R
TE
Enabled
Hi-Z
0
X
X
X
R
Hi-Z
IN
2881f
11
LTM2881
APPLICATIONS INFORMATION
Overview
The DC/DC converter is connected to a low dropout reg-
ulator ꢁLDOꢂ to provide a regulated low noise 5V output.
TheLTM2881μModuletransceiverprovidesagalvanically-
isolated robust RS485/RS422 interface, powered by an
integrated, regulated DC/DC converter, complete with
decoupling capacitors. A switchable termination resistor
is integrated at the receiver input to provide proper
termination to the RS485 bus. The LTM2881 is ideal for
use in networks where grounds can take on different
voltages. Isolation in the LTM2881 blocks high voltage
differences and eliminates ground loops and is extremely
tolerant of common mode transients between ground
potentials. Error free operation is maintained through
common mode events greater than 30kV/ꢀs providing
excellent noise isolation.
The internal power solution is sufficient to support the
transceiverinterfaceatitsmaximumspecifiedloadanddata
rate, and external pins are supplied for extra decoupling
ꢁoptionalꢂ and heat dissipation. The logic supplies, V
CC
and V have a 2.2μF decoupling capacitance to GND
L
and the isolated supply V
has a 2.2μF decoupling
CC2
capacitancetoGND2withintheμModulepackage.Surplus
current is available to external applications. The amount
of surplus current is dependent upon the implementation
and current delivered to the RS485 driver and line load.
An example of available surplus current is shown in the
Typical Performance Characteristics graph, V
Current vs Temperature.
Surplus
CC2
DC/DC Converter
Driver
The LTM2881 contains a fully integrated isolated DC/DC
converter, including the transformer, so that no external
components are necessary. The logic side contains a full-
bridge driver, running about 2MHz, and is AC-coupled
to a single transformer primary. A series DC blocking
capacitor prevents transformer saturation due to driver
duty cycle imbalance. The transformer scales the primary
voltage, and is rectified by a full-wave voltage doubler.
This topology eliminates transformer saturation caused
by secondary imbalances.
The driver provides full RS485 and RS422 compatibility.
When enabled, if DI is high, Y–Z is positive. When the
driver is disabled, both outputs are high impedance with
less than 10μA of leakage current over the entire common
mode range of –7V to 12V, with respect to GND2.
2881f
12
LTM2881
APPLICATIONS INFORMATION
Driver Overvoltage and Overcurrent Protection
The driver outputs are protected from short circuits to
the conditions of an idle bus. Further network biasing
constructed to condition transient noise during an idle
state is unnecessary due to the common mode transient
rejection of the LTM2881. The failsafe detector monitors
A and B in parallel with the receiver and detects the state
of the bus when A-B is above the input failsafe threshold
for longer than about 3μs with a hysteresis of 25mV. This
failsafe feature is guaranteed to work for inputs spanning
the entire common mode range of –7V to 12V.
any voltage within the absolute maximum range of ꢁV
CC2
current
–15Vꢂ to ꢁGND2 +15Vꢂ levels. The maximum V
CC2
in this condition is 250mA. If the pin voltage exceeds
about 10V, current limit folds back to about half of the
peak value to reduce overall power dissipation and avoid
damaging the part.
The device also features thermal shutdown protection
that disables the driver and receiver output in case of
excessive power dissipation ꢁSee Note 4 in the Electrical
Characteristics sectionꢂ.
The receiver output is internally driven high ꢁto V ꢂ or low
L
ꢁtoGNDꢂwithnoexternalpull-upneeded.Whenthereceiver
is disabled the RO pin becomes Hi-Z with leakage of less
than 1μA for voltages within the supply range.
SLO Mode
TheLTM2881featuresalogic-selectablereducedslewrate
mode ꢁSLO modeꢂ that softens the driver output edges to
reduce EMI emissions from equipment and data cables.
The reduced slew rate mode is entered by taking the SLO
pin low to GND2, where the data rate is limited to about
250kbps. Slew limiting also mitigates the adverse effects
ofimperfecttransmissionlineterminationcausedbystubs
or mismatched cables.
0
6.25
12.5
Figures 9a and 9b show the frequency spectrums of the
LTM2881driveroutputsinnormalandSLOmodeoperating
at 250kbps. SLO mode significantly reduces the high
frequency harmonics.
FREQUENCY 1.25MHz/DIV
2881 F09a
Figure 9a. Frequency Spectrum SLO Mode 125kHz Input
Receiver and Failsafe
With the receiver enabled, when the absolute value of the
differentialvoltagebetweentheAandBpinsisgreaterthan
200mV, the state of RO will reflect the polarity of ꢁA-Bꢂ.
During data communication the receiver detects the state
of the input with symmetric thresholds around 0V. The
symmetric thresholds preserve duty cycle for attenuated
signalswithslowtransitionratesonhighcapacitivebusses,
or long cable lengths. The receiver incorporates a failsafe
feature that guarantees the receiver output to be a logic-
high during an idle bus, when the inputs are shorted, left
open or terminated, but not driven. The failsafe feature
eliminatestheneedforsystemlevelintegrationofnetwork
pre-biasing by guaranteeing a logic-high on RO under
0
6.25
12.5
FREQUENCY 1.25MHz/DIV
2881 F09b
Figure 9b. Normal Mode Frequency Spectrum 125kHz Input
2881f
13
LTM2881
APPLICATIONS INFORMATION
Receiver Input Resistance
ThereceiverinputresistancefromAorBtoGND2isgreater
than96kpermittinguptoatotalof256receiverspersystem
withoutexceedingtheRS485receiverloadingspecification.
The input resistance of the receiver is unaffected by
enabling/disablingthereceiverorbypowering/unpowering
thepart. TheequivalentinputresistancelookingintoAand
B is shown in Figure 10.
2881 F11
Figure 11. Curve Trace ꢁetween A and ꢁ with Termination
Enabled and Disabled
A
>96k
60ꢃ
130
128
126
124
122
120
118
116
114
112
110
TE
60ꢃ
B
2881 F10
>96k
Figure 10. Equivalent Input Resistance into A and ꢁ
Switchable Termination
–10
–5
0
5
10
15
Proper cable termination is very important for
signal fidelity. If the cable is not terminated with its
characteristic impedance, reflections will distort the
signal waveforms.
COMMON MODE VOLTAGE ꢁVꢂ
2881 G11
Figure 12. Termination Resistance vs Common Mode Voltage
150
140
130
120
10
The integrated switchable termination resistor provides
logic control of the line termination for optimal perfor-
mance when configuring transceiver networks.
PHASE
0
When the TE pin is high, the termination resistor is
enabled and the differential resistance from A to B is
120Ω. Figure 11 shows the I/V characteristics between
pins A and B with the termination resistor enabled and
disabled. The resistance is maintained over the entire
RS485 common mode range of –7V to 12V as shown in
Figure 12. The integrated termination resistor has a high
frequency response which does not limit performance at
the maximum specified data rate. Figure 13 shows the
magnitudeandphaseoftheterminationimpedanceversus
frequency. The termination resistor cannot be enabled by
TE if the device is unpowered, ON is low or the LTM2881
is in thermal shutdown.
–10
–20
MAGNITUDE
110
100
–30
–40
0.1
1
10
FREQUENCY ꢁMHzꢂ
2881 F13
Figure 13. Termination Magnitude and Phase vs Frequency
2881f
14
LTM2881
APPLICATIONS INFORMATION
Supply Current
• If the LTM2881 voltage supply is hot plugged without
additionalprotection,damagemayoccur.RefertoLinear
Technology Application Note 88, entitled “Ceramic
Capacitors Can Cause Overvoltage Transients” for a
detailed discussion of this problem. To protect against
hot plug transients use tantalum for aforementioned
additional capacitor.
Thestaticsupplycurrentisdominatedbypowerdeliveredto
theterminationresistance.Powersupplycurrentincreases
with data rate due to capacitive loading. Figure 14 shows
supply current versus data rate for three different loads
for the circuit configuration of Figure 4.
250
230
• Do not place copper on the PCB between the inner
rows of pads. This area must remain open to withstand
the rated isolation voltage. The PCB may also be slotted
inthisareatoinsurecontaminationdoesnotcompromise
the isolation voltage.
210
LTM2881-3
190
170
150
130
110
90
R=54 CL=1000p
R=54 CL=100p
R=54 CL=0
LTM2881-5
R=54 CL=1000p
R=54 CL=100p
R=54 CL=0
70
V
GND2
CC
50
ON
V
CC2
0.1
1
10
DATA RATE ꢁMbpsꢂ
V
L
2881 F14
RO
A
B
Z
Y
Figure 14. Supply Current vs Data Rate
RE
DE
DI
PCꢁ Layout Isolation Considerations
TE
SLO
ThehighintegrationoftheLTM2881makesthePCBboard
layout very simple. However, to optimize its electrical
isolation characteristics and thermal performance, some
layout considerations are still necessary. Figure 15 is a
suggested layout for good thermal performance and to
optimize isolation characteristics.
DO1
GND
DL1
9.48mm
2881 F15
Figure 15. PCꢁ Recommended Layout
• Use large PCB copper areas for high current paths,
including V , GND, V , and GND2. It helps to
CC
CC2
minimize the PCB conduction loss and thermal stress.
• The LTM2881 includes 2.2μF ceramic decoupling
capacitors on V to GND, V to GND, and V to
CC2
CC
L
GND2 supply pins. Further decoupling capacitance
ꢁ10μFꢂcanbeaddedwithinone-quarterinchawayfrom
the V , V , and/or V pin.
CC2
CC
L
2881f
15
LTM2881
APPLICATIONS INFORMATION
Cable Length versus Data Rate
10k
1k
For a given data rate, the maximum transmission distance
is bounded by the cable properties. A typical curve of
cable length versus data rate compliant with the RS485
standard is shown in Figure 16. Three regions of this
curve reflect different performance limiting factors in data
transmission. In the flat region of the curve, maximum
distance is determined by resistive loss in the cable. The
downwardslopingregionrepresentslimitsindistanceand
rate due to the AC losses in the cable. The solid vertical
line represents the specified maximum data rate in the
RS485 standard. The dashed line at 250kbps shows the
maximum data rate when SLO is low. The dashed line at
20Mbps shows the maximum data rate when SLO is high.
LOW-EMI MODE
MAX DATA RATE
NORMAL
MODE MAX
DATA RATE
100
10
RS485 MAX
DATA RATE
10k
100k
1M
10M
100M
2881 F16
DATA RATE ꢁbpsꢂ
Figure 16. Cable Length vs Data Rate
2881f
16
LTM2881
TYPICAL APPLICATIONS
Full-Duplex RS485 Connection
V
V
CC
CC
LTM2881
PWR
V
L
A
RO
B
RE
TE
DE
DI
Y
Z
GND
GND2
2881 TA02
Isolated System Fault Detection
V
CC
V
CC
LTM2881
V
L
A
B
RO
RE
TE
DE
Y
DI
Z
330k
D
D
OUT
IN
GND
GND2
FAULT
2881 TA03
Switched 5V Power with Isolated CMOS Logic Connection with Low Voltage Interface
V
V
CC
CC
REGULATED 5V
SWITCHED 5V
1.8V
V
CC2
PWR
V
L
A
RO
IRLML6402
B
Z
RE
LTM2881
TE
DE
330k
DI
D
OFF ON
D
OUT
IN
GND
GND2
CMOS OUTPUT
CMOS INPUT
2881 TA04
2881f
17
LTM2881
TYPICAL APPLICATIONS
4-Wire Full Duplex Self ꢁiasing for Unshielded CAT5 Connection
V
V
V
CCB
CC
V
CC
CC
LTM2881
LTM2881
V
DE
L
PWR
PWR
V
L
A
B
Y
51Ω
51Ω
RO
DI
Z
RE
10nF
DE
DI
RE
Y
Z
A
B
51Ω
51Ω
RO
10nF
GND
GND2
GND2
GND
2881 TA04a
BUS INHERITED
B
2881f
18
LTM2881
PACKAGE DESCRIPTION
LGA Package
32-Lead (15mm × 11.25mm × 2.8mm)
ꢁReference LTC DWG # 05-08-1773 Rev θꢂ
DETAIL A
8
2.69 – 2.95
7
6
5
4
3
2
1
PAD 1
aaa
Z
A
B
C
D
E
F
PAD “A1”
CORNER
4
12.70
BSC
15.00
BSC
MOLD
CAP
G
H
J
SUBSTRATE
0.290 – 0.350
2.400 – 2.600
DETAIL B
K
L
PADS
1.27
BSC
X
Y
SEE NOTES
11.25
BSC
8.89
BSC
3
DETAIL B
PACKAGE TOP VIEW
PACKAGE BOTTOM VIEW
0.630 0.025 Ø 32x
0.630 0.025 Ø 32x
eee
S
X
Y
eee S X Y
DETAIL c
DETAIL A
NOTES:
DETAIL C
6.350
5.080
1. DIMENSIONING AND TOLERANCING PER ASME Y14.5M-1994
2. ALL DIMENSIONS ARE IN MILLIMETERS
3
4
LAND DESIGNATION PER JESD MO-222
DETAILS OF PAD #1 IDENTIFIER ARE OPTIONAL,
BUT MUST BE LOCATED WITHIN THE ZONE INDICATED.
THE PAD #1 IDENTIFIER MAY BE EITHER A MOLD OR
MARKED FEATURE
0.000
LTMXXXXXX
μModule
COMPONENT
PIN “A1”
5. PRIMARY DATUM -Z- IS SEATING PLANE
6. THE TOTAL NUMBER OF PADS: 32
TRAY PIN 1
BEVEL
SYMꢁOL TOLERANCE
PACKAGE IN TRAY LOADING ORIENTATION
5.080
6.350
LGA 32 0308 REV Ø
aaa
bbb
eee
0.10
0.10
0.05
SUGGESTED PCB LAYOUT
TOP VIEW
2881f
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However,noresponsibilityisassumedforitsuse.LinearTechnologyCorporationmakesnorepresentation
that the interconnection of its circuits as described herein will not infringe on existing patent rights.
19
LTM2881
TYPICAL APPLICATION
Multi-Node Network with End Termination and Single Ground
Connection on Isolation ꢁus
V
CCC
V
CCA
V
V
CC
CC
LTM2881
LTM2881
PWR
PWR
V
V
L
L
A
A
RO
RO
B
B
RE
RE
TE
TE
V
CC2
V
CC1
CABLE SHIELD
OR GROUND RETURN
DE
DI
DE
DI
Y
Z
Y
Z
GND
GND2
GND2
GND
A
C
ISOLATION BARRIER
LTM2881
B
2881 TA05
B
RELATED PARTS
PART NUMꢁER
LTC1535
LT1785
DESCRIPTION
COMMENTS
2500V Isolation in Surface Mount Package
Isolated RS485 Transceiver
RMS
60V Fault-Protected Transceiver
60V Fault-Protected Transceiver
Half Duplex
Full Duplex
LT1791
LTC2861
20Mbps RS485 Transceivers with Integrated Switchable Termination
Full Duplex 15kV ESD
2881f
LT 1109 • PRINTED IN USA
LinearTechnology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
20
●
●
© LINEAR TECHNOLOGY CORPORATION 2009
ꢁ408ꢂ 432-1900 FAX: ꢁ408ꢂ 434-0507 www.linear.com
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