LTM2881_10 [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： | LTM2881_10 |
| 厂家： | 
Linear |
| 描述： | Complete Isolated RS485/RS422 μModule Transceiver + Power |
| 文件： | 总22页 (文件大小：456K) |
| 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
Isolated RS485/RS422 Transceiver: 2500V
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.
RMS
n
Isolated DC Power: 5V at Up to 200mA
n
No External Components Required
n
20Mbps or Low EMI 250kbps Data Rate
n
High ESD: ±±5kV HꢀM on Transceiver Interface
High Common Mode Transient Immunity: 30kV/μs
Integrated Selectable ±20Ω Termination
n
n
n
n
n
n
n
n
n
n
n
n
Coupled inductors and an isolation power transformer
provide2500V
ofisolationbetweenthelinetransceiver
RMS
3.3V (LTM2881-3) or 5.0V (LTM2881-5) Operation
1.62V to 5.5V Logic Supply Pin for Flexible Digital Interface
Common Mode Working Voltage: 560V
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 (< 10µA)
and the logic interface. This device is ideal for systems
where the ground loop is broken allowing for large com-
mon mode voltage variation. Uninterrupted communica-
tion is guaranteed for common mode transients greater
than 30kV/μs.
PEAK
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 × 2.8mm)
Surface Mount BGA and LGA Packages
applicaTions
Enhanced ESD protection allows this part to withstand up
to 15kV(humanbodymodel)onthetransceiverinterface
pins to isolated supplies and 10kV through the isolation
barrier to logic supplies without latch-up or damage.
n
Isolated RS485/RS422 Interface
n
Industrial Networks
n
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
LTM288± Operating Through 35kV/μs CM Transients
3.3V
MULTIPLE SWEEPS
OF COMMON MODE
TRANSIENTS
V
CC
LTM2881
AVAILABLE CURRENT:
150mA (LTM2881-5)
100mA (LTM2881-3)
V
5V
CC2
A
PWR
500V/DIV
V
L
RO
B
RE
DI
TWISTED-PAIR
CABLE
RO
TE
DE
1V/DIV
1V/DIV
Y
Z
DI
2881 TA01a
50ns/DIV
GND
GND2
2881 TA01
2881fa
ꢀ
LTM2881
absoluTe MaxiMuM raTings
pin conFiguraTion
(Note ±)
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, Z) 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 sec)....... 245°C
GND2
K
L
D
IN
SLO
Y
Z
B
A
V
CC2
BGA PACKAGE
LGA PACKAGE
32-PIN (15mm s 11.25mm s 3.42mm)
32-PIN (15mm s 11.25mm s 2.8mm)
T
JA
JCTOP
JCBOTTOM
Q
= 125°C,
T
= 125°C,
JMAX
JMAX
= 31.1°C/W,
Q
= 32.2°C/W,
Q
JA
Q
= 27.2°C/W,
Q
= 27.3°C/W,
JCTOP
Q
= 20.9°C/W,
Q
= 19.5°C/W,
JCBOTTOM
= 26.4°C/W,
Q
= 25.1°C/W,
JB
WEIGHT = 1g
JB
WEIGHT = 1g
orDer inForMaTion
LEAD FREE FINISH
LTM2881CY-3#PBF
LTM2881IY-3#PBF
LTM2881CY-5#PBF
LTM2881IY-5#PBF
LTM2881CV-3#PBF
LTM2881IV-3#PBF
LTM2881CV-5#PBF
LTM2881IV-5#PBF
TRAY
PART MARKING*
LTM2881Y-3
LTM2881Y-3
LTM2881Y-5
LTM2881Y-5
LTM2881V-3
LTM2881V-3
LTM2881V-5
LTM2881V-5
PACKAGE DESCRIPTION
TEMPERATURE RANGE
0°C to 70°C
LTM2881CY-3#PBF
LTM2881IY-3#PBF
LTM2881CY-5#PBF
LTM2881IY-5#PBF
LTM2881CV-3#PBF
LTM2881IV-3#PBF
LTM2881CV-5#PBF
LTM2881IV-5#PBF
32-Pin (15mm × 11.25mm × 3.42mm) BGA
32-Pin (15mm × 11.25mm × 3.42mm) BGA
32-Pin (15mm × 11.25mm × 3.42mm) BGA
32-Pin (15mm × 11.25mm × 3.42mm) BGA
32-Pin (15mm × 11.25mm × 2.8mm) LGA
32-Pin (15mm × 11.25mm × 2.8mm) LGA
32-Pin (15mm × 11.25mm × 2.8mm) LGA
32-Pin (15mm × 11.25mm × 2.8mm) LGA
–40°C to 85°C
0°C to 70°C
–40°C to 85°C
0°C to 70°C
–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/
2881fa
ꢁ
LTM2881
elecTrical characTerisTics The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. LTM288±-3 VCC = 3.3V, LTM288±-5 VCC = 5.0V, VL = 3.3V, GND = GND2 =
0V, ON = VL unless otherwise noted.
SYMꢀOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
Power Supply
l
l
V
V
Supply Voltage
CC
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
V
Supply Current in Off Mode
Supply Current in On Mode
ON = 0V
0
µA
CCPOFF
CCS
CC
CC
l
l
LTM2881-3 DE = 0V, RE = V , No Load
LTM2881-5 DE = 0V, RE = V , No Load
20
15
25
19
mA
mA
L
L
l
V
Regulated V
Output Voltage, Loaded
Output Voltage, No Load
LTM2881-3 DE = 0V, RE = V , I
= 100mA
= 150mA
4.7
4.7
5.0
5.0
V
V
CC2
CC2
L
LOAD
LOAD
LTM2881-5 DE = 0V, RE = V , I
L
V
Regulated V
Efficiency
DE = 0V, RE = V , No Load
4.8
5.0
62
5.35
250
V
%
CC2NOLOAD
CC2
L
I
= 100mA, LTM2881-5 (Note 2)
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 1)
R = 27Ω (RS485) (Figure 1)
R = 50Ω (RS422) (Figure 1)
V
CC2
V
CC2
V
CC2
V
V
V
1.5
2
l
∆|V
|
OD
Difference in Magnitude of Driver Differential R = 27Ω or R = 50Ω (Figure 1)
Output Voltage for Complementary Output
States
0.2
V
l
l
V
Driver Common Mode Output Voltage
R = 27Ω or R = 50Ω (Figure 1)
R = 27Ω or R = 50Ω (Figure 1)
3
V
V
OC
∆|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 Impedance) Output DE = 0V, (Y or Z) = –7V, +12V
Current on Y and Z
10
µA
OZD
Maximum Driver Short-Circuit Current
–7V ≤ (Y or Z) ≤ 12V (Figure 2)
–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 3)
R
TE
Receiver Termination Resistance Enabled
Receiver Input Current (A, B)
TE = V , V = 2V, V = –7V, 0V, 10V
108
156
125
L
AB
B
(Figure 8)
I
IN
ON = 0V V = 0V or 5V, V = 12V
µA
µA
V
CC2
IN
(Figure 3)
ON = 0V V = 0V or 5V, V = –7V
–100
–0.2
CC2
IN
(Figure 3)
V
TH
Receiver Differential Input Threshold Voltage –7V ≤ B ≤ 12V
(A-B)
0.2
∆V
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
2881fa
ꢂ
LTM2881
elecTrical characTerisTics The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. LTM288±-3 VCC = 3.3V, LTM288±-5 VCC = 5.0V, VL = 3.3V, GND = GND2 =
0V, ON = VL unless otherwise noted.
SYMꢀOL
PARAMETER
CONDITIONS
MIN
TYP
0
MAX
UNITS
µA
l
I
Logic Input Current
Logic Input Hysteresis
Output High Voltage
1
INL
V
HYS
V
OH
(Note 2)
150
mV
V
l
l
Output High, I
= –4mA
V –0.4
L
LOAD
(Sourcing), 5.5V ≥ V ≥ 3V
L
Output High, I
= –1mA
V –0.4
L
V
LOAD
(Sourcing), 1.62V ≤ V < 3V
L
l
l
V
Output Low Voltage
Output Low, I
= 4mA
0.4
0.4
V
V
OL
LO AD
(Sinking), 5.5V ≥ V ≥ 3V
L
Output High, I
(Sinking), 1.62V ≤ V < 3V
= 1mA
LOAD
L
l
l
I
I
Three-State (High Impedance) Output Current RE = V , 0V ≤ RO ≤ V
1
µA
OZR
L
L
on RO
Short-Circuit Current
0V ≤ (RO or D ) ≤ 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. LTM288±-3 VCC = 3.3V, LTM288±-5 VCC = 5.0V, VL = 3.3V, GND = GND2 =
0V, ON = VL unless otherwise noted.
SYMꢀOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
Driver SLO = V
CC2
f
Maximum Data Rate
Driver Input to Output
(Note 3)
20
Mbps
ns
MAX
l
l
l
l
l
t
t
R
= 54Ω, C = 100pF
60
1
85
8
PLHD
PHLD
DIFF
L
(Figure 4)
∆t
Driver Input to Output Difference
R
= 54Ω, C = 100pF
ns
ns
ns
ns
PD
DIFF
L
|t
– t
PHLD
|
(Figure 4)
PLHD
t
Driver Output Y to Output Z
R
= 54Ω, C = 100pF
1
8
SKEWD
DIFF
L
(Figure 4)
t
t
Driver Rise or Fall Time
R
= 54Ω, C = 100pF
4
12.5
170
RD
FD
DIFF
L
(Figure 4)
t
t
, t
,
Driver Output Enable or Disable Time
R = 500Ω, C = 50pF
ZLD ZHD
L
L
, t
(Figure 5)
LZD HZD
Driver SLO = GND2
f
Maximum Data Rate
Driver Input to Output
(Note 3)
250
kbps
µs
MAX
t
t
R
= 54Ω, C = 100pF
1
1.55
500
500
1.5
PLHD
PHLD
DIFF
L
(Figure 4)
∆t
Driver Input to Output Difference
R
= 54Ω, C = 100pF
50
ns
ns
µs
ns
PD
DIFF
L
|t
– t
PHLD
|
(Figure 4)
PLHD
t
Driver Output Y to Output Z
R
= 54Ω, C = 100pF
200
0.9
SKEWD
DIFF
L
(Figure 4)
l
l
t
t
Driver Rise or Fall Time
R
DIFF
= 54Ω, C = 100pF
RD
FD
L
(Figure 4)
t
t
, t
, t
,
Driver Output Enable or Disable Time
R = 500Ω, C = 50pF
400
ZLD ZHD
LZD HZD
L
L
(Figure 5)
2881fa
ꢃ
LTM2881
swiTching characTerisTics The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. LTM288±-3 VCC = 3.3V, LTM288±-5 VCC = 5.0V, VL = 3.3V, GND = GND2 =
0V, ON = VL unless otherwise noted.
SYMꢀOL
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 6)
R
F
t
C = 15pF
SKEWR
L
|t
- t
|
(Figure 6)
PLHR PHLR
t
RR
t
FR
Receiver Output Rise or Fall Time
C = 15pF
3
12.5
50
L
(Figure 6)
t
t
, t
, t
,
Receiver Output Enable Time
R =1kΩ, C = 15pF
ZLR ZHR
LZR HZR
L
L
(Figure 7)
t
, t
Termination Enable or Disable Time
RE = 0V, DE = 0V, V = 2V, V = 0V
100
RTEN RTZ
AB
B
(Figure 8)
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 4.5V)
isolaTion characTerisTics TA = 25°C, LTM288±-3 VCC = 3.3V, LTM288±-5 VCC = 5.0V, VL = 3.3V unless
otherwise noted.
SYMꢀOL
PARAMETER
CONDITIONS
MIN
2500
4400
30
TYP
MAX
UNITS
V
ISO
Rated Dielectric Insulation Voltage
1 Minute (Derived from 1 Second Test)
V
RMS
1 Second
(Note 2)
(Note 2)
V
DC
Common Mode Transient Immunity
Maximum Working Insulation Voltage
Partial Discharge
kV/µs
V
IORM
560
V
PEAK
V
= 1050 V
(Note 2)
<5
pC
PR
PEAK
9
Input to Output Resistance
Input to Output Capacitance
Creepage Distance
(Note 2)
(Note 2)
(Note 2)
>10
Ω
pF
6
9.48
mm
Note ±: 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.
2881fa
ꢄ
LTM2881
Typical perForMance characTerisTics TA = 25°C, LTM288±-3 VCC = 3.3V, LTM288±-5
VCC = 5.0V, VL = 3.3V unless otherwise noted.
Driver Propagation Delay
Receiver Skew vs Temperature
Driver Skew vs Temperature
vs Temperature
2.0
1.5
2.0
1.5
80
75
70
65
60
55
50
1.0
1.0
0.5
0.5
0
0
–0.5
–1.0
–0.5
–1.0
–50
–25
0
25
50
75
100
–50
–25
0
25
50
75
100
–50
–25
0
25
50
75
100
TEMPERATURE (°C)
TEMPERATURE (°C)
TEMPERATURE (°C)
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 (°C)
OUTPUT CURRENT (mA)
TEMPERATURE (°C)
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 = 54Ω (LTM2881-3)
R = 100Ω (LTM2881-3)
R = 54Ω (LTM2881-5)
R = 100Ω (LTM2881-5)
60
40
R = ∞ (LTM2881-3)
R = ∞ (LTM2881-5)
20
SINK
90
–50
0
0.1
0
1
2
3
4
5
–25
0
25
50
75
100
1
10
OUTPUT CURRENT (mA)
TEMPERATURE (°C)
DATA RATE (Mbps)
2881 G07
2881 G08
2881 G09
2881fa
ꢅ
LTM2881
Typical perForMance characTerisTics TA = 25°C, LTM288±-3 VCC = 3.3V, LTM288±-5
VCC = 5.0V, VL = 3.3V unless otherwise noted.
VCC2 Surplus Current
vs Temperature
V
CC Supply Current vs Temperature
V
CC2 vs Load Current
at ILOAD = ±00mA on VCC2
350
300
250
200
150
100
50
250
200
150
100
50
6
LTM2881-3
LTM2881-5
LTM2881-5 (RS485 60mA)
5
4
LTM2881-3
LTM2881-5
LTM2881-5 (RS485 90mA)
LTM2881-3 (RS485 60mA)
3
2
LTM2881-3 (RS485 90mA)
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
TEMPERATURE (°C)
TEMPERATURE (°C)
V
LOAD CURRENT (mA)
CC2
2881 G10
2881 G11
2881 G12
VCC2 Power Efficiency
VCC2 Load Step (±00mA)
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
100µs/DIV
200µs/DIV
0
50
100
150
200
I
OUTPUT CURRENT (mA)
CC2
2881 G13
2881fa
ꢆ
LTM2881
pin FuncTions
LOGIC SIDE (V , V , GND)
ISOLATED SIDE (V , GND2)
CC2
CC
L
D
(Pin A±): General Purpose Logic Output. Logic
D (Pin L±): 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 120Ω) between pins A
and B.
SLO (Pin L2): Driver Slew Rate Control. A low input, rela-
tive 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 high), then a low on DI forces the driver noninverting
output (Y) low and the inverting output (Z) high. A high
on DI, with the driver outputs enabled, forces the driver
noninverting output (Y) high and inverting output (Z) 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 imped-
ance 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 low) 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. Internally bypassed to GND2 with 2.2µF.
GND2 (Pins K±-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. Internally bypassed
to GND with 2.2µF.
ON(PinA8):Enable. Enablespoweranddatacommunica-
tion 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 ꢀ±-ꢀ5): Circuit Ground.
V
(Pinsꢀ6-ꢀ8):SupplyVoltage. Recommendedoperat-
CC
ing voltage is 3V to 3.6V for LTM2881-3 and 4.5V to 5.5V
for LTM2881-5. Internally bypassed to GND with 2.2µF.
2881fa
ꢇ
LTM2881
block DiagraM
V
2.2µF
CC
V
CC2
5V
REG
ISOLATED
DC/DC
CONVERTER
2.2µF
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 BD
= 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 ±. 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
2881fa
ꢈ
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
OD
1/2 V
OD
2881 F04a
90%
90%
10%
0
0
(Y-Z)
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
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
L
2881 F05a
CC2
0V
2881 F05b
t
t
HZD
ZHD
Figure 5. Driver Enable and Disable Timing Measurements
t
R
t
F
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
2881fa
ꢀ0
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
2881fa
ꢀꢀ
LTM2881
applicaTions inForMaTion
Overview
The internal power solution is sufficient to support the
transceiverinterfaceatitsmaximumspecifiedloadanddata
rate, and external pins are supplied for extra decoupling
The LTM2881 µModule transceiver provides a galvani-
cally-isolated robust RS485/RS422 interface, powered by
an integrated, regulated DC/DC converter, complete with
decouplingcapacitors.Aswitchableterminationresistoris
integrated at the receiver input to provide proper termina-
tion 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
commonmodetransientsbetweengroundpotentials.Error
freeoperationismaintainedthroughcommonmodeevents
greater than 30kV/μs providing excellent noise isolation.
(optional)andheatdissipation.Thelogicsupplies,V and
CC
V have a 2.2µF decoupling capacitance to GND and the
L
isolated supply V has a 2.2µF decoupling capacitance
CC2
to GND2 within the µModule package.
V
Output
CC2
Theon-boardDC/DCconverterprovidesisolated5Vpower
to output V . V is capable of suppling up to 1W of
CC2 CC2
power at 5V in the LTM2881-5 option and up to 600mW
of power in the LTM2881-3 option. This surplus current is
available to external applications. The amount of surplus
currentisdependentupontheimplementationandcurrent
delivered to the RS485 driver and line load. An example
of available surplus current is shown in the Typical Per-
µModule Technology
The LTM2881 utilizes isolator µModule technology to
translate signals and power across an isolation barrier.
Signals on either side of the barrier are encoded into
pulses and translated across the isolation boundary using
coreless transformers formed in the µModule substrate.
This system, complete with data refresh, error checking,
safe shutdown on fail, and extremely high common mode
immunity, provides a robust solution for bidirectional
signal isolation. The µModule technology provides the
means to combine the isolated signaling with our RS485
transceiver and powerful isolated DC/DC converter in one
small package.
formance Characteristics graph, V
Surplus Current vs
CC2
Temperature. Figure 19 demonstrates a method of using
the V
output directly and with a switched power path
that is controlled with the isolated RS485 data channel.
CC2
Driver
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.
DC/DC Converter
Driver Overvoltage and Overcurrent Protection
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 outputs are protected from short circuits to
any voltage within the absolute maximum range of (V
CC2
cur-
–15V) to (GND2 +15V) levels. The maximum V
CC2
rent 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 section).
The DC/DC converter is connected to a low dropout reg-
ulator (LDO) to provide a regulated low noise 5V output.
2881fa
ꢀꢁ
LTM2881
applicaTions inForMaTion
0
6.25
12.5
0
6.25
12.5
FREQUENCY (MHz)
FREQUENCY (MHz)
2881 F09b
2881 F09a
Figure 9a. Frequency Spectrum SLO Mode ±25kHz Input
Figure 9b. Normal Mode Frequency Spectrum ±25kHz Input
SLO Mode
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.
TheLTM2881featuresalogic-selectablereducedslewrate
mode (SLO mode) 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.
The receiver output is internally driven high (to V ) or low
L
(toGND)withnoexternalpull-upneeded.Whenthereceiver
is disabled the RO pin becomes Hi-Z with leakage of less
than 1µA for voltages within the supply range.
Figures 9a and 9b show the frequency spectrums of the
LTM2881 driver outputs in normal and SLO mode operat-
ing at 250kbps. SLO mode significantly reduces the high
frequency harmonics.
Receiver Input Resistance
The receiver input resistance from A or B to GND2 is
greater than 96k permitting up to a total of 256 receivers
per system without exceeding the RS485 receiver load-
ing specification. The input resistance of the receiver is
unaffected by enabling/disabling the receiver or by power-
ing/unpowering the part. The equivalent input resistance
looking into A and B is shown in Figure 10.
Receiver and Failsafe
With the receiver enabled, when the absolute value of the
differential voltage between the A and B pins is greater
than 200mV, the state of RO will reflect the polarity of (A-
B). During data communication the receiver detects the
state of the input with symmetric thresholds around 0V.
The symmetric thresholds preserve duty cycle for attenu-
ated signals with slow transition rates on high capacitive
busses, 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,leftopenorterminated,butnotdriven.Thefailsafe
feature eliminates the need for system level integration of
network pre-biasing by guaranteeing a logic-high on RO
under the conditions of an idle bus. Further network bias-
ing constructed to condition transient noise during an idle
A
>96k
60Ω
TE
60Ω
B
2881 F10
>96k
Figure ±0. Equivalent Input Resistance into A and ꢀ
2881fa
ꢀꢂ
LTM2881
applicaTions inForMaTion
Switchable Termination
phase of the termination impedance versus frequency.
The termination resistor cannot be enabled by TE if the
device is unpowered, ON is low or the LTM2881 is in
thermal shutdown.
Propercableterminationisveryimportantforsignalfidelity.
If the cable is not terminated with its characteristic imped-
ance, reflections will distort the signal waveforms.
Supply Current
The integrated switchable termination resistor provides
logic control of the line termination for optimal perfor-
mance when configuring transceiver networks.
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. Supply current
increases with additional external applications drawing
WhentheTEpinishigh,theterminationresistorisenabled
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 re-
sponsewhichdoesnotlimitperformanceatthemaximum
specified data rate. Figure 13 shows the magnitude and
current from V
.
CC2
130
128
126
124
122
120
118
116
114
112
110
–10
–5
0
5
10
15
COMMON MODE VOLTAGE (V)
2881 G11
2881 F11
Figure ±±. Curve Trace ꢀetween A and ꢀ with Termination
Enabled and Disabled
Figure ±2. Termination Resistance vs Common Mode Voltage
150
140
130
120
10
250
230
PHASE
0
210
LTM2881-3
190
170
150
130
110
90
R=54 CL=1000p
R=54 CL=100p
R=54 CL=0
–10
–20
MAGNITUDE
LTM2881-5
R=54 CL=1000p
R=54 CL=100p
R=54 CL=0
110
100
–30
–40
70
50
0.1
1
10
0.1
1
10
FREQUENCY (MHz)
DATA RATE (Mbps)
2881 F13
2881 F14
Figure ±3. Termination Magnitude and Phase vs Frequency
Figure ±4. Supply Current vs Data Rate
2881fa
ꢀꢃ
LTM2881
applicaTions inForMaTion
PCꢀ Layout Considerations
antennastructure,whichcanradiatedifferentialvoltages
formed between GND and GND2. If ground planes are
used, minimize their area, and use contiguous planes,
any openings or splits can increase RF emissions.
The high integration of the LTM2881 makes PCB layout
very simple. However, to optimize its electrical isolation
characteristics, EMI, and thermal performance, some
layout considerations are necessary. The PCB layout in
Figure 15 shows a recommended configuration for a low
EMI RS485 application.
• For large ground planes a small capacitance (≤ 330pF)
from GND to GND2, either discrete or embedded within
the substrate, provides a low impedance current return
path for the module parasitic capacitance, minimizing
anyhighfrequencydifferentialvoltagesandsubstantially
reducingradiatedemissions.Discretecapacitanceisnot
as effective due to parasitic ESL; in addition consider
voltage rating, leakage, and clearance for component
selection. Embedding the capacitance within the PCB
substrateprovidesanearidealcapacitorandeliminates
theothercomponentselectionissues,howeverthePCB
must be 4 layers and the use of a slot is not compatible.
Exercise care in applying either technique to insure the
voltage rating of the barrier is not compromised.
• Under heavily loaded conditions, V and GND current
CC
can exceed 300mA. Use sufficient copper on the PCB to
ensure resistive losses do not cause the supply voltage
to drop below the minimum allowed level. Similarly,
size the V
and GND2 conductors to support any
CC2
external load current. These heavy copper traces will
also help to reduce thermal stress and improve the
thermal conductivity.
• Input and Output decoupling is not required, since
these components are integrated within the package.
If an additional decoupling capacitor is used a value of
6.8µF to 22µF is recommended. The recommendation
forEMIsensitiveapplicationsistoincludeanadditional
low ESL ceramic capacitor of 1µF to 4.7µF, placed close
to the power and ground terminals. Alternatively, use a
number of smaller value parallel capacitors to reduce
ESL and achieve the same net capacitance.
C1
V
= V
L
= ON
CC
RO
A
B
Z
Y
RE
DE
DI
• Hot plugging the LTM2881 voltage supply without ad-
ditional protection may cause device damage. Refer to
LinearTechnologyApplicationNote88,entitled“Ceramic
Capacitors Can Cause Overvoltage Transients” for a
detailed discussion of this problem. To protect against
hotplugtransientsusea6.8µFtantalumastheadditional
decoupling capacitor.
TE
GND
• Do not place copper on the PCB between the inner col-
umnsofpads. Thisareamustremainopentowithstand
the rated isolation voltage. Slot the PCB in this area to
facilitate cleaning and ensure contamination does not
compromise the isolation voltage.
• The recommendation for non-EMI critical applications
is to use solid ground planes for GND and GND2 for
optimizing signal fidelity, thermal performance, and to
minimize RF emissions due to uncoupled PCB trace
conduction. The drawback of using ground planes,
where EMI is of concern, is the creation of a dipole
2881 F15
Figure ±5. PCꢀ Recommended Layout
2881fa
ꢀꢄ
LTM2881
applicaTions inForMaTion
Cable Length versus Data Rate
RF, Magnetic Field Immunity
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.
The LTM2881 has been independently evaluated and has
successfully passed the RF and magnetic field immunity
testing requirements per European Standard EN 55024,
in accordance with the following test standards:
EN 61000-4-3 Radiated, Radio-Frequency,
Electromagnetic Field Immunity
EN 61000-4-8 Power Frequency Magnetic Field
Immunity
EN 61000-4-9 Pulsed Magnetic Field Immunity
Tests were performed using an unshielded test card de-
signed per the data sheet PCB layout recommendations.
Specific limits per test are detailed in Table 1.
10k
Table ±. Test Frequency Field Strength
EN 61000-4-3, Annex D 80MHz to 1GHz
1.4MHz to 2GHz
10V/m
3V/m
1V/m
LOW-EMI MODE
MAX DATA RATE
2GHz to 2.7GHz
1k
100
10
EN61000-4-8, Level 4 50Hz and 60Hz
EN61000-4-8, Level 5 60Hz
EN61000-4-9, Level 5 Pulse
*Non IEC Method
30A/m
100A/m*
1000A/m
NORMAL
MODE MAX
DATA RATE
RS485 MAX
DATA RATE
10k
100k
1M
10M
100M
DATA RATE (bps)
2881 F16
Figure ±6. Cable Length vs Data Rate
2881fa
ꢀꢅ
LTM2881
Typical applicaTions
V
V
CC
CC
LTM2881
V
L
A
B
RO
RE
TE
DE
Y
DI
Z
330k
D
IN
D
OUT
GND
GND2
FAULT
2881 F17
Figure ±7. Isolated System Fault Detection
V
V
CC
CC
LTM2881
PWR
V
L
A
B
RO
RE
TE
DE
DI
Y
Z
GND
GND2
2881 F18
Figure ±8. Full-Duplex RS485 Connection
2881fa
ꢀꢆ
LTM2881
Typical applicaTions
V
V
CC
REGULATED 5V
SWITCHED 5V
1.8V
V
CC
CC2
A
PWR
V
L
RO
IRLML6402
B
RE
LTM2881
TE
DE
330k
DI
D
OFF ON
Z
D
OUT
IN
GND
GND2
CMOS OUTPUT
CMOS INPUT
2881 F19
Figure ±9. Switched 5V Power with Isolated CMOS Logic Connection with Low Voltage Interface
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 F20
BUS INHERITED
B
Figure 20. 4-Wire Full Duplex Self ꢀiasing for Unshielded CAT5 Connection
2881fa
ꢀꢇ
LTM2881
package DescripTion
/ / b b b
Z
4 . 4 4 5
3 . 1 7 5
1 . 9 0 5
0 . 6 3 5
0 . 6 3 5
0 . 0 0 0
1 . 9 0 5
3 . 1 7 5
4 . 4 4 5
a a a
Z
2881fa
ꢀꢈ
LTM2881
package DescripTion
Z
b b b
Z
4 . 4 4 5
3 . 1 7 5
1 . 9 0 5
0 . 6 3 5
0 . 6 3 5
1 . 9 0 5
3 . 1 7 5
4 . 4 4 5
a a a
Z
2881fa
ꢁ0
LTM2881
revision hisTory
REV
DATE
DESCRIPTION
PAGE NUMꢀER
A
3/10
Changes to Features, Description and Typical Application
Add BGA Package to Pin Configuration, Order Information and Package Description Sections
Changes to LGA Package in Pin Configuration Section
Changes to Electrical Characteristics Section
Changes to Graphs G09, G13, G14
1
2, 19
2
3
6, 7
8
Update to Pin Functions
Update to Applications Information
12
13
14
15
16
22
Change to X-Axis on Figures 9a and 9b
Update to Supply Current Section
“PCB Layout Isolation Considerations” Section Replaced
RF, Magnetic Field Immunity Section Added
Changes to Related Parts
2881fa
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.
ꢁꢀ
LTM2881
Typical applicaTion
V
CCC
V
CCA
V
CC
V
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 F21
B
Figure 2±. Multi-Node Network with End Termination
and Single Ground Connection on Isolation ꢀus
relaTeD parTs
PART NUMꢀER
LTM2882
LTC1535
LT1785
DESCRIPTION
COMMENTS
Dual Isolated RS232 µModule Transceiver + Power
Isolated RS485 Transceiver
1Mbps, 10kV HBM ESD, 2500V
RMS
2500V
Isolation in Surface Mount Package
RMS
60V Fault-Protected Transceiver
Half Duplex
Full Duplex
LT1791
60V Fault-Protected Transceiver
LTC2861
20Mbps RS485 Transceivers with Integrated Switchable Termination
Full Duplex 15kV ESD
2881fa
LT 0310 REV A • PRINTED IN USA
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
ꢁꢁ
●
●
LINEAR TECHNOLOGY CORPORATION 2009
(408) 432-1900 FAX: (408) 434-0507 www.linear.com
相关型号：
LTM2882CV-3#PBF
LTM2882 - Dual Isolated RS232 uModule Transceiver + Power; Package: LGA; Pins: 32; Temperature Range: 0°C to 70°C
Linear
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