LINER LTM2881IV-5 Complete isolated rs485/rs422 î¼module transceiver power Datasheet

LTM2881
Complete Isolated
RS485/RS422 µModule
Transceiver + Power
DESCRIPTION
FEATURES
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Isolator μModule Technology
Isolated RS485/RS422 Transceiver: 2500VRMS
Integrated Isolated DC/DC Converter: 1W, 62% Efficiency
No External Components Required
20Mbps or Low EMI 250kbps Data Rate
High ESD: ±15kV HBM on Transceiver Interface
High Common Mode Transient Immunity: 30kV/μs
Integrated Selectable 120Ω Termination
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: 560VPEAK
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)
General Purpose CMOS Isolated Channel
Small, Low Profile (15mm × 11.25mm × 2.8mm)
Surface Mount LGA Package
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Coupled inductors and an isolation power transformer provide
2500VRMS of isolation between the line transceiver and the
logic interface. This device is ideal for systems where the
ground loop is broken allowing for large common mode
voltage variation. Uninterrupted communication is guaranteed
for common mode transients greater than 30kV/μs.
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.
Enhanced ESD protection allows this part to withstand up
to ±15kV (human body model) on the transceiver interface
pins to isolated supplies and ±10kV through the isolation
barrier to logic supplies without latch-up or damage.
APPLICATIONS
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The LTM®2881 is a complete galvanically isolated fullduplex 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.
Isolated RS485/RS422 Interface
Industrial Networks
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
VCC
LTM2881
PWR
VL
RO
500V/DIV
ISOLATION BARRIER
A
RE
TE
DE
DI
B
TWISTED-PAIR
CABLE
Y
Z
GND
DI
RO
1V/DIV
1V/DIV
50ns/DIV
2881 TA01a
GND2
2881 TA01
2881f
1
LTM2881
ABSOLUTE MAXIMUM RATINGS
PIN CONFIGURATION
(Note 1)
VCC to GND .................................................. –0.3V to 6V
VCC2 to GND2 ............................................... –0.3V to 6V
VL to GND .................................................... –0.3V to 6V
Interface Voltages
(A, B, Y, Z) to GND2 ........................ VCC2 –15V to 15V
Signal Voltages ON, RO, DI, DE,
RE, TE, DOUT to GND ......................... –0.3V to VL +0.3V
Signal Voltages SLO,
DIN to GND2 ....................................–0.3V to VCC2 +0.3V
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
TOP VIEW
1
2
DOUT TE
3
4
5
6
7
8
DI DE RE RO VL ON
A
B
VCC
GND
C
D
E
F
G
H
J
GND2
K
L
DIN SLO Y
Z
B
A
VCC2
LGA PACKAGE
32-PIN (15mm s 11.25mm s 2.8mm)
TJMAX = 125°C, θJA = 32°C/W WEIGHT = 1g
ORDER INFORMATION
LEAD FREE FINISH
TRAY
PART MARKING*
PACKAGE DESCRIPTION
TEMPERATURE RANGE
LTM2881CV-3#PBF
LTM2881CV-3#PBF
LTM2881V-3
32-Pin (15mm × 11.25mm × 2.8mm) LGA
0°C to 70°C
LTM2881IV-3#PBF
LTM2881IV-3#PBF
LTM2881V-3
32-Pin (15mm × 11.25mm × 2.8mm) LGA
–40°C to 85°C
LTM2881CV-5#PBF
LTM2881CV-5#PBF
LTM2881V-5
32-Pin (15mm × 11.25mm × 2.8mm) LGA
0°C to 70°C
LTM2881IV-5#PBF
LTM2881IV-5#PBF
LTM2881V-5
32-Pin (15mm × 11.25mm × 2.8mm) LGA
–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.
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
l
l
3.0
4.5
3.3
5.0
3.6
5.5
l
1.62
UNITS
VCC
VCC Supply Voltage
LTM2881-3
LTM2881-5
VL
VL Supply Voltage
5.5
V
ICCPOFF
VCC Supply Current in Off Mode
ON = 0V
l
0
10
μA
ICCS
VCC Supply Current in On Mode
LTM2881-3 DE = 0V, RE = VL , No Load
LTM2881-5 DE = 0V, RE = VL , No Load
l
l
20
15
25
19
mA
mA
VCC2
Regulated VCC2 Output Voltage, Loaded
LTM2881-3 DE = 0V, RE = VL, ILOAD = 100mA
LTM2881-5 DE = 0V, RE = VL, ILOAD = 180mA
l
VCC2NOLOAD
Regulated VCC2 Output Voltage, No Load
DE = 0V, RE = VL , No Load
Efficiency
ICC2 = 100mA (Note 2)
VCC2 Short-Circuit Current
DE = 0V, RE = VL , VCC2 = 0V
l
Differential Driver Output Voltage
R = ∞ (Figure 1)
R = 27Ω (RS485) (Figure 1)
R = 50Ω (RS422) (Figure 1)
l
l
l
R = 27Ω or R = 50Ω (Figure 1)
l
Power Supply
ICC2S
4.7
4.7
5.0
5.0
4.8
5.0
V
V
V
V
5.35
V
250
mA
VCC2
VCC2
VCC2
V
V
V
0.2
V
50
%
Driver
|VOD|
Δ|VOD|
Difference in Magnitude of Driver Differential
Output Voltage for Complementary Output
States
1.5
2
VOC
Driver Common Mode Output Voltage
R = 27Ω or R = 50Ω (Figure 1)
l
3
V
Δ|VOC|
Difference in Magnitude of Driver
Common Mode Output Voltage for
Complementary Output States
R = 27Ω or R = 50Ω (Figure 1)
l
0.2
V
IOZD
Driver Three-State (High Impedance) Output
Current on Y and Z
DE = 0V, (Y or Z) = –7V, +12V
l
±10
μA
IOSD
Maximum Driver Short-Circuit Current
– 7V ≤ (Y or Z) ≤ 12V (Figure 2)
l
– 250
250
mA
RIN
Receiver Input Resistance
RE = 0V or VL , VIN = –7V, –3V, 3V, 7V,
12V (Figure 3)
l
96
125
RTE
Receiver Termination Resistance Enabled
TE = VL , VAB = 2V, VB = – 7V, 0V, 10V
(Figure 8)
l
108
120
IIN
Receiver Input Current (A, B)
ON = 0V VCC2 = 0V or 5V, VIN = 12V
(Figure 3)
l
ON = 0V VCC2 = 0V or 5V, VIN = –7V
(Figure 3)
l
–100
l
–0.2
Receiver
VTH
Receiver Differential Input Threshold Voltage
(A-B)
–7V ≤ B ≤ 12V
ΔVTH
Receiver Input Failsafe Hysteresis
B = 0V
Receiver Input Failsafe Threshold
B = 0V
kΩ
156
Ω
125
μA
μA
0.2
25
–0.2
–0.05
V
mV
0
V
Logic
VIL
Logic Input Low Voltage
1.62V ≤ VL ≤ 5.5V
l
Logic Input High Voltage
DIN
SLO
DI, TE, DE, ON, RE:
VL ≥ 2.35V
1.62V ≤ VL < 2.35V
l
l
0.67•VCC2
2
V
V
l
l
0.67•VL
0.75•VL
V
V
VIH
0.4
V
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.
SYMBOL
PARAMETER
IINL
Logic Input Current
VHYS
Logic Input Hysteresis
(Note 2)
VOH
Output High Voltage
Output High, ILOAD = –4mA
(Sourcing), 5.5V ≥ VL ≥ 3V
Output High, ILOAD = –1mA
(Sourcing), 1.62V ≤ VL < 3V
l
VL –0.4
V
l
VL –0.4
V
Output Low, ILO AD = 4mA
(Sinking), 5.5V ≥ VL ≥ 3V
Output High, ILOAD = 1mA
(Sinking), 1.62V ≤ VL < 3V
l
0.4
V
l
0.4
V
VOL
Output Low Voltage
CONDITIONS
MIN
l
TYP
MAX
0
±1
150
UNITS
μA
mV
IOZR
Three-State (High Impedance) Output Current RE = VL , 0V ≤ RO ≤ VL
on RO
l
±1
μA
IOSR
Short-Circuit Current
l
±85
mA
0V ≤ (RO or DOUT) ≤ VL
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.
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
Driver SLO = VCC2
fMAX
Maximum Data Rate
(Note 3)
tPLHD
tPHLD
Driver Input to Output
RDIFF = 54Ω, CL = 100pF
(Figure 4)
l
20
60
85
Mbps
ns
ΔtPD
Driver Input to Output Difference
|tPLHD – tPHLD|
RDIFF = 54Ω, CL = 100pF
(Figure 4)
l
1
8
ns
tSKEWD
Driver Output Y to Output Z
RDIFF = 54Ω, CL = 100pF
(Figure 4)
l
1
±8
ns
tRD
tFD
Driver Rise or Fall Time
RDIFF = 54Ω, CL = 100pF
(Figure 4)
l
4
12.5
ns
tZLD , tZHD ,
tLZD , tHZD
Driver Output Enable or Disable Time
RL = 500Ω, CL = 50pF
(Figure 5)
l
170
ns
Driver SLO = GND2
fMAX
Maximum Data Rate
(Note 3)
250
kbps
tPLHD
tPHLD
Driver Input to Output
RDIFF = 54Ω, CL = 100pF
(Figure 4)
1
1.55
μs
ΔtPD
Driver Input to Output Difference
|tPLHD – tPHLD|
RDIFF = 54Ω, CL = 100pF
(Figure 4)
50
500
ns
tSKEWD
Driver Output Y to Output Z
RDIFF = 54Ω, CL = 100pF
(Figure 4)
±200
±500
ns
tRD
tFD
Driver Rise or Fall Time
RDIFF = 54Ω, CL = 100pF
(Figure 4)
l
0.9
1.5
μs
tZLD , tZHD ,
tLZD , tHZD
Driver Output Enable or Disable Time
RL = 500Ω, CL = 50pF
(Figure 5)
l
400
ns
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.
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
tPLHR
tPHLR
Receiver Input to Output
CL = 15pF, VCM = 2.5V, |VAB| = 1.4V,
tR and tF < 4ns, (Figure 6)
l
100
140
ns
tSKEWR
Differential Receiver Skew
|tPLHR - tPHLR|
CL = 15pF
(Figure 6)
l
1
8
ns
tRR
tFR
Receiver Output Rise or Fall Time
CL = 15pF
(Figure 6)
l
3
12.5
ns
tZLR , tZHR ,
tLZR , tHZR
Receiver Output Enable Time
RL =1kΩ, CL = 15pF
(Figure 7)
l
50
ns
tRTEN , tRTZ
Termination Enable or Disable Time
RE = 0V, DE = 0V, VAB = 2V, VB = 0V
(Figure 8)
l
100
μs
CL = 15pF,
tR and tF < 4ns
l
60
100
ns
l
325
800
μs
Receiver
Generic Logic Input
tPLHL1
tPHLL1
DIN to DOUT Input to Output
Power Supply Generator
VCC2 –GND2 Supply Start-Up Time
(0V to 4.5V)
ON
ISOLATION CHARACTERISTICS
otherwise noted.
SYMBOL
PARAMETER
VISO
Rated Dielectric Insulation Voltage
TA = 25°C, LTM2881-3 VCC = 3.3V, LTM2881-5 VCC = 5.0V, VL = 3.3V unless
CONDITIONS
MIN
TYP
MAX
UNITS
1 Minute (Derived from 1 Second Test)
2500
VRMS
1 Second
±4400
VDC
(Note 2)
30
Maximum Working Insulation Voltage
(Note 2)
560
Partial Discharge
VPR = 1050 VPEAK (Note 2)
Common Mode Transient Immunity
VIORM
VL, No Load
kV/μs
VPEAK
<5
>109
pC
Ω
Input to Output Resistance
(Note 2)
Input to Output Capacitance
(Note 2)
6
pF
Creepage Distance
(Note 2)
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
and is not tested directly.
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.
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
Driver Skew vs Temperature
2.0
80
1.5
1.5
75
1.0
0.5
0
–0.5
DRIVER PROP DELAY (ns)
2.0
DRIVER SKEW (ns)
RECEIVER SKEW (ns)
Receiver Skew vs Temperature
1.0
0.5
0
–25
0
25
50
TEMPERATURE (°C)
75
–1.0
–50
100
–25
0
25
50
TEMPERATURE (°C)
75
2881 G01
4.5
126
4.0
124
122
120
118
116
2.0
1.5
0.5
0
100
OUTPUT LOW
R = 54Ω
2
0
10
20
30
40
50
OUTPUT CURRENT (mA)
60
0
–50
70
–25
0
25
50
TEMPERATURE (°C)
75
100
2881 G06
Receiver Propagation Delay
vs Temperature
Supply Current vs Data Rate
120
200
180
3
2
1
115
160
SUPPLY CURRENT (mA)
RECEIVER PROP DELAY (ns)
SOURCE
OUTPUT VOLTAGE (V)
R = 100Ω
3
2881 G05
4
110
105
100
R = 54 (–3)
140
120
100
R = 100 (–3)
R = 54 (–5)
80
60
R = 100 (–5)
40
95
20
SINK
0
2
3
4
OUTPUT CURRENT (mA)
R=∞
4
1
2881 G04
Receiver Output Voltage vs
Output Current (Source and Sink)
100
5
OUTPUT HIGH
2.5
112
1
75
6
3.0
1.0
0
0
25
50
TEMPERATURE (°C)
Driver Differential Output Voltage
vs Temperature
3.5
114
75
–25
2881 G03
OUTPUT VOLTAGE (V)
128
OUTPUT VOLTAGE (V)
RESISTANCE (Ω)
5.0
0
25
50
TEMPERATURE (°C)
50
–50
100
Driver Output Low/High Voltage
vs Output Current
130
–25
60
2881 G02
RTERM vs Temperature
110
–50
65
55
–0.5
–1.0
–50
70
5
2881 G07
90
–50
–25
0
25
50
TEMPERATURE (°C)
75
100
2881 G08
0
0.1
R = ∞ (–3)
R = ∞ (–5)
1
DATA RATE (Mbps)
10
2881 G09
2881f
6
LTM2881
TYPICAL PERFORMANCE CHARACTERISTICS
VCC = 5.0V, VL = 3.3V unless otherwise noted.
VCC2 Surplus Current
vs Temperature
VCC Supply Current vs Temperature
at ILOAD = 100mA on VCC2
VCC2 vs Load Current
250
350
6
250
200
LTM2881-5
150
100
200
LTM2881-5
LTM2881-5 (RS485 60mA)
150
LTM2881-5 (RS485 90mA)
LTM2881-3 (RS485 60mA)
100
5
VOLTAGE (V)
SURPLUS CURRENT (mA)
LTM2881-3
300
ICC CURRENT (mA)
TA = 25°C, LTM2881-3 VCC = 3.3V, LTM2881-5
LTM2881-3
4
3
50
50
LTM2881-3 (RS485 90mA)
0
–50
–25
0
25
50
TEMPERATURE (°C)
75
100
0
–50
2
–25
0
25
50
TEMPERATURE (°C)
75
100
10
2881 G11
2881 G10
VCC2 Power Efficiency
20
40 60 80 100 120 140 160 180
VCC2 LOAD CURRENT (mA)
2881 G12
VCC2 Load Step (100mA)
VCC2 Noise
70
LTM2881-5
EFFICIENCY (%)
60
VCC2
100mV/DIV
50
LTM2881-3
10mV/DIV
40
ILOAD
50mA/DIV
30
20
1000μs/DIV
2881 G14
200μs/DIV
2881 G15
10
0
150
50
100
ICC2 OUTPUT CURRENT (mA)
200
2881 G13
2881f
7
LTM2881
PIN FUNCTIONS
LOGIC SIDE (VCC , VL, GND)
ISOLATED SIDE (VCC2, GND2)
DOUT (Pin A1): General Purpose Logic Output. Logic
output connected through isolation path to DIN . Under
the condition of an isolation communication failure DOUT
is in a high impedance state.
DIN (Pin L1): General Purpose Isolated Logic Input. Logic
input on the isolated side relative to VCC2 and GND2. A
logic high on DIN will generate a logic high on DOUT. A
logic low on DIN will generate a logic low on DOUT.
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,
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 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.
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.
RE (Pin A5): Receiver Enable. A logic low enables the
receiver output. A logic high disables RO to a high
impedance state.
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.
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.
B (Pin L5): Inverting Receiver Input. Impedance is > 96kΩ
in receive mode with TE low or unpowered.
A (Pin L6): Non Inverting Receiver Input. Impedance is
> 96kΩ in receive mode with TE low or unpowered.
VCC2 (Pins L7-L8): Isolated Supply Voltage. Internally
generated from VCC by an isolated DC/DC converter and
regulated to 5V.
GND2 (Pins K1-K8): Isolated Side Circuit Ground. The
pads should be connected to the isolated ground and/or
cable shield.
VL (Pin A7): Logic Supply. Interface supply voltage for
pins RO, RE, TE, DI, DE, DOUT, and ON. Recommended
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 B1-B5): Circuit Ground.
VCC (Pins B6-B8): Supply Voltage. Recommended
operating voltage is 3V to 3.6V for LTM2881-3 and 4.5V
to 5.5V for LTM2881-5.
2881f
8
LTM2881
BLOCK DIAGRAM
VCC
2.2μF
VCC2
5V
REG
ISOLATED
DC/DC
CONVERTER
2.2μF
VL
2.2μF
A
RO
RX
B
RE
ISOLATED
COMM
INTERFACE
DE
ISOLATED
COMM
INTERFACE
120Ω
DI
Y
DX
ON
Z
SLO
TE
DIN
DOUT
GND
GND2
2881 BD
= LOGIC SIDE COMMON
= ISOLATED SIDE COMMON
TEST CIRCUITS
Y
GND
OR
VL
DI
Y
+
DRIVER
R
GND
OR
VL
VOD
–
Z
R
+
–
DI
IOSD
DRIVER
VOC
Z
2881 F01
–7V TO 12V
2881 F02
Figure 1. Driver DC Characteristics
Figure 2. Driver Output Short-Circuit Current
IIN
VIN
+
–
+
–
A OR B
RECEIVER
B OR A
2881 F03
V
RIN = IN
IIN
Figure 3. Receiver Input Current and Input Resistance
2881f
9
LTM2881
TEST CIRCUITS
VL
Y
DI
tPLHD
DI
tPHLD
0V
tSKEWD
CL
DRIVER
RDIFF
Y, Z VOD
1/2 VOD
CL
Z
90%
2881 F04a
(Y-Z)
10%
90%
0
0
10%
tRD
tFD
2881 F04b
Figure 4. Driver Timing Measurement
RL
Y
VL
OR
GND
DI
CL
GND
OR
VCC2
DRIVER
RL
DE
Z
CL
VL
DE
1/2 VL
0V
tZLD
tLZD
VCC2
Y OR Z
1/2 VCC2
Z OR Y
1/2 VCC2
0.5V
VCC2
OR
GND
2881 F05a
0.5V
0V
2881 F05b
tZHD
tHZD
Figure 5. Driver Enable and Disable Timing Measurements
tR
±VAB/2
VCM
±VAB/2
VAB
A-B
–VAB
A
RECEIVER
B
RO
CL
2881 F06a
VL
RO
0
90%
10%
tF
90%
0
10%
tPLHR
90%
1/2 VL
10%
tRR
tPHLR
1/2 VL
90%
10%
tFR
2881 F06b
Figure 6. Receiver Propagation Delay Measurements
2881f
10
LTM2881
TEST CIRCUITS
VL
RE
1/2 VL
0V
A
0V OR VCC2
RL
RO
RECEIVER
B
VCC2 OR 0V
VL
OR
GND
CL
RE
tZLR
VL
tLZR
1/2 VL
RO
0.5V
VOL
VOH
0.5V
1/2 VL
RO
0V
2881 F07a
2881 F07b
tZHR
tHZR
Figure 7. Receiver Enable/Disable Time Measurements
IA
RTE =
VAB
IA
VL
A
RO
+
–
RECEIVER
TE
VAB
1/2 VL
0V
IA
B
+
–
TE
tRTEN
tRTZ
90%
10%
VB
2881 F08
Figure 8. Termination Resistance and Timing Measurements
FUNCTIONAL TABLE
LOGIC INPUTS
MODE
A, B
Y, Z
RO
DC/DC
CONVERTER
TERMINATOR
RIN
Hi-Z
Enabled
On
Off
ON
RE
TE
DE
1
0
0
0
Receive
1
0
0
1
Transceiver
RIN
Driven
Enabled
On
Off
1
1
0
1
Transmit
RIN
Driven
Hi-Z
On
Off
1
0
1
0
Receive + Term On
RTE
Hi-Z
Enabled
On
On
0
X
X
X
Off
RIN
Hi-Z
Hi-Z
Off
Off
2881f
11
LTM2881
APPLICATIONS INFORMATION
Overview
The LTM2881 μModule transceiver provides a galvanicallyisolated 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/μs providing
excellent noise isolation.
The DC/DC converter is connected to a low dropout regulator (LDO) to provide a regulated low noise 5V output.
The internal power solution is sufficient to support the
transceiver interface at its maximum specified load and data
rate, and external pins are supplied for extra decoupling
(optional) and heat dissipation. The logic supplies, VCC
and VL have a 2.2μF decoupling capacitance to GND
and the isolated supply VCC2 has a 2.2μF decoupling
capacitance to GND2 within the μModule package. 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, VCC2 Surplus
Current vs Temperature.
DC/DC Converter
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 fullbridge 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.
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.
2881f
12
LTM2881
APPLICATIONS INFORMATION
Driver Overvoltage and Overcurrent Protection
The driver outputs are protected from short circuits to
any voltage within the absolute maximum range of (VCC2
–15V) to (GND2 +15V) levels. The maximum VCC2 current
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 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.
The receiver output is internally driven high (to VL) or low
(to GND) with no external pull-up needed. When the receiver
is disabled the RO pin becomes Hi-Z with leakage of less
than ±1μA for voltages within the supply range.
The LTM2881 features a logic-selectable reduced slew rate
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
of imperfect transmission line termination caused by stubs
or mismatched cables.
Figures 9a and 9b show the frequency spectrums of the
LTM2881 driver outputs in normal and SLO mode operating
at 250kbps. SLO mode significantly reduces the high
frequency harmonics.
Y-Z 10dB/DIV
SLO Mode
0
6.25
FREQUENCY 1.25MHz/DIV
12.5
2881 F09a
Figure 9a. Frequency Spectrum SLO Mode 125kHz Input
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 attenuated
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 logichigh during an idle bus, when the inputs are shorted, left
open or terminated, but not driven. The failsafe feature
eliminates the need for system level integration of network
pre-biasing by guaranteeing a logic-high on RO under
Y-Z 10dB/DIV
Receiver and Failsafe
0
6.25
FREQUENCY 1.25MHz/DIV
12.5
2881 F09b
Figure 9b. Normal Mode Frequency Spectrum 125kHz Input
2881f
13
LTM2881
APPLICATIONS INFORMATION
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 loading specification.
The input resistance of the receiver is unaffected by
enabling/disabling the receiver or by powering/unpowering
the part. The equivalent input resistance looking into A and
B is shown in Figure 10.
A
2881 F11
Figure 11. Curve Trace Between A and B with Termination
Enabled and Disabled
>96k
60Ω
130
128
TE
126
>96k
2881 F10
RESISTANCE (Ω)
60Ω
B
Figure 10. Equivalent Input Resistance into A and B
124
122
120
118
116
114
112
Switchable Termination
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.
110
–10
15
2881 G11
Figure 12. Termination Resistance vs Common Mode Voltage
The integrated switchable termination resistor provides
logic control of the line termination for optimal performance when configuring transceiver networks.
150
10
MAGNITUDE (Ω)
PHASE
140
0
130
–10
120
MAGNITUDE
–20
110
100
0.1
PHASE (DEGREES)
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
magnitude and 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.
–5
0
5
10
COMMON MODE VOLTAGE (V)
–30
–40
1
FREQUENCY (MHz)
10
2881 F13
Figure 13. Termination Magnitude and Phase vs Frequency
2881f
14
LTM2881
APPLICATIONS INFORMATION
Supply Current
The static supply current is dominated by power delivered to
the termination resistance. Power supply current increases
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
• 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
in this area to insure contamination does not compromise
the isolation voltage.
230
190
170
LTM2881-3
R=54 CL=1000p
R=54 CL=100p
R=54 CL=0
150
130
110
90
LTM2881-5
R=54 CL=1000p
R=54 CL=100p
R=54 CL=0
70
VCC
50
0.1
1
DATA RATE (Mbps)
10
2881 F14
Figure 14. Supply Current vs Data Rate
GND2
VCC2
ON
VL
RO
A
RE
B
SLOT
SUPPLY CURRENT (mA)
210
• If the LTM2881 voltage supply is hot plugged without
additional protection, damage may occur. Refer to Linear
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.
DE
PCB Layout Isolation Considerations
The high integration of the LTM2881 makes the PCB board
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.
Z
DI
Y
TE
SLO
DO1
9.48mm
DL1
GND
2881 F15
Figure 15. PCB Recommended Layout
• Use large PCB copper areas for high current paths,
including VCC , GND, VCC2 , and GND2. It helps to
minimize the PCB conduction loss and thermal stress.
• The LTM2881 includes 2.2μF ceramic decoupling
capacitors on VCC to GND, VL to GND, and VCC2 to
GND2 supply pins. Further decoupling capacitance
(10μF) can be added within one-quarter inch away from
the VCC, VL , and/or VCC2 pin.
2881f
15
LTM2881
APPLICATIONS INFORMATION
Cable Length versus Data Rate
CABLE LENGTH (FT)
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
downward sloping region represents limits in distance and
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.
10k
LOW-EMI MODE
MAX DATA RATE
1k
NORMAL
MODE MAX
DATA RATE
100
RS485 MAX
DATA RATE
10
10k
100k
1M
10M
DATA RATE (bps)
100M
2881 F16
Figure 16. Cable Length vs Data Rate
2881f
16
LTM2881
TYPICAL APPLICATIONS
Full-Duplex RS485 Connection
VCC
VCC
LTM2881
PWR
VL
A
ISOLATION BARRIER
RO
RE
TE
DE
DI
B
Y
Z
GND
GND2
2881 TA02
Isolated System Fault Detection
VCC
VCC
LTM2881
VL
A
ISOLATION BARRIER
RO
RE
TE
DE
DI
330k
B
Y
Z
DIN
DOUT
FAULT
GND
GND2
2881 TA03
Switched 5V Power with Isolated CMOS Logic Connection with Low Voltage Interface
VCC
1.8V
VCC
VL
RO
ISOLATION BARRIER
TE
DE
DI
GND
IRLML6402
B
LTM2881
330k
Z
DIN
DOUT
CMOS OUTPUT
REGULATED 5V
SWITCHED 5V
A
RE
OFF ON
PWR
VCC2
GND2
CMOS INPUT
2881 TA04
2881f
17
LTM2881
TYPICAL APPLICATIONS
4-Wire Full Duplex Self Biasing for Unshielded CAT5 Connection
VCC
VCCB
ISOLATION BARRIER
RO
RE
DE
DI
LTM2881
LTM2881
PWR
A
Y
51Ω
B
Z
10nF
Y
51Ω
A
51Ω
Z
GND
GND2
VCC
PWR
ISOLATION BARRIER
VCC
VL
VL
DE
DI
RE
RO
B
51Ω
10nF
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 θ)
DETAIL A
2.69 – 2.95
8
aaa Z
7
6
5
4
3
2
1
PAD 1
A
PAD “A1”
CORNER
B
4
C
D
E
15.00
BSC
12.70
BSC
MOLD
CAP
F
G
SUBSTRATE
H
0.290 – 0.350
2.400 – 2.600
J
Z
bbb Z
DETAIL B
K
L
X
aaa Z
11.25
BSC
Y
8.89
BSC
DETAIL B
PACKAGE TOP VIEW
4.445
3.175
1.905
0.635
0.635
1.905
3
0.630 ±0.025 Ø 32x
eee S X Y
3.175
PADS
SEE NOTES
PACKAGE BOTTOM VIEW
0.630 ±0.025 Ø 32x
4.445
1.27
BSC
eee S X Y
DETAIL c
DETAIL A
6.350
5.080
DETAIL C
NOTES:
1. DIMENSIONING AND TOLERANCING PER ASME Y14.5M-1994
2. ALL DIMENSIONS ARE IN MILLIMETERS
0.000
3
LAND DESIGNATION PER JESD MO-222
4
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
5. PRIMARY DATUM -Z- IS SEATING PLANE
6. THE TOTAL NUMBER OF PADS: 32
5.080
6.350
COMPONENT
PIN “A1”
LTMXXXXXX
μModule
TRAY PIN 1
BEVEL
SYMBOL TOLERANCE
aaa
0.10
bbb
0.10
eee
0.05
PACKAGE IN TRAY LOADING ORIENTATION
LGA 32 0308 REV Ø
SUGGESTED PCB LAYOUT
TOP VIEW
2881f
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 representation
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 Bus
VCCC
VCCA
VCC
LTM2881
PWR
VL
ISOLATION BARRIER
CABLE SHIELD
OR GROUND RETURN
Y
Z
Z
GND2
GND2
A
A
RE
TE
VCC2
DE
DI
GND
C
Z
GND
B
Y
VL
RO
PWR
GND2
DI
B
Y
DE
A
B
VCC1
RE
TE
A
PWR
ISOLATION BARRIER
RO
VCC
LTM2881
ISOLATION BARRIER
GND
DI
DE
TE
RE
RO
VCC
VL
VCCB
LTM2881
B
2881 TA05
B
RELATED PARTS
PART NUMBER
DESCRIPTION
COMMENTS
LTC1535
Isolated RS485 Transceiver
2500VRMS Isolation in Surface Mount Package
LT1785
±60V Fault-Protected Transceiver
Half Duplex
LT1791
±60V Fault-Protected Transceiver
Full Duplex
LTC2861
20Mbps RS485 Transceivers with Integrated Switchable Termination
Full Duplex 15kV ESD
2881f
20 Linear Technology Corporation
LT 1109 • PRINTED IN USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 ● FAX: (408) 434-0507
●
www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2009
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