LTC1535
Isolated RS485 Transceiver
Features
Description
UL Rated Isolated RS485: 2500VRMS
UL Recognized
File #E151738
n Eliminates Ground Loops
n 250kBd Maximum Data Rate
n Self-Powered with 420kHz Converter
n Half- or Full-Duplex
n Fail-Safe Output High for Open or
Shorted Receiver Inputs
n Short-Circuit Current Limit
n Slow Slew Rate Control
n 68kΩ Input Impedance Allows Up to 128 Nodes
n Thermal Shutdown
n 8kV ESD Protection On Driver Outputs and
Receiver Inputs
n Available in 28-Lead SW Package
The LTC®1535 is an isolated RS485 full-duplex differential
line transceiver. Isolated RS485 is ideal for systems where
the ground loop is broken to allow for much larger common mode voltage ranges. An internal capacitive isolation barrier provides 2500VRMS of isolation between the
line transceiver and the logic level interface. The powered
side contains a 420kHz push-pull converter to power the
isolated RS485 transceiver. Internal full-duplex communication occurs through the capacitive isolation barrier.
The transceiver meets RS485 and RS422 requirements.
n
®
Applications
n
n
n
n
Isolated RS485 Receiver/Driver
RS485 with Large Common Mode Voltage
Breaking RS485 Ground Loops
Multiple Unterminated Line Taps
The driver and receiver feature three-state outputs, with
the driver maintaining high impedance over the entire
common mode range. The drivers have short-circuit current limits in both directions and a slow slew rate select
to minimize EMI or reflections. The 68kΩ receiver input
allows up to 128 node connections. A fail-safe feature
defaults to a high output state when the receiver inputs
are open or shorted.
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks of Analog
Devices, Inc. All other trademarks are the property of their respective owners.
Typical Application
**
CTX02-14659
1/2 BAT54C
+
10µF
2
2
VCC
10µF
1
+
VCC
ST1
1/2 BAT54C
3
ST2
2
11
14
GND2
VCC2
420kHz
A
1
LOGIC COMMON
28
RO
RO
R
B
1
FLOATING RS485 COMMON
2
RE
27
DE
26
DI
25
** EATON (888) 414-2645
4
RO2
RE
Y
DE
D
DI
Z
SLO
GND
1
16
15
TWISTED-PAIR
CABLE
17
13
12
18
1535 TA01
1535fc
For more information www.linear.com/LTC1535
1
LTC1535
Absolute Maximum Ratings
Pin Configuration
(Note 1)
VCC to GND..................................................................6V
VCC2 to GND2...............................................................8V
Control Input Voltage to GND........ – 0.3V to (VCC + 0.3V)
Driver Input Voltage to GND...........–0.3V to (VCC + 0.3V)
Driver Output Voltage
(Driver Disabled) to GND2................(VCC2 – 13V) to 13V
Driver Output Voltage
(Driver Enabled) to GND2................. (VCC2 – 13V) to 10V
Receiver Input Voltage to GND2............................... ±14V
Receiver Output Voltage................–0.3V to (VCC + 0.3V)
Operating Temperature Range
LTC1535C...........................................0°C ≤ TA ≤ 70°C
LTC1535I....................................... –40°C ≤ TA ≤ 85°C
Storage Temperature Range...................– 65°C to 150°C
Lead Temperature (Soldering, 10 sec).................... 300°C
Order Information
TOP VIEW
VCC 1
28 RO
ST1 2
27 RE
ST2 3
26 DE
GND 4
25 DI
GND2 11
18 SLO
Z 12
17 RO2
Y 13
16 A
VCC2 14
15 B
SW PACKAGE
28-LEAD PLASTIC SO
TJMAX = 125°C, θJA = 125°C/W
http://www.linear.com/product/LTC1535#orderinfo
LEAD FREE FINISH
TAPE AND REEL
PART MARKING*
PACKAGE DESCRIPTION
TEMPERATURE RANGE
LTC1535CSW#PBF
LTC1535CSW#TRPBF
1535
28-Lead Plastic SO
0°C to 70°C
LTC1535ISW#PBF
LTC1535ISW#TRPBF
1535
28-Lead Plastic SO
–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/
For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/. Some packages are available in 500 unit reels through
designated sales channels with #TRMPBF suffix.
2
1535fc
For more information www.linear.com/LTC1535
LTC1535
Electrical
Characteristics
The
l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VCC = 5V, VCC2 = 5V unless otherwise noted.
SYMBOL PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
VCC
VCC Supply Range
l
VCC2
VCC2 Supply Range
l
4.5
5.5
V
4.5
7.5
V
ICC
VCC Supply Current
Transformer Not Driven (Note 10)
l
13
28
mA
ICC2
VCC2 Supply Current
R = 27Ω, Figure 2
No Load
l
l
63
7
73
12
mA
mA
VOD1
Differential Driver Output
No Load
l
5
V
VOD2
Differential Driver Output
R = 50Ω (RS422) (Note 2), VCC2 = 4.5V
R = 27Ω(RS485), Figure 2, VCC2 = 4.5V
l
l
2
1.5
2
VOC
Driver Output Common Mode Voltage
DC Level, R = 50Ω, Figure 2
l
2.0
2.5
3.0
V
IOSD1
Driver Short-Circuit Current
VOUT = HIGH
VOUT = LOW
Driver Enabled (DE = 1)
–7V ≤ VCM ≤ 10V
–7V ≤ VCM ≤ 10V
l
l
60
60
100
100
150
150
mA
mA
VIH
Logic Input High Voltage
DE, DI, RE
SLO
l
l
2
4
1.7
2.2
VIL
Logic Input Low Voltage
DE, DI, RE
SLO
l
l
IIN
Input Current (A, B)
(Note 3)
1.7
1.8
V
V
V
V
0.8
1
V
V
VIN = 12V
l
0.25
mA
VIN = –7V
l
–0.20
mA
VTH
Receiver Input Threshold
–7V ≤ VCM ≤ 12V, (Note 4)
l
–200
–90
–10
mV
∆VTH
Receiver Input Hysteresis
–7V ≤ VCM ≤ 12V
0°C ≤ TA ≤ 70°C
l
10
30
70
mV
– 40°C ≤ TA ≤ 85°C
l
5
30
70
mV
l
50
68
100
kΩ
RIN
Receiver Input Impedance
VIOC
Receiver Input Open Circuit Voltage
VOH
RO Output High Voltage
IRO = – 4mA, VCC = 4.5V
l
VOL
RO Output Low Voltage
IRO = 4mA, VCC = 4.5V
l
IOZ
Driver Output Leakage
Driver Disabled (DE = 0)
VOH2
RO2 Output High Voltage
IRO2 = – 4mA, VCC = 4.5V
l
VOL2
RO2 Output Low Voltage
IRO2 = 4mA, VCC = 4.5V
l
fSW
DC Converter Frequency
l
RSWH
DC Converter Impedance High
RSWL
DC Converter Impedance Low
IREL
RE Output Low Current
IREH
RE Output High Current
VUVL
3.7
3.4
V
4.0
V
0.4
0.8
1
3.7
V
µA
3.9
V
0.4
0.8
V
420
590
kHz
l
4
6
Ω
l
2.5
5
Ω
290
RE Sink Current, Fault = 0
l
–40
–50
–80
µA
RE Source Current, Fault = 1
l
80
100
130
µA
Undervoltage Low Threshold
RE Fault = 1, (Note 5)
l
3.70
4.00
4.25
V
VUVH
Undervoltage High Threshold
RE Fault = 0, (Note 5)
l
4.05
4.20
4.40
V
VISO
Isolation Voltage
1 Minute, (Note 6)
1 Second
2500
3000
VRMS
VRMS
1535fc
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3
LTC1535
Electrical
Characteristics
The
l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VCC = 5V, VCC2 = 5V, R = 27Ω (RS485) unless otherwise noted.
SYMBOL PARAMETER
CONDITIONS
MIN
TYP
MAX
250
285
UNITS
tSJ
Data Sample Jitter
Figure 8, (Note 7)
l
fMAX
Max Baud Rate
Jitter = 10% Max, SLO = 1, (Note 8)
l
tPLH
Driver Input to Output
DE = 1, SLO = 1, Figure 4, Figure 6
DE = 1, SLO = 0, Figure 4, Figure 6
l
l
600
1300
855
1560
ns
ns
tPHL
Driver Input to Output
DE = 1, SLO = 1, Figure 4, Figure 6
DE = 1, SLO = 1, Figure 4, Figure 6
l
l
600
1300
855
1560
ns
ns
tr, tf
Driver Rise or Fall Time
DE = 1, SLO = 1, Figure 4, Figure 6
DE = 1, SLO = 0, VCC = VCC2 = 4.5V
l
l
20
500
100
1000
ns
ns
tZH
Driver Enable to Output
DI = 1, SLO = 1, Figure 5, Figure 7
l
1000
1400
ns
tZL
Driver Enable to Output
DI = 0, SLO = 1, Figure 5, Figure 7
l
1000
1400
ns
tLZ
Driver Disable to Output
DI = 0, SLO = 1, Figure 5, Figure 7
l
700
1300
ns
tHZ
Driver Disable to Output
DI = 1, SLO = 1, Figure 5, Figure 7
l
700
1300
ns
tPLH
Receiver Input to RO
RE = 0, Figure 3, Figure 8
l
600
855
ns
tPHL
Receiver Input to RO
RE = 0, Figure 3, Figure 8
l
600
855
ns
tPLH
Receiver Input to RO2
RE = 0, Figure 3, Figure 8
30
ns
tPHL
Receiver Input to RO2
RE = 0, Figure 3, Figure 8
30
ns
tr, tf
Receiver Rise or Fall Time
RE = 0, Figure 3, Figure 8
20
ns
tLZ
Receiver Disable to Output
Figure 3, Figure 9
30
ns
tHZ
Receiver Disable to Output
Figure 3, Figure 9
30
ns
tSTART
Initial Start-Up Time
(Note 9)
1200
ns
tTOF
Data Time-Out Fault
(Note 9)
1200
ns
ST1, ST2 Duty Cycle
0°C ≤ TA ≤ 70°C
–40°C ≤ TA ≤ 85°C
Note 1: Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
Note 2: RS422 50Ω specification based on RS485 27Ω test.
Note 3: IIN is tested at VCC2 = 5V, guaranteed by design from
GND2 ≤ VCC2 ≤ 5.25V.
Note 4: Input fault conditions on the RS485 receiver are detected with a
fixed receiver offset. The offset is such that an input short or open will
result in a high data output.
Note 5: The low voltage detect faults when VCC2 or VCC drops below
VUVL and re-enables when greater than VUVH . The fault can be monitored
through the weak driver output on RE.
4
l
l
250
150
410
ns
kBd
56
57
%
%
Note 6: Value derived from 1 second test.
Note 7: The input signals are internally sampled and encoded. The internal
sample rate determines the data output jitter since the internal sampling is
asynchronous with respect to the external data. Nominally, a 4MHz internal
sample rate gives 250ns of sampling uncertainty in the input signals.
Note 8: The maximum baud rate is 250kBd with 10% sampling jitter.
Lower baud rates have lower jitter.
Note 9: Start-up time is the time for communication to recover after a fault
condition. Data time-out is the time a fault is indicated on RE after data
communication has stopped.
Note 10: ICC measured with no load, ST1 and ST2 floating.
1535fc
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LTC1535
Typical Performance Characteristics
VCC Supply Current
vs Temperature
VCC CURRENT (mA)
90
EATON
CTX02-14659
TRANSFORMER
110
90
RL = 120Ω
80
70
RL = OPEN
60
50
–50 –25
0
6.5
VCC2 = 6V
80
RL = 54Ω
100
70
6.0
VCC2 = 5V
60
50
VCC2 = 4.5V
40
30
20
10
–50 –25
25 50 75 100 125 150
TEMPERATURE (°C)
0
60
55
Driver Differential Output
Rise/Fall Time vs Temperature
800
VCC2 = 5V, 4.5V
SLO = VCC2
RL = 54Ω
700
SLO = 0V
RL = 54Ω
VCC2 = 5V
600
50
45
40
500
VCC2 = 4.5V
400
300
30
25
–50 –25
25 50 75 100 125 150
TEMPERATURE (°C)
0
200
–50 –25
25 50 75 100 125 150
TEMPERATURE (°C)
1535 G04
600
Receiver Output Low Voltage
vs Temperature
4
1.0
VCC = 5V
0.9
VCC2 = 6V
300
VCC2 = 5V
2
VCC2 = 4.5V
1
1535 G07
0
–50 –25
0
VCC = 4.5V
0.7
VCC = 5V
0.6
0.5
0.4
0.3
0.2
SLO = VCC2
RL = 54Ω
25 50 75 100 125 150
TEMPERATURE (°C)
I = 8mA
0.8
3
OUTPUT VOLTAGE (V)
OUTPUT VOLTAGE (V)
400
25 50 75 100 125 150
TEMPERATURE (°C)
1535 G06
Driver Differential Output Voltage
vs Temperature
500
0
1535 G05
Switcher Frequency
vs Temperature
FREQUENCY (kHz)
25 50 75 100 125 150
TEMPERATURE (°C)
35
VCC = VCC2 = 4.5V
SLO = VCC2
RL = 54Ω
0
0
1535 G03
TIME (ns)
TIME (ns)
fMAX (kHz)
400
200
–50 –25
EATON
CTX02-14659
TRANSFORMER
4.5
–50 –25
25 50 75 100 125 150
TEMPERATURE (°C)
65
0
RL = 54Ω, VCC = 4.5V
Driver Differential Output
Rise/Fall Time vs Temperature
500
100
–50 –25
5.5
1535 G02
Maximum Baud Rate
vs Temperature
300
fDI = 250kHz
SLO = 0V
RL = OPEN, VCC = 5V
RL = 54Ω, VCC = 5V
5.0
fDI = fMAX
SLO = 0V
RL = 54Ω
1535 G01
200
VCC2 Supply Voltage
vs Temperature
VCC2 VOLTAGE (V)
120
VCC = 5V
VCC2 CURRENT (mA)
130
VCC2 Supply Current
vs Temperature
0.1
25 50 75 100 125 150
TEMPERATURE (°C)
1535 G08
0
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
1535 G09
1535fc
For more information www.linear.com/LTC1535
5
LTC1535
Typical Performance Characteristics
Receiver Output High Voltage
vs Temperature
Driver Differential Output Voltage
vs Output Current
5
VCC = 5V
4.0
VCC = 4.5V
3.5
0
4
3
2
0
25 50 75 100 125 150
TEMPERATURE (°C)
VCC = 4.5V
0
10
20 30 40 50 60 70
OUTPUT CURRENT (mA)
1535 G10
5
5
VCC = 5V
1
0
0 10 20 30 40 50 60 70 80 90 100 110
OUTPUT CURRENT (mA)
1535 G13
6
VCC = 5V
2
0
90
VCC = 5.5V
0 10 20 30 40 50 60 70 80 90 100 110
OUTPUT CURRENT (mA)
1535 G12
Receiver Output Voltage
vs Load Current
5.0
3
2
1
4.5
TA = 25°C
VCC = 5V
4.5
4
OUTPUT VOLTAGE (V)
OUTPUT VOLTAGE (V)
OUTPUT VOLTAGE (V)
4
2
80
TA = 25°C
RL = 60Ω
VCC = 4.5V
VCC = 4.5V
1
Driver Differential Output Voltage
vs VCC2 Supply Voltage
TA = 25°C
VCC = 6V
3
1535 G11
Driver Output Low Voltage
vs Output Current
3
TA = 25°C
VCC = 5V
1
3.0
–50 –25
TA = 25°C
VCC = 5.5V
4
OUTPUT VOLTAGE (V)
OUTPUT VOLTAGE (V)
5
I = 8mA
OUTPUT VOLTAGE (V)
4.5
Driver Output High Voltage
vs Output Current
OUTPUT HIGH, SOURCING
4.0
1.0
OUTPUT LOW, SINKING
0.5
5
5.5
6
6.5
7
VCC2 SUPPLY VOLTAGE (V)
7.5
1535 G14
0
0
1
2
3
4
5
6
7
LOAD CURRENT (mA)
8
9
1535 G15
1535fc
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LTC1535
Pin Functions
Power Side
Isolated Side
VCC (Pin 1): 5V Supply. Bypass to GND with 10µF
capacitor.
GND2 (Pin 11): Isolated Side Power Ground.
ST1 (Pin 2): DC Converter Output 1 to DC Transformer.
ST2 (Pin 3): DC Converter Output 2 to DC Transformer.
GND (Pin 4): Ground.
Z (Pin 12): Differential Driver Inverting Output.
Y (Pin 13): Differential Driver Noninverting Output.
VCC2 (Pin 14): 5V to 7.5V Supply from DC Transformer.
Bypass to GND2 with 10µF capacitor.
DI (Pin 25): Transmit Data TTL Input to the Isolated Side
RS485 Driver. Do not float.
B (Pin 15): Differential Receiver Inverting Input.
DE (Pin 26): Transmit Enable TTL Input to the Isolated
Side RS485 Driver. A high level enables the driver. Do
not float.
RO2 (Pin 17): Isolated Side Receiver TTL Output. This
output is always enabled and is unaffected by RE.
RE (Pin 27): Receive Data Output Enable TTL Input. A low
level enables the receiver. This pin also provides a fault
output signal. (See Figure 11.)
A (Pin 16): Differential Receiver Noninverting Input.
SLO (Pin 18): Slow Slew Rate Control of RS485 Driver.
A low level forces the driver outputs into slow slew rate
mode.
RO (Pin 28): Receive Data TTL Output.
Block Diagram
POWER SIDE
1
ISOLATED SIDE
1.3
+
1.3
2
3
ST1
ST2
11
14
GND2
VCC2
12.75k
63.5k A
420kHz
1
28
VCC
DECODE
27.25k
ENCODE
R
RO
12.75k
27.25k
27
RE
25
4
DE
RO2
Y
DECODE
D
Z
EN
DI
SLO
EN
GND
B
63.5k
FAULT
ENCODE
26
16
FAULT
15
17
13
12
18
100k
VCC2
1535 BD
1535fc
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7
LTC1535
Test Circuit
ILOAD
IEXT
**
CTX02-14659
VCC2
1/2 BAT54C
+
IVCC2
10µF
2
1/2 BAT54C
2
VCC
10µF
1
+
ST1
VCC
2
3
ST2
11
14
GND2
VCC2
420kHz
A
1
28
RO
RO
R
B
fRO = MAX
BAUD
RATE
27
26
25
RO2
RE
Y
DE
D
DI
Z
SLO
GND
4
1
16
15
17
FLOATING RS485 COMMON
1
2
RL
Z
12
C1
50pF
18
1535 F01
LOGIC COMMON
Y
13
C2
50pF
2
2
SLOW SLEW
RATE JUMPER
** EATON (888) 414-2645
2
Figure 1. Self-Oscillation at Maximum Data Rate (Test Configuration for the First Six Typical Performance Characteristics Curves)
Y
VOD
VOC
CRL
R
Z
S1
TEST POINT
RECEIVER
OUTPUT
R
1k
VCC
S2
1535 F03
1535 F02
Figure 2. Driver DC Test Load
Figure 3. Receiver Timing Test Load
3V
S1
DE
DI
1k
Y
R
Z
R
CL1
VCC
500Ω
OUTPUT
UNDER TEST
CL
CL2
S2
1535 F05
1535 F04
Figure 4. Driver Timing Test Circuit
8
Figure 5. Driver Timing Test Load
1535fc
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LTC1535
switching time waveforms
3V
tr ≤ 10ns, tf ≤ 10ns
1.5V
DI
1.5V
0V
tPLH
Z
Y
tPHL
VO
VO
0V
–VO
80%
20%
tr
80%
20%
VDIFF = V(Y) – V(Z)
tSJ
tf
tSJ
1535 F06
Figure 6. Driver Propagation Delays
3V
tr ≤ 10ns, tf ≤ 10ns
1.5V
DE
1.5V
0V
Y, Z
tLZ
tZL
5V
2.3V
OUTPUT NORMALLY LOW
0.5V
2.3V
OUTPUT NORMALLY HIGH
0.5V
VOL
Y, Z
VOH
0V
tHZ
tZH
1535 F07
tSJ
tSJ
Figure 7. Driver Enable and Disable Times
RO
tSJ
VOH
tSJ
1.5V
VOL
tPHL
VOD2
A–B
–VOD2
1.5V
OUTPUT
tPLH
tr ≤ 10ns, tf ≤ 10ns
0V
0V
INPUT
1535 F08
Figure 8. Receiver Propagation Delays
3V
RE
1.5V
RO
1.5V
tr ≤ 10ns, tf ≤ 10ns
0V
tZL
5V
1.5V
tLZ
OUTPUT NORMALLY LOW
tSJ
RO
1.5V
0V
tZH
0.5V
tSJ
OUTPUT NORMALLY HIGH
0.5V
tHZ
tSJ
1535 F09
tSJ
Figure 9. Receiver Enable and Disable Times
1535fc
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9
LTC1535
Applications Information
Isolation Barrier and Sampled Communication
Push-Pull DC/DC Converter
The LTC1535 uses the SW-28 isolated lead frame package to provide capacitive isolation barrier between the
logic interface and the RS485 driver/receiver pair. The
barrier provides 2500VRMS of isolation. Communication
between the two sides uses the isolation capacitors in a
multiplexed way to communicate full-duplex data across
this barrier (see Figure 20 and Block Diagram). The data
is sampled and encoded before transmitting across the
isolation barrier, which will add sampling jitter and delay
to the signals (see Figures 13 and 14). The sampling jitter
is approximately 250ns with a nominal delay of 600ns. At
250kBd rate, this represents 6.2% total jitter. The nominal
DE signal to the driver output delay is 875ns ±125ns,
which is longer due to the encoding. Communication
start-up time is approximately 1µs to 2µs. A time-out fault
will occur if communication from the isolated side fails.
Faults can be monitored on the RE pin.
The powered side contains a full-bridge open-loop driver,
optimized for use with a single primary and center-tapped
secondary transformer. Figure 10 shows the DC/DC converter in a configuration that can deliver up to 100mA of
current to the isolated side using a Eaton CTX02-14659
transformer.
The maximum baud rate can be determined by connecting in self-oscillation mode as shown in Figure 1. In this
configuration, with SLO = VCC2 , the oscillation frequency
is set by the internal sample rate. With SLO = 0V, the frequency is reduced by the slower output rise and fall times.
Table 1 lists examples of transformers which are suitable for use in the LTC1535’s DC/DC converter using the
circuit topology shown in Figure 10. While this secondary circuit topology is recommended, other secondary
circuit topologies are possible which allows for different
Because the DC/DC converter is open-loop, care in choosing low impedance parts is important for good regulation.
Care must also be taken to not exceed the VCC2 recommended maximum voltage of 7.5V when there is very
light loading. The isolated side contains a low voltage
detect circuit to ensure that communication across the
barrier will only occur when there is sufficient isolated
supply voltage. If the output of the DC/DC converter is
overloaded, the supply voltage will trip the low voltage
detection at 4.2V. For higher voltage stand-off, the Eaton
CTX02-14608 transformer may be used.
ILOAD
VCC2 vs ILOAD
IEXT
**
CTX02-14659
8
1/2 BAT54C
+
IVCC2
10µF
6
1/2 BAT54C
2
VCC
10µF
+
1
VCC
4
GND
3
ST1
ST2
420kHz
VCC2 (V)
2
2
11
14
GND2
VCC2
VCC = 5.5V
VCC = 4.5V
2
0
1
1
1535 F10
LOGIC COMMON
FLOATING RS485 COMMON
1
2
VCC = 5V
4
0
50
100
TOTAL LOAD CURRENT, ILOAD (mA)
150
1535 F10a
** EATON (888) 414-2645
Figure 10
10
1535fc
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LTC1535
Applications Information
transformer configurations. The DC/DC converter driver’s
Thévenin equivalent resistance is approximately 4Ω and
the transformer’s volt-second rating should be greater
than 7µVs.
Driver Output and Slow Slew Rate Control
The LTC1535 uses a proprietary driver output stage
that allows a common mode voltage range that extends
beyond the power supplies. Thus, the high impedance
state is maintained over the full RS485 common mode
range. The output stage provides 100mA of short-circuit
current limiting in both the positive and negative directions. Thus, even under short-circuit conditions, the supply voltage from the open-loop DC converter will remain
high enough for proper communication across the isolation barrier. The driver output will be disabled in the
event of a thermal shutdown and a fault condition will be
indicated through the RE weak output.
The CMOS level SLO pin selects slow or fast slew rates
on the RS485 driver output (see Figures 15, 16, 17, 18 for
typical waveforms). The SLO input has an internal 100k
pull-up resistor. When SLO is low, the driver outputs are
slew rate limited to reduce high frequency edges. Left
open or tied high, SLO defaults to fast edges. The part
draws more current during slow slew rate edges.
Monitoring Faults on RE
The RE pin can be used to monitor the following fault
conditions: low supply voltages, thermal shutdown or
a time-out fault when there is no data communication
across the barrier. During a fault, the receiver output,
RO, defaults to a high state (see Table 2). Open circuit or
short-circuit conditions on the twisted pair do not cause
a fault indication. However, the RS485 receiver defaults
to a high output state when the receiver input is open or
short-circuited.
The RE pin has a weak current drive output mode for
indicating fault conditions. This fault state can be polled
using a bidirectional microcontroller I/O line or by using
the circuit in Figure 11, where the control to RE is threestated and the fault condition read back from the RE pin.
The weak drive has 100µA pull-up current to indicate a
fault and 50µA pull-down current for no fault. This allows
the RE pin to be polled without disabling RE on nonfault
conditions.
Both sides contain a low voltage detect circuit. A
voltage less than 4.2V on the isolated side disables
communication.
VCC
RO
RE
VCC
RE
LTC1535
DI
POLL
FAULT
DE
FAULT
GND
BUFFER
POLL
FAULT
FAULT INDICATED WHEN RE IS THREE-STATED
1535 F11
Figure 11. Detecting Fault Conditions
1535fc
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11
LTC1535
Applications Information
Table 1. Examples of Transformers Compatible with the LTC1535
MANUFACTURER
WE-Midcom
PART NUMBER
750311542
750031160
31160R
Eaton
Murata Power Solutions
DC ISOLATION VOLTAGE
(1 SECOND)
PHONE NUMBER/WEBSITE
1.25kV
(800) 643-2661
http://www.we-online.com
1.25kV
760390014
3.125kVAC
750313638
6.25kVAC
CTX02-14659
CTX02-14659-R
CTX02-14608
500V
3.75kVAC
78253/55JC
1.5kV
78253/55JVC
4kV
Minntronix
4810796R
3kVAC
Pulse Electronics
P1597NL
500V
PH9085.034NL
2.5kV
S-167-5779
100V
Sumida (Japan)
(888) 414-2645
http://www.murata-ps.com
(605) 884-0195
http://minntronix.com/
http://www.pulseelectronics.com/
03-3667-3320
http://www.sumida.com/
Table 2. Fault Mode Behavior
VCC > VUVH
VCC2 > VUVH
On
VCC < VUVL
VCC2 > VUVH
On
VCC > VUVH
VCC2 < VUVL
On
VCC < VUVL
VCC2 > VUVL
On
THERMAL
SHUTDOWN
Off
RE = 0V
Active
Forced-High
Forced-High
Forced High
Forced-High
RE = VCC
Hi-Z
Hi-Z
Hi-Z
Hi-Z
Hi-Z
Active
Hi-Z
Hi-Z
Hi-Z
Hi-Z
FUNCTION (PINS)
DC/DC Converter (2, 3)
RO (28)
RE = Floating
RO2 (17)
Active
Active
Active
Active
Active
Driver Outputs
Y and Z (13, 12)
Active
Hi-Z
Hi-Z
Hi-Z
Hi-Z
Communications Across Isolation Barrier
Active
Disabled
Disabled
Disabled
Disabled
Low
High
High
High
High
Fault Indicator on RE (27)
Table 3. Driver Function Table
Table 4. Receiver Function Table
INPUTS
OUTPUTS
INPUTS
OUTPUTS
RE
DE
DI
Y
Z
RE
DE
A-B
X
1
1
1
0
0
X
≥ VTH(MAX
X
1
0
0
1
0
X
X
0
X
Z
2
0
X
0
1
Note: Z = high impedance, X = don’t care
RO
R02
1
1
≤ VTH(MIN)
0
0
Inputs Open
1
1
X
Inputs Shorted
1
1
X
≥ VTH(MAX)
Z
1
1
X
≤ VTH(MIN)
Z
0
1
X
Inputs Open
Z
1
1
X
Inputs Shorted
Z
1
Note: Z = high impedance, X = don’t care
12
1535fc
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LTC1535
Applications Information
High Voltage Considerations
The LTC1535 eliminates ground loops on data commun
ication lines. However, such isolation can bring potentially
dangerous voltages onto the circuit board. An example
would be accidental faulting to 117V AC at some point
on the cable which is then conducted to the PC board.
Figure 12 shows how to detect and warn the user or
installer that a voltage fault condition exists on the twisted
pair or its shield. A small (3.2mm) glow lamp is connected
between GND2 (the isolated ground) and the equipment’s
safety “earth” ground. If a potential of more than 75V
AC is present on the twisted pair or shield, B1 will light,
indicating a wiring fault. Resistors R3 and R4 are used
to ballast the current in B1. Two resistors are necessary
because they can only stand off 200V each, as well as for
power dissipation. As shown, the circuit can withstand a
direct fault to a 440V 3-phase system.
Other problems introduced by floating the twisted pair
include the collection of static charge on the twisted
pair, its shield and the attached circuitry. Resistors R1
and R2 provide a path to shunt static charge safely to
ground. Again, two resistors are necessary to withstand
high voltage faults. Electrostatic spikes, electromagnetically induced transients and radio frequency pickup are
shunted by addition capacitor C1.
Receiver Inputs Fail-Safe
The LTC1535 features an input common mode range
covering the entire RS485 specified range of –7V to 12V.
Differential signals of greater than ± 200mV within the
specified input common mode range will be converted
to TTL compatible signals at the receiver outputs, RO
and RO2. A small amount of input hysteresis is included
to minimize the effects of noise on the line signals. If
the receiver inputs are floating or shorted, a designedin receiver offset guarantees a fail-safe logic high at the
receiver outputs. If a fail-safe logic low is desired, connect
as shown in Figure 19.
A
Y
TWISTED-PAIR
NETWORK
LTC1535
B
GND2
Z
2
2
2
R1*
470k
R2*
470k
C1***
10nF
R3**
100k
R4**
100k
B1
CN2R (JKL)
EQUIPMENT SAFETY GROUND
EARTH GROUND
* IRC WCR1206
** IRC WCR1210
*** PANASONIC ECQ-U2A103MV
FLOATING RS485 COMMON
2
1535 F12
Figure 12. Detecting Fault Conditions
For more information www.linear.com/LTC1535
1535fc
13
LTC1535
Applications Information
DI
DI
Y-Z
Y-Z
1535 F14
1535 F13
Figure 13. Driver Propagation Delay
with Sample Jitter. SLO = VCC2
Figure 14. Driver Propagation Delay
with Sample Jitter. SLO = 0V
Z
Z
Y
Y
1535 F15
1535 F16
Figure 16. Driver Output.
R = 27Ω, VCC2 = 5V, SLO = 0V
Figure 15. Driver Output.
R = 27Ω, VCC2 = 5V, SLO = VCC2
Y-Z
Y-Z
1535 F17
Figure 17. Driver Differential Output.
R = 27Ω, VCC2 = 5V, SLO = VCC2
14
1535 F18
Figure 18. Driver Differential Output.
R = 27Ω, VCC2 = 5V, SLO = 0V
1535fc
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LTC1535
Typical Application
3V
DE
DI
Y
R
Z
R
CL1
CL2
1535 TA02
Figure 19. Fail-Safe Logic “0”
RO
RE
DE
DI
A
B
Y
Z
LTC1535
TTL INPUT
RO
RE
DE
DI
30k
LTC1535
A
B
Y
Z
TTL INPUT
30k
1535 TA02b
(20a) Noninverting
(20b) Inverting
Figure 20. Configuring Receiver for TTL Level Input. Y and Z Outputs Are TTL Compatible with No Modification
Full-Duplex Connection
**
CTX02-14659
1/2 BAT54C
+
10µF
2
1/2 BAT54C
2
VCC
1
+
10µF
VCC
3
ST1
ST2
2
11
14
GND2
VCC2
420kHz
A
1
28
RO
27
RE
VCC
1
26
25
DI
RO
R
RO2
RE
Y
DE
D
DI
Z
SLO
GND
4
B
1
16
120Ω
15
17
13
120Ω
12
18
1535 TA02c
LOGIC COMMON
FLOATING RS485 COMMON
1
2
** EATON (888) 414-2645
1535fc
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15
.420
MIN
16
For more information www.linear.com/LTC1535
2
3
.030 ±.005
TYP
.050 BSC
0° – 8° TYP
N/2
.325 ±.005
.045 ±.005
1
.050
(1.270)
BSC
.093 – .104
(2.362 – 2.642)
NOTE 3
N
28
2
27
3
26
4
25
INCHES
(MILLIMETERS)
2. DRAWING NOT TO SCALE
3. PIN 1 IDENT, NOTCH ON TOP AND CAVITIES ON THE BOTTOM OF PACKAGES ARE THE MANUFACTURING OPTIONS.
THE PART MAY BE SUPPLIED WITH OR WITHOUT ANY OF THE OPTIONS
4. THESE DIMENSIONS DO NOT INCLUDE MOLD FLASH OR PROTRUSIONS.
MOLD FLASH OR PROTRUSIONS SHALL NOT EXCEED .006" (0.15mm)
NOTE:
1. DIMENSIONS IN
.016 – .050
(0.406 – 1.270)
NOTE 3
.291 – .299
(7.391 – 7.595)
NOTE 4
.010 – .029 × 45°
(0.254 – 0.737)
RECOMMENDED SOLDER PAD LAYOUT
.009 – .013
(0.229 – 0.330)
.005
(0.127)
RAD MIN
1
N
(Reference LTC DWG # 05-08-1690 Rev A)
.014 – .019
(0.356 – 0.482)
TYP
.697 – .712
(17.70 – 18.08)
NOTE 4
SW Package
Variation: SW28(16)
28-Lead Plastic Small Outline (Wide .300 Inch)
11
18
12
17
13
16
.037 – .045
(0.940 – 1.143)
.394 – .419
(10.007 – 10.643)
S28 (WIDE) REV A 0915
.004 – .012
(0.102 – 0.305)
14
N/2
15
LTC1535
Package Description
Please refer to http://www.linear.com/product/LTC1535#packaging for the most recent package drawings.
1535fc
LTC1535
Revision History
(Revision history begins at Rev B)
REV
DATE
DESCRIPTION
PAGE NUMBER
B
12/09
Update Manufacturer’s Information on Typical Application and Figure 10
Revise Receiver Input Hysteresis Conditions
3
Revise Block Diagram
7
Revise Figure 1.
8
Update Tables 1 and 3
C
8/17
1, 10
Updated External Transformer Recommendations
12
1, 5, 8, 10,
12, 15
1535fc
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 information
circuits as described
herein will not infringe on existing patent rights.
For more
www.linear.com/LTC1535
17
LTC1535
Typical Application
Complete, Isolated 24-Bit Data Acquisition System
1/2 BAT54C
LT1761-5
+
T1
10µF
16V
TANT
IN
OUT
SHDN
BYP
10µF
1µF
GND
+
10µF
10V
TANT
2
+
1/2 BAT54C
RO ST1
RE
DE
DI VCC1
“SDO”
“SCK”
LOGIC 5V
1
10µF
10V
TANT
+
ST2
LTC1535
G1
1
1
VCC2
G2
2
ISOLATION
BARRIER
1
2
A
B
Y
Z
= LOGIC COMMON
2
10µF
CERAMIC
10µF
10V
TANT
LTC2402
FO
SCK
SDO
CS
GND
1k
2
2
VCC
FSSET
CH1
CH0
ZSSET
1535 TA05
= FLOATING COMMON
2
T1 = EATON CTX02-14659
(888) 414-2645
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18
1535fc
LT 0817 REV C • PRINTED IN USA
For more information www.linear.com/LTC1535
www.linear.com/LTC1553
LINEAR TECHNOLOGY CORPORATION 2009
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