LTC1535
1
1535fc
For more information www.linear.com/LTC1535
Typical applicaTion
FeaTures DescripTion
Isolated RS485 Transceiver
The LT C
®
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 com-
mon mode voltage ranges. An internal capacitive isola-
tion 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 commu-
nication occurs through the capacitive isolation barrier.
The transceiver meets RS485 and RS422 requirements.
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 cur-
rent 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.
applicaTions
n 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
n Isolated RS485 Receiver/Driver
n RS485 with Large Common Mode Voltage
n Breaking RS485 Ground Loops
n Multiple Unterminated Line Taps
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.
**
D
Y
Z
SLO
2
1
1
R
A
B
RO2
1535 TA01
VCC
RO
RE
DE
DI
GND
LOGIC COMMON
2
FLOATING RS485 COMMON
** EATON (888) 414-2645
420kHz
28
27
26
25
4
17
15
16
18
12
13
1411
1
+
+
GND2
1/2 BAT54C
1/2 BAT54C
VCC2
ST1 ST2
32
VCC
RO
RE
DE
DI
1
10µF
10µF
2
CTX02-14659
TWISTED-PAIR
CABLE
LTC1535
2
1535fc
For more information www.linear.com/LTC1535
pin conFiguraTionabsoluTe MaxiMuM raTings
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
(Note 1)
1
2
3
4
11
12
13
14
28
27
26
25
18
17
16
15
VCC
ST1
ST2
GND
GND2
Z
Y
VCC2
RO
RE
DE
DI
SLO
RO2
A
B
SW PACKAGE
28-LEAD PLASTIC SO
TOP VIEW
TJMAX = 125°C, θJA = 125°C/W
orDer inForMaTion
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.
http://www.linear.com/product/LTC1535#orderinfo
LTC1535
3
1535fc
For more information www.linear.com/LTC1535
elecTrical characTerisTics
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
VCC VCC Supply Range l4.5 5.5 V
VCC2 VCC2 Supply Range l4.5 7.5 V
ICC VCC Supply Current Transformer Not Driven (Note 10) l13 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 l5 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
V
V
VOC Driver Output Common Mode Voltage DC Level, R = 50Ω, Figure 2 l2.0 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
V
V
VIL Logic Input Low Voltage DE, DI, RE
SLO
l
l
1.7
1.8
0.8
1
V
V
IIN Input Current (A, B) (Note 3) VIN = 12V l0.25 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 l10 30 70 mV
–40°C ≤ TA ≤ 85°C l5 30 70 mV
RIN Receiver Input Impedance l50 68 100 kΩ
VIOC Receiver Input Open Circuit Voltage 3.4 V
VOH RO Output High Voltage IRO = –4mA, VCC = 4.5V l3.7 4.0 V
VOL RO Output Low Voltage IRO = 4mA, VCC = 4.5V l0.4 0.8 V
IOZ Driver Output Leakage Driver Disabled (DE = 0) 1 µA
VOH2 RO2 Output High Voltage IRO2 = –4mA, VCC = 4.5V l3.7 3.9 V
VOL2 RO2 Output Low Voltage IRO2 = 4mA, VCC = 4.5V l0.4 0.8 V
fSW DC Converter Frequency l290 420 590 kHz
RSWH DC Converter Impedance High l4 6 Ω
RSWL DC Converter Impedance Low l2.5 5 Ω
IREL RE Output Low Current RE Sink Current, Fault = 0 l–40 –50 –80 µA
IREH RE Output High Current RE Source Current, Fault = 1 l80 100 130 µA
VUVL Undervoltage Low Threshold RE Fault = 1, (Note 5) l3.70 4.00 4.25 V
VUVH Undervoltage High Threshold RE Fault = 0, (Note 5) l4.05 4.20 4.40 V
VISO Isolation Voltage 1 Minute, (Note 6)
1 Second
2500
3000
VRMS
VRMS
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.
LTC1535
4
1535fc
For more information www.linear.com/LTC1535
elecTrical characTerisTics
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
tSJ Data Sample Jitter Figure 8, (Note 7) l250 285 ns
fMAX Max Baud Rate Jitter = 10% Max, SLO = 1, (Note 8) l250 410 kBd
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, tfDriver Rise or Fall Time DE = 1, SLO = 1, Figure 4, Figure 6
DE = 1, SLO = 0, VCC = VCC2 = 4.5V
l
l
150
20
500
100
1000
ns
ns
tZH Driver Enable to Output DI = 1, SLO = 1, Figure 5, Figure 7 l1000 1400 ns
tZL Driver Enable to Output DI = 0, SLO = 1, Figure 5, Figure 7 l1000 1400 ns
tLZ Driver Disable to Output DI = 0, SLO = 1, Figure 5, Figure 7 l700 1300 ns
tHZ Driver Disable to Output DI = 1, SLO = 1, Figure 5, Figure 7 l700 1300 ns
tPLH Receiver Input to RO RE = 0, Figure 3, Figure 8 l600 855 ns
tPHL Receiver Input to RO RE = 0, Figure 3, Figure 8 l600 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, tfReceiver 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
l
l
56
57
%
%
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 = 27 (RS485) unless otherwise noted.
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.
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.
LTC1535
5
1535fc
For more information www.linear.com/LTC1535
Typical perForMance characTerisTics
Maximum Baud Rate
vs Temperature
Driver Differential Output
Rise/Fall Time vs Temperature
Driver Differential Output
Rise/Fall Time vs Temperature
Switcher Frequency
vs Temperature
Driver Differential Output Voltage
vs Temperature
Receiver Output Low Voltage
vs Temperature
VCC Supply Current
vs Temperature
VCC2 Supply Current
vs Temperature
VCC2 Supply Voltage
vs Temperature
TEMPERATURE (°C)
–50 –25 0 25 50 75 100 125 150
VCC CURRENT (mA)
1535 G01
130
120
110
100
90
80
70
60
50
RL = 54Ω
VCC = 5V EATON
CTX02-14659
TRANSFORMER
RL = 120Ω
RL = OPEN
TEMPERATURE (°C)
–50 –25 0 25 50 75 100 125 150
V
CC2
CURRENT (mA)
1535 G02
90
80
70
60
50
40
30
20
10
VCC2 = 6V
VCC2 = 5V
VCC2 = 4.5V
fDI = fMAX
SLO = 0V
RL = 54Ω
TEMPERATURE (°C)
–50 –25 0 25 50 75 100 125 150
VCC2 VOLTAGE (V)
1535 G03
6.5
6.0
5.5
5.0
4.5
RL = 54Ω, VCC = 5V
RL = 54Ω, VCC = 4.5V
fDI = 250kHz
SLO = 0V
EATON
CTX02-14659
TRANSFORMER
RL = OPEN, VCC = 5V
TEMPERATURE (°C)
–50 –25 0 25 50 75 100 125 150
f
MAX
(kHz)
1535 G04
500
400
300
200
100
VCC = VCC2 = 4.5V
SLO = VCC2
RL = 54Ω
TEMPERATURE (°C)
–50 –25 0 25 50 75 100 125 150
TIME (ns)
1535 G05
65
60
55
50
45
40
35
30
25
VCC2 = 5V, 4.5V
SLO = VCC2
RL = 54Ω
TEMPERATURE (°C)
–50 –25 0 25 50 75 100 125 150
TIME (ns)
1535 G06
800
700
600
500
400
300
200
SLO = 0V
RL = 54Ω
VCC2 = 5V
VCC2 = 4.5V
TEMPERATURE (°C)
–50 –25 0 25 50 75 100 125 150
FREQUENCY (kHz)
1535 G07
600
500
400
300
200
VCC = 5V
TEMPERATURE (°C)
–50 –25 0 25 50 75 100 125 150
OUTPUT VOLTAGE (V)
1535 G08
4
3
2
1
0
VCC2 = 6V
VCC2 = 5V
VCC2 = 4.5V
SLO = VCC2
RL = 54Ω
TEMPERATURE (°C)
–50 –25 0 25 50 75 100 125 150
OUTPUT VOLTAGE (V)
1535 G09
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
VCC = 5V
VCC = 4.5V
I = 8mA
LTC1535
6
1535fc
For more information www.linear.com/LTC1535
Typical perForMance characTerisTics
Driver Output Low Voltage
vs Output Current
Driver Differential Output Voltage
vs VCC2 Supply Voltage
Receiver Output Voltage
vs Load Current
Receiver Output High Voltage
vs Temperature
Driver Differential Output Voltage
vs Output Current
Driver Output High Voltage
vs Output Current
TEMPERATURE (°C)
–50 –25 0 25 50 75 100 125 150
OUTPUT VOLTAGE (V)
1535 G10
4.5
4.0
3.5
3.0
VCC = 5V
VCC = 4.5V
I = 8mA
OUTPUT CURRENT (mA)
0 10 20 30 40 50 60 70 80 90
OUTPUT VOLTAGE (V)
1535 G11
5
4
3
2
1
0
VCC = 5.5V
VCC = 4.5V
VCC = 5V
TA = 25°C
OUTPUT CURRENT (mA)
0 10 20 30 40 50 60 70 80 90 100 110
OUTPUT VOLTAGE (V)
1535 G12
5
4
3
2
1
0
VCC = 5.5V
VCC = 4.5V
VCC = 5V
TA = 25°C
OUTPUT CURRENT (mA)
OUTPUT VOLTAGE (V)
1535 G13
5
4
3
2
1
0
VCC = 6V
VCC = 4.5V
VCC = 5V
TA = 25°C
0 10 20 30 40 50 60 70 80 90 100 110
VCC2 SUPPLY VOLTAGE (V)
4.5 5 5.5 6 6.5 7 7.5
OUTPUT VOLTAGE (V)
1535 G14
5
4
3
2
1
TA = 25°C
RL = 60Ω
LOAD CURRENT (mA)
0 1 23456789
OUTPUT VOLTAGE (V)
1535 G15
5.0
4.5
4.0
1.0
0.5
0
TA = 25°C
VCC = 5V
OUTPUT HIGH, SOURCING
OUTPUT LOW, SINKING
LTC1535
7
1535fc
For more information www.linear.com/LTC1535
pin FuncTions
POWER SIDE
VCC (Pin 1): 5V Supply. Bypass to GND with 10µF
capacitor.
ST1 (Pin 2): DC Converter Output 1 to DC Transformer.
ST2 (Pin 3): DC Converter Output 2 to DC Transformer.
GND (Pin 4): Ground.
DI (Pin 25): Transmit Data TTL Input to the Isolated Side
RS485 Driver. Do not float.
DE (Pin 26): Transmit Enable TTL Input to the Isolated
Side RS485 Driver. A high level enables the driver. Do
not float.
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.)
RO (Pin 28): Receive Data TTL Output.
ISOLATED SIDE
GND2 (Pin 11): Isolated Side Power 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.
B (Pin 15): Differential Receiver Inverting Input.
A (Pin 16): Differential Receiver Noninverting Input.
RO2 (Pin 17): Isolated Side Receiver TTL Output. This
output is always enabled and is unaffected by RE.
SLO (Pin 18): Slow Slew Rate Control of RS485 Driver.
A low level forces the driver outputs into slow slew rate
mode.
block DiagraM
1
1.3
1.3
POWER SIDE ISOLATED SIDE
D
Y
Z
SLO
100k
VCC2
27.25k
63.5k
12.75k
27.25k 63.5k
12.75k
DECODE
EN
FAULT
R
A
B
RO2
1535 BD
VCC
RO
RE
DE
DI
GND
ENCODE
DECODE
420kHz
FAULT
ENCODE
EN
28
27
26
25
4
17
15
16
18
12
13
1411
1
+
GND2 VCC2
ST1 ST2
32
LTC1535
8
1535fc
For more information www.linear.com/LTC1535
TesT circuiT
2
**
D
Y
Z
Y
Z
SLO
2
1
1
R
A
B
RO2
1535 F01
VCC
RO
RE
DE
DI
GND
LOGIC COMMON
2
2
FLOATING RS485 COMMON ** EATON (888) 414-2645
SLOW SLEW
RATE JUMPER
420kHz
28
27
26
25
4
17
15
16
18
12
13
1411
1
+
+
GND2
1/2 BAT54C
1/2 BAT54C
VCC2
ST1 ST2
32
VCC
RO
1
10µF
10µF
2
CTX02-14659
ILOAD IEXT
VCC2
IVCC2
RL
C2
50pF
2
C1
50pF
fRO = MAX
BAUD
RATE
Figure 1. Self-Oscillation at Maximum Data Rate (Test Configuration for the First Six Typical Performance Characteristics Curves)
VOD
Y
Z
R
R
VOC
1535 F02
RECEIVER
OUTPUT
CRL 1k
S1
S2
TEST POINT
VCC
1k
1535 F03
3V
DE
Y
Z
DI
R
R
CL1
CL2
1535 F04
OUTPUT
UNDER TEST
CL
S1
S2
VCC
500Ω
1535 F05
Figure 2. Driver DC Test Load Figure 3. Receiver Timing Test Load
Figure 4. Driver Timing Test Circuit Figure 5. Driver Timing Test Load
LTC1535
9
1535fc
For more information www.linear.com/LTC1535
swiTching TiMe waveForMs
DI
3V
1.5V
tPLH
trtSJ tSJ
VO
tr ≤ 10ns, tf ≤ 10ns
80%
20%
0V
Z
Y
VO
–VO
0V 80%
1.5V
tPHL
20%
tf
VDIFF = V(Y) – V(Z)
1535 F06
1.5V
2.3V
2.3V
tZH
tZL
1.5V
tLZ
0.5V
0.5V
tHZ
OUTPUT NORMALLY LOW
OUTPUT NORMALLY HIGH
3V
0V
DE
5V
VOL
VOH
0V
Y, Z
Y, Z
1535 F07
tr ≤ 10ns, tf ≤ 10ns
tSJ tSJ
t
SJ
t
SJ
1.5V
tPHL
RO
–VOD2
A – B 0V 0V
1.5V
tPLH
OUTPUT
INPUT
VOD2
VOL
VOH
1535 F08
tr ≤ 10ns, tf ≤ 10ns
tSJ
tSJ tSJ
1.5V
tZL
tZH
tSJ
1.5V
1.5V
1.5V
tLZ
0.5V
0.5V
tHZ
OUTPUT NORMALLY LOW
OUTPUT NORMALLY HIGH
0V
RE
5V
0V
RO
RO
1535 F09
tr ≤ 10ns, tf ≤ 10ns
Figure 6. Driver Propagation Delays
Figure 7. Driver Enable and Disable Times
Figure 8. Receiver Propagation Delays
Figure 9. Receiver Enable and Disable Times
LTC1535
10
1535fc
For more information www.linear.com/LTC1535
applicaTions inForMaTion
Isolation Barrier and Sampled Communication
The LTC1535 uses the SW-28 isolated lead frame pack-
age 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 maximum baud rate can be determined by connect-
ing 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 fre-
quency is reduced by the slower output rise and fall times.
Push-Pull DC/DC Converter
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 con-
verter in a configuration that can deliver up to 100mA of
current to the isolated side using a Eaton CTX02-14659
transformer.
Because the DC/DC converter is open-loop, care in choos-
ing low impedance parts is important for good regulation.
Care must also be taken to not exceed the VCC2 recom-
mended 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.
Table 1 lists examples of transformers which are suit
-
able for use in the LTC1535’s DC/DC converter using the
circuit topology shown in Figure 10. While this second-
ary circuit topology is recommended, other secondary
circuit topologies are possible which allows for different
**
2
1
1
1535 F10
VCC
GND
LOGIC COMMON
2
FLOATING RS485 COMMON ** EATON (888) 414-2645
420kHz
4
1411
1
+
+
GND2
1/2 BAT54C
1/2 BAT54C
ILOAD
VCC2
ST1 ST2
32
VCC
1
10µF
10µF
2
CTX02-14659
IEXT
IVCC2
TOTAL LOAD CURRENT, ILOAD (mA)
0 50 100 150
V
CC2
(V)
1535 F10a
8
6
4
2
0
VCC = 5.5V
VCC = 5V
VCC = 4.5V
Figure 10
VCC2 vs ILOAD
LTC1535
11
1535fc
For more information www.linear.com/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 direc-
tions. Thus, even under short-circuit conditions, the sup-
ply voltage from the open-loop DC converter will remain
high enough for proper communication across the iso-
lation 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 three-
stated 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.
RE
POLL
FAULT
FAULT INDICATED WHEN RE IS THREE-STATED
VCC
1535 F11
POLL
FAULT
BUFFER
RE
RO
DI
DE GND
LTC1535
FAULT
VCC
Figure 11. Detecting Fault Conditions
LTC1535
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For more information www.linear.com/LTC1535
applicaTions inForMaTion
Table 1. Examples of Transformers Compatible with the LTC1535
MANUFACTURER PART NUMBER
DC ISOLATION VOLTAGE
(1 SECOND) PHONE NUMBER/WEBSITE
WE-Midcom 750311542 1.25kV (800) 643-2661
http://www.we-online.com
750031160
31160R
1.25kV
760390014 3.125kVAC
750313638 6.25kVAC
Eaton CTX02-14659
CTX02-14659-R
500V (888) 414-2645
CTX02-14608 3.75kVAC
Murata Power Solutions 78253/55JC 1.5kV http://www.murata-ps.com
78253/55JVC 4kV
Minntronix 4810796R 3kVAC (605) 884-0195
http://minntronix.com/
Pulse Electronics P1597NL 500V http://www.pulseelectronics.com/
PH9085.034NL 2.5kV
Sumida (Japan) S-167-5779 100V 03-3667-3320
http://www.sumida.com/
Table 2. Fault Mode Behavior
FUNCTION (PINS)
VCC > VUVH
VCC2 > VUVH
VCC < VUVL
VCC2 > VUVH
VCC > VUVH
VCC2 < VUVL
VCC < VUVL
VCC2 > VUVL
THERMAL
SHUTDOWN
DC/DC Converter (2, 3) On On On On Off
RO (28) RE = 0V Active Forced-High Forced-High Forced High Forced-High
RE = VCC Hi-Z Hi-Z Hi-Z Hi-Z Hi-Z
RE = Floating Active Hi-Z Hi-Z Hi-Z Hi-Z
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
Fault Indicator on RE (27) Low High High High High
Table 3. Driver Function Table
INPUTS OUTPUTS
RE DE DI Y Z
X 1 1 1 0
X 1 0 0 1
X 0 X Z 2
Note: Z = high impedance, X = don’t care
Table 4. Receiver Function Table
INPUTS OUTPUTS
RE DE A-B RO R02
0 X ≥ VTH(MAX 1 1
0 X ≤ VTH(MIN) 0 0
0 X Inputs Open 1 1
0 X Inputs Shorted 1 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
LTC1535
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For more information www.linear.com/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, electromagneti-
cally 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 designed-
in 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.
TWISTED-PAIR
NETWORK
2
EQUIPMENT SAFETY GROUND
EARTH GROUND
2
2
FLOATING RS485 COMMON
* IRC WCR1206
** IRC WCR1210
*** PANASONIC ECQ-U2A103MV
B1
CN2R (JKL)
1535 F12
R1*
470k
R2*
470k
R3**
100k
R4**
100k
C1***
10nF
2
Y
A
B
Z
GND2
LTC1535
Figure 12. Detecting Fault Conditions
LTC1535
14
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For more information www.linear.com/LTC1535
applicaTions inForMaTion
Figure 13. Driver Propagation Delay
with Sample Jitter. SLO = VCC2
Figure 14. Driver Propagation Delay
with Sample Jitter. SLO = 0V
Figure 15. Driver Output.
R = 27, VCC2 = 5V, SLO = VCC2
Figure 16. Driver Output.
R = 27, VCC2 = 5V, SLO = 0V
Figure 17. Driver Differential Output.
R = 27, VCC2 = 5V, SLO = VCC2
Figure 18. Driver Differential Output.
R = 27, VCC2 = 5V, SLO = 0V
DI
Y-Z
1535 F13
DI
Y-Z
1535 F14
Z
Y
1535 F15
Z
Y
1535 F16
Y-Z
1535 F17
Y-Z
1535 F18
LTC1535
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For more information www.linear.com/LTC1535
Typical applicaTion
3V
DE
Y
Z
DI
R
R
CL1
CL2
1535 TA02
A
B
Y
Z
TTL INPUTRO
RE
DE
DI
LTC1535 30k
A
B
Y
Z
TTL INPUT
RO
RE
DE
DI
LTC1535
1535 TA02b
30k
120Ω
**
D
Y
Z
SLO
2
1
1
1
R
A
120Ω
B
RO2
1535 TA02c
VCC
RO
RE
DE
DI
GND
LOGIC COMMON
2
FLOATING RS485 COMMON ** EATON (888) 414-2645
420kHz
28
27
26
25
4
17
15
16
18
12
13
1411
1
+
+
GND2
1/2 BAT54C
1/2 BAT54C
VCC2
ST1 ST2
32
VCC
RO
RE
VCC
DI
1
10µF
10µF
2
CTX02-14659
Figure 20. Configuring Receiver for TTL Level Input. Y and Z Outputs Are TTL Compatible with No Modification
(20a) Noninverting (20b) Inverting
Figure 19. Fail-Safe Logic “0”
Full-Duplex Connection
LTC1535
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For more information www.linear.com/LTC1535
package DescripTion
S28 (WIDE) REV A 0915
0° – 8° TYP
NOTE 3
.009 – .013
(0.229 – 0.330)
.016 – .050
(0.406 – 1.270)
.291 – .299
(7.391 – 7.595)
NOTE 4
× 45°
.010 – .029
(0.254 – 0.737)
.037 – .045
(0.940 – 1.143)
.004 – .012
(0.102 – 0.305)
.093 – .104
(2.362 – 2.642)
.050
(1.270)
BSC .014 – .019
(0.356 – 0.482)
TYP
NOTE 3
.697 – .712
(17.70 – 18.08)
NOTE 4
12 3
12 3 N/2
N
4
.394 – .419
(10.007 – 10.643)
2526
11 12
18 17 16 15
14
N/2
13
2728
N
.325 ±.005
.045 ±.005.050 BSC
RECOMMENDED SOLDER PAD LAYOUT
.420
MIN
.005
(0.127)
RAD MIN
INCHES
(MILLIMETERS)
NOTE:
1. DIMENSIONS IN
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)
SW Package
Variation: SW28(16)
28-Lead Plastic Small Outline (Wide .300 Inch)
(Reference LTC DWG # 05-08-1690 Rev A)
.030 ±.005
TYP
Please refer to http://www.linear.com/product/LTC1535#packaging for the most recent package drawings.
LTC1535
17
1535fc
For more information www.linear.com/LTC1535
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.
revision hisTory
REV DATE DESCRIPTION PAGE NUMBER
B 12/09 Update Manufacturer’s Information on Typical Application and Figure 10
Revise Receiver Input Hysteresis Conditions
Revise Block Diagram
Revise Figure 1.
Update Tables 1 and 3
1, 10
3
7
8
12
C 8/17 Updated External Transformer Recommendations 1, 5, 8, 10,
12, 15
(Revision history begins at Rev B)
LTC1535
18
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For more information www.linear.com/LTC1535
LINEAR TECHNOLOGY CORPORATION 2009
LT 0817 REV C • PRINTED IN USA
www.linear.com/LTC1553
relaTeD parTs
Typical applicaTion
PART NUMBER DESCRIPTION COMMENTS
LTM2881 Isolated RS485/RS422 μModule Transceiver + Power 20Mbps 2500VRMS Isolation with Power in LGA/BGA Package
LTM2885 6500VRMS Isolated RS485/RS422 μModule Transceiver + Power 20Mbps 6500VRMS Isolation with Power in LGA/BGA Package
LTC2862A ±60V Fault Protected 3V to 5.5V RS485/RS422 Transceiver ±60V Tolerant, ±40kV HBM ESD, IEC Level 4 ESD and EFT,
±25V Common Mode Range, 20Mbps or 250kbps
LTC2861 20Mbps RS485 T
ransceivers with Integrated Switchable
Termination Full Duplex
±15kV ESD
LTC2856/LTC2857/
LTC2858
20Mbps and Slew Rate Limited 15kV RS485/RS422 Transceivers Low EMI 250kbps, Micropower Shutdown
LT1785/LT1791 ±60V Fault Protected RS485 Transceiver, Half/Full-Duplex ±15kV ESD Protection, Industry Standard Pinout
LTC2870/LTC2871 RS232/RS485 Multiprotocol Transceivers with Integrated
Termination
20Mbps RS485 and 500kbps RS232, ±26kV ESD,
3V to 5.5V Operation
Complete, Isolated 24-Bit Data Acquisition System
+
+
FO
SCK
SDO
CS
GND
VCC
FSSET
CH1
CH0
ZSSET
LTC2402
1535 TA05
LT1761-5
GND
10µF
10V
TANT
10µF
10V
TANT
+10µF
16V
TANT
+
10µF
10V
TANT
10µF
1µF
T1
1/2 BAT54C
1/2 BAT54C
ISOLATION
BARRIER
= LOGIC COMMON
= FLOATING COMMON
T1 = EATON CTX02-14659
(888) 414-2645
1k
2
21 2
1
1
1
2
2
22
10µF
CERAMIC
A
B
Y
Z
RO
RE
DE
DI
VCC2
ST2
G1
VCC1 G2
ST1
“SDO”
“SCK”
LOGIC 5V
IN OUT
SHDN BYP
LTC1535