LTC1535
1
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TYPICAL APPLICATION
FEATURES DESCRIPTION
Isolated RS485 Transceiver
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.
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 refl ections. 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 Linear
Technology Corporation. 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
** COOPER (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
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PIN CONFIGURATION ABSOLUTE 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 specifi ed with wider operating temperature ranges. *The temperature grade is identifi ed 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 specifi
cations, go to: http://www.linear.com/tapeandreel/
LTC1535
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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 l5V
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
V
OUT = 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
ΔVTH Receiver Input Hysteresis –7V ≤ VCM ≤ 12V 0°C ≤ TA ≤ 70°C l10 30 70 mV
–40°C ≤ TA ≤ 85°C l53070 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 l46 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 specifi cations which apply over the full operating
temperature range, otherwise specifi cations are at TA = 25°C. VCC = 5V, VCC2 = 5V unless otherwise noted.
LTC1535
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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
l150
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 specifi cations which apply over the full operating
temperature range, otherwise specifi cations are at TA = 25°C. VCC = 5V, VCC2 = 5V, R = 27Ω (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 specifi cation 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
fi xed 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 reenables 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 fl oating.
LTC1535
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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 COOPER
CTX02-14659
TRANSFORMER
RL = 120Ω
RL = OPEN
TEMPERATURE (°C)
–50 –25 0 25 50 75 100 125 150
VCC2 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
COOPER
CTX02-14659
TRANSFORMER
RL = OPEN, VCC = 5V
TEMPERATURE (°C)
–50 –25 0 25 50 75 100 125 150
fMAX (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
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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 102030405060708090
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 102030405060708090100110
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 102030405060708090100110
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)
012 3 4 5 6 7 8 9
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
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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 fl oat.
DE (Pin 26): Transmit Enable TTL Input to the Isolated
Side RS485 Driver. A high level enables the driver. Do
not fl oat.
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
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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 ** COOPER (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 Confi guration 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
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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
tSJ
tSJ
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
3V
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
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APPLICATIONS INFORMATION
Isolation Barrier and Sampled Communication
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 maximum baud rate can be determined by connecting
in self-oscillation mode as shown in Figure 1. In this
confi guration, 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.
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
converter in a confi guration that can deliver up to 100mA
of current to the isolated side using a Cooper CTX02-14659
transformer.
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 suffi cient 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 Cooper CTX02-14608
transformer may be used.
**
2
1
1
1535 F10
VCC
GND
LOGIC COMMON
2
FLOATING RS485 COMMON ** COOPER (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
VCC2 (V)
1535 F10a
8
6
4
2
0
VCC = 5.5V
VCC = 5V
VCC = 4.5V
Figure 10
VCC2 vs ILOAD
LTC1535
11
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APPLICATIONS INFORMATION
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 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
12
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APPLICATIONS INFORMATION
Table 1. List of Transformers Designed for LTC1535
MANUFACTURER PART NUMBER
DC ISOLATION
VOLTAGE
(1 SECOND) PHONE NUMBER
Cooper CTX02-14659 500V (888) 414-2645
Cooper CTX02-14608 3.75kVAC (888) 414-2645
Epcos AG (Germany)
(USA)
B78304-A1477-A3 500V (0 89) 636-2 80 00
(800) 888-7724
Midcom 31160R 1.25kV (605) 886-4385
Minntronix 4810796R 3kVAC (605) 884-0195
Pulse FEE (France) P1597 500V (33) 3 84 35 04 04
Sumida (Japan) S-167-5779 100V 03-3667-3320
Transpower TTI7780-SM 500V (775) 852-0140
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
Communiactions 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
X1 1 1 0
X1 0 0 1
X0 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 11
0 X ≤ VTH(MIN) 00
0 X Inputs Open 1 1
0 X Inputs Shorted 1 1
1 X ≥ VTH(MAX) Z1
1 X ≤ VTH(MIN) Z0
1 X Inputs Open Z 1
1 X Inputs Shorted Z 1
Note: Z = high impedance, X = don’t care
LTC1535
13
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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 fl oating 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 resisitors 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 specifi ed range of –7V to 12V.
Differential signals of greater than ±200mV within the
specifi ed input common mode range will be converted
to TTL compatible signals at the receiver outputs, RO
and RO2. A small amount of input hyteresis is included
to minimize the effects of noise on the line signals. If
the receiver inputs are fl oating 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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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 = 27Ω, VCC2 = 5V, SLO = VCC2
Figure 16. Driver Output.
R = 27Ω, VCC2 = 5V, SLO = 0V
Figure 17. Driver Differential Output.
R = 27Ω, VCC2 = 5V, SLO = VCC2
Figure 18. Driver Differential Output.
R = 27Ω, 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
15
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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 ** COOPER (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. Confi guring Receiver for TTL Level Input. Y and Z Outputs Are TTL Compatible with No Modifi cation
(20a) Noninverting (20b) Inverting
Figure 19. Fail-Safe Logic “0”
Full-Duplex Connection
LTC1535
16
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PACKAGE DESCRIPTION
SW Package
28-Lead Plastic Small Outline Isolation Barrier (Wide .300 Inch)
(Reference LTC DWG # 05-08-1690)
SW28 (ISO) 1103
0° – 8° TYP
NOTE 2
.009 – .013
(0.229 – 0.330)
.016 – .050
(0.406 – 1.270)
.291 – .299**
(7.391 – 7.595)
s 45°
.010 – .029
(0.254 – 0.737)
.004 – 0.012
(0.102 – 0.305)
.093 – .104
(2.362 – 2.642)
.050
(1.270)
BSC .014 – .019
(0.356 – 0.482)
TYP
NOTE 2
.697 – .712*
(17.70 – 18.08)
1234
.394 – .419
(10.007 – 10.643)
2526
11 12
18 17 16 15
1413
2728
2. 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.
DIMENSION DOES NOT INCLUDE MOLD FLASH. MOLD FLASH SHALL NOT EXCEED .006" (0.152mm) PER SIDE
DIMENSION DOES NOT INCLUDE INTERLEAD FLASH. INTERLEAD FLASH SHALL NOT EXCEED .010" (0.254mm) PER SIDE
*
**
INCHES
(MILLIMETERS)
NOTE:
1. DIMENSIONS IN
LTC1535
17
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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
(Revision history begins at Rev B)
LTC1535
18
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Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2009
LT 1209 REV B • PRINTED IN USA
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TYPICAL APPLICATION
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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 = COOPER 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
