LINER LTC1535 Isolated rs485 transceiver Datasheet

LTC1535
Isolated RS485 Transceiver
U
DESCRIPTIO
FEATURES
■
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.
UL Rated Isolated RS485: 2500VRMS
UL Recognized
File #E151738
Eliminates Ground Loops
250kBd Maximum Data Rate
Self-Powered with 420kHz Converter
Half- or Full-Duplex
Fail-Safe Output High for Open or
Shorted Receiver Inputs
Short-Circuit Current Limit
Slow Slew Rate Control
68kΩ Input Impedance Allows Up to 128 Nodes
Thermal Shutdown
8kV ESD Protection On Driver Outputs and
Receiver Inputs
Available in 28-Lead SW Package
®
■
■
■
■
■
■
■
■
■
■
■
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.
U
APPLICATIO S
■
■
■
Isolated RS485 Receiver/Driver
RS485 with Large Common Mode Voltage
Breaking RS485 Ground Loops
Multiple Unterminated Line Taps
, LTC and LT are registered trademarks of Linear Technology Corporation.
U
■
TYPICAL APPLICATIO
**
CTX02-14659
1/2 BAT54C
+
10µF
2
1/2 BAT54C
2
VCC
1
+
VCC
ST1
3
ST2
2
11
14
GND2
VCC2
420kHz
10µF
A
1
LOGIC COMMON
28
RO
RO
R
B
1
FLOATING RS485 COMMON
2
** TRANSFORMER
COOPER (561) 241-7876
RO2
RE
27
DE
26
25
DI
4
1
RE
Y
DE
D
DI
GND
Z
SLO
16
15
TWISTED-PAIR
CABLE
17
13
12
18
1535 TA01
1535fa
1
LTC1535
U
W W
W
ABSOLUTE
AXI U RATI GS
U
W
U
PACKAGE/ORDER I FOR ATIO
(Note 1)
TOP VIEW
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
VCC 1
28 RO
ST1 2
27 RE
ST2 3
26 DE
GND 4
25 DI
ORDER PART
NUMBER
LTC1535CSW
LTC1535ISW
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
Consult LTC Marketing for parts specified with wider operating temperature ranges.
ELECTRICAL CHARACTERISTICS
The ● 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
VCC
VCC Supply Range
CONDITIONS
MIN
VCC2
VCC2 Supply Range
7.5
V
ICC
VCC Supply Current
Transformer Not Driven (Note 10)
●
13
28
mA
ICC2
VCC2 Supply Current
R = 27Ω, Figure 2
No Load
●
●
63
7
73
12
mA
mA
VOD1
Differential Driver Output
No Load
●
VOD2
Differential Driver Output
R = 50Ω (RS422) (Note 2), VCC2 = 4.5V
R = 27Ω(RS485), Figure 2, VCC2 = 4.5V
●
●
2
1.5
2
●
4.5
●
4.5
TYP
MAX
UNITS
5.5
V
5
V
V
V
VOC
Driver Output Common Mode Voltage
DC Level, R = 50Ω, Figure 2
●
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
●
●
60
60
100
100
150
150
mA
mA
VIH
Logic Input High Voltage
DE, DI, RE
SLO
●
●
2
4
1.7
2.2
VIL
Logic Input Low Voltage
DE, DI, RE
SLO
●
●
IIN
Input Current (A, B)
(Note 3)
VTH
Receiver Input Threshold
–7V ≤ VCM ≤ 12V, (Note 4)
∆VTH
Receiver Input Hysteresis
–7V ≤ VCM ≤ 12V
RIN
Receiver Input Impedance
1.7
1.8
V
V
0.8
1
V
V
VIN = 12V
●
0.25
mA
VIN = – 7V
●
–0.20
mA
●
–200
–90
–10
mV
0°C ≤ TA ≤ 0°C
●
10
30
70
mV
– 40°C ≤ TA ≤ 85°C
●
5
30
70
mV
●
50
68
100
kΩ
1535fa
2
LTC1535
ELECTRICAL CHARACTERISTICS
The ● 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
VIOC
Receiver Input Open Circuit Voltage
CONDITIONS
MIN
VOH
RO Output High Voltage
IRO = – 4mA, VCC = 4.5V
●
VOL
RO Output Low Voltage
IRO = 4mA, VCC = 4.5V
●
IOZ
Driver Output Leakage
Driver Disabled (DE = 0)
VOH2
RO2 Output High Voltage
IRO2 = – 4mA, VCC = 4.5V
●
VOL2
RO2 Output Low Voltage
IRO2 = 4mA, VCC = 4.5V
●
fSW
DC Converter Frequency
●
RSWH
DC Converter Impedance High
●
RSWL
DC Converter Impedance Low
2.5
5
Ω
IREL
RE Output Low Current
RE Sink Current, Fault = 0
●
– 40
– 50
– 80
µA
IREH
RE Output High Current
RE Source Current, Fault = 1
●
80
100
130
µA
VUVL
Undervoltage Low Threshold
RE Fault = 1, (Note 5)
●
3.70
4.00
4.25
V
VUVH
Undervoltage High Threshold
RE Fault = 0, (Note 5)
●
4.05
4.20
4.40
V
VISO
Isolation Voltage
1 Minute, (Note 6)
1 Second
3.7
TYP
MAX
UNITS
3.4
V
4.0
V
0.4
0.8
V
1
µA
3.7
3.9
V
0.4
0.8
V
290
420
590
kHz
4
6
●
2500
3000
Ω
VRMS
VRMS
U
SWITCHI G CHARACTERISTICS
The ● 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
250
410
tSJ
Data Sample Jitter
Figure 8, (Note 7)
●
fMAX
Max Baud Rate
Jitter = 10% Max, SLO = 1, (Note 8)
●
tPLH
Driver Input to Output
DE = 1, SLO = 1, Figure 4, Figure 6
DE = 1, SLO = 0, Figure 4, Figure 6
●
●
600
1300
855
1560
ns
ns
tPHL
Driver Input to Output
DE = 1, SLO = 1, Figure 4, Figure 6
DE = 1, SLO = 0, Figure 4, Figure 6
●
●
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
●
●
20
500
100
1000
ns
ns
150
UNITS
ns
kBd
tZH
Driver Enable to Output
DI = 1, SLO = 1, Figure 5, Figure 7
●
1000
1400
ns
tZL
Driver Enable to Output
DI = 0, SLO = 1, Figure 5, Figure 7
●
1000
1400
ns
tLZ
Driver Disable to Output
DI = 0, SLO = 1, Figure 5, Figure 7
●
700
1300
ns
tHZ
Driver Disable to Output
DI = 1, SLO = 1, Figure 5, Figure 7
●
700
1300
ns
tPLH
Receiver Input to RO
RE = 0, Figure 3, Figure 8
●
600
855
ns
tPHL
Receiver Input to RO
RE = 0, Figure 3, Figure 8
●
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
●
●
56
57
%
%
1535fa
3
LTC1535
ELECTRICAL CHARACTERISTICS
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 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 floating.
U W
TYPICAL PERFOR A CE CHARACTERISTICS
VCC Supply Current vs
Temperature
VCC2 Supply Current vs
Temperature
90
VCC = 5V
COOPER
CTX02-14659
TRANSFORMER
120
80
VCC2 CURRENT (mA)
VCC CURRENT (mA)
110
RL = 54Ω
100
90
RL = 120Ω
80
70
RL = OPEN
60
50
–50 –25
0
6.5
VCC2 = 6V
70
6.0
VCC2 = 5V
60
50
VCC2 = 4.5V
40
30
20
0
55
25 50 75 100 125 150
TEMPERATURE (°C)
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
TIME (ns)
TIME (ns)
400
300
0
1535 G03
65
60
fMAX (kHz)
4.5
–50 –25
25 50 75 100 125 150
TEMPERATURE (°C)
Driver Differential Output Rise/
Fall Time vs Temperature
500
45
40
500
VCC2 = 4.5V
400
35
VCC = VCC2 = 4.5V
SLO = VCC2
RL = 54Ω
0
RL = 54Ω, VCC = 4.5V
1535 G02
Maximum Baud Rate vs
Temperature
100
–50 –25
5.5
COOPER
CTX02-14659
TRANSFORMER
1535 G01
200
RL = 54Ω, VCC = 5V
5.0
fDI = fMAX
SLO = 0V
RL = 54Ω
10
–50 –25
25 50 75 100 125 150
TEMPERATURE (°C)
fDI = 250kHz
SLO = 0V
RL = OPEN, VCC = 5V
VCC2 VOLTAGE (V)
130
VCC2 Supply Voltage vs
Temperature
300
30
25 50 75 100 125 150
TEMPERATURE (°C)
1535 G04
25
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
1535 G05
200
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
1535 G06
1535fa
4
LTC1535
U W
TYPICAL PERFOR A CE CHARACTERISTICS
Switcher Frequency vs
Temperature
Driver Differential Output Voltage
vs Temperature
600
Receiver Output Low Voltage vs
Temperature
4
1.0
VCC = 5V
400
300
VCC2 = 5V
2
VCC2 = 4.5V
1
0
25 50 75 100 125 150
TEMPERATURE (°C)
0
5
TA = 25°C
VCC = 5.5V
TA = 25°C
OUTPUT VOLTAGE (V)
OUTPUT VOLTAGE (V)
OUTPUT VOLTAGE (V)
4
VCC = 5V
3
2
VCC = 4.5V
0
25 50 75 100 125 150
TEMPERATURE (°C)
0
10
20 30 40 50 60 70
OUTPUT CURRENT (mA)
VCC = 4.5V
1
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
TA = 25°C
VCC = 5V
4.5
4
OUTPUT VOLTAGE (V)
OUTPUT VOLTAGE (V)
OUTPUT VOLTAGE (V)
0
90
TA = 25°C
RL = 60Ω
4
0
80
5
TA = 25°C
VCC = 5V
VCC = 5V
2
Driver Differential Output Voltage
vs VCC2 Supply Voltage
5
2
VCC = 4.5V
1535 G11
Driver Output Low Voltage vs
Output Current
3
3
1
1535 G10
VCC = 6V
25 50 75 100 125 150
TEMPERATURE (°C)
Driver Output High Voltage vs
Output Current
1
0
0
1535 G09
4
3.0
–50 –25
0.3
0
–50 –25
25 50 75 100 125 150
TEMPERATURE (°C)
5
I = 8mA
3.5
0.4
Driver Differential Output Voltage
vs Output Current
4.5
VCC = 4.5V
0.5
1535 G08
Receiver Output High Voltage vs
Temperature
4.0
VCC = 5V
0.6
0.1
1535 G07
VCC = 5V
0.7
0.2
SLO = VCC2
RL = 54Ω
0
–50 –25
VCC = 4.5V
0.8
3
OUTPUT VOLTAGE (V)
OUTPUT VOLTAGE (V)
FREQUENCY (kHz)
500
200
–50 –25
I = 8mA
0.9
VCC2 = 6V
3
2
OUTPUT HIGH, SOURCING
4.0
1.0
OUTPUT LOW, SINKING
0.5
0 10 20 30 40 50 60 70 80 90 100 110
OUTPUT CURRENT (mA)
1535 G13
1
4.5
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
1535fa
5
LTC1535
U
U
U
PI FU CTIO S
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.
Y (Pin 13): Differential Driver Noninverting Output.
ST2 (Pin 3): DC Converter Output 2 to DC Transformer.
VCC2 (Pin 14): 5V to 7.5V Supply from DC Transformer.
Bypass to GND2 with 10µF capacitor.
GND (Pin 4): Ground.
Z (Pin 12): Differential Driver Inverting Output.
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.
W
BLOCK DIAGRA
POWER SIDE
1
ISOLATED SIDE
1.3
+
2
3
ST1
ST2
11
14
GND2
VCC2
12.75k
63.5k A
420kHz
16
27.25k
1
28
VCC
DECODE
ENCODE
R
RO
12.75k
B
27.25k
27
RE
RO2
FAULT
Y
ENCODE
26
25
4
DE
DECODE
D
Z
EN
DI
SLO
EN
GND
FAULT
15
63.5k
17
13
12
18
100k
VCC2
1535 BD
1535fa
6
LTC1535
TEST CIRCUITS
ILOAD
IEXT
**
CTX02-14659
VCC2
1/2 BAT54C
+
IVCC2
10µF
2
1/2 BAT54C
2
VCC
1
+
ST1
VCC
2
3
ST2
11
14
GND2
VCC2
420kHz
10µF
A
1
28
RO
RO
R
fRO = MAX
BAUD
RATE
B
RO2
27
26
25
RE
Y
DE
D
DI
SLO
GND
4
Z
1
16
15
17
Y
13
C1
50pF
18
LOGIC COMMON
FLOATING RS485 COMMON
2
C2
50pF
2
1535 F01
1
RL
Z
12
2
SLOW SLEW
RATE JUMPER
** TRANSFORMER
COOPER (561) 241-7876
2
Figure 1. Self-Oscillation at Maximum Data Rate
(Test Configuration for the First Six Typical Performance Characteristics Curves)
Y
R
VOD
S1
TEST POINT
RECEIVER
OUTPUT
1k
VCC
VOC
1k
CRL
S2
R
1535 F03
Z
1535 F02
Figure 2. Driver DC Test Load
Figure 3. Receiver Timing Test Load
3V
DE
Y
R
DI
Z
S1
CL1
R
CL2
OUTPUT
UNDER TEST
VCC
500Ω
S2
CL
1535 F04
1535 F05
Figure 4. Driver Timing Test Circuit
Figure 5. Driver Timing Test Load
1535fa
7
LTC1535
U
W
W
SWITCHI G TI E WAVEFOR S
3V
tr ≤ 10ns, tf ≤ 10ns
1.5V
DI
1.5V
0V
t PLH
t PHL
Z
VO
Y
VO
0V
–VO
80%
tr
80%
20%
VDIFF = V(Y) – V(Z)
20%
t SJ
1535 F06
tf
t SJ
Figure 6. Driver Propagation Delays
3V
tr ≤ 10ns, tf ≤ 10ns
1.5V
DE
1.5V
0V
t LZ
t ZL
5V
Y, Z
2.3V
OUTPUT NORMALLY LOW
0.5V
2.3V
OUTPUT NORMALLY HIGH
0.5V
VOL
VOH
Y, Z
0V
t HZ
t ZH
1535 F07
t SJ
t SJ
Figure 7. Driver Enable and Disable Times
t SJ
t SJ
VOH
1.5V
RO
1.5V
OUTPUT
VOL
tr ≤ 10ns, tf ≤ 10ns
t PHL
VOD2
A–B
–VOD2
0V
t PLH
0V
INPUT
1535 F08
Figure 8. Receiver Propagation Delays
3V
1.5V
RE
1.5V
tr ≤ 10ns, tf ≤ 10ns
0V
tZL
5V
RO
1.5V
t LZ
OUTPUT NORMALLY LOW
t SJ
RO
1.5V
0.5V
t SJ
OUTPUT NORMALLY HIGH
0.5V
0V
t HZ
tZH
t SJ
1535 F09
t SJ
Figure 9. Receiver Enable and Disable Times
1535fa
8
LTC1535
U
U
W
U
APPLICATIO S I FOR ATIO
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 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 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 Cooper
CTX02-14608 transformer may be used.
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.
IEXT
ILOAD
**
CTX02-14659
VCC2 vs ILOAD
1/2 BAT54C
IVCC2
+
8
10µF
2
1/2 BAT54C
2
VCC
1
+
VCC
3
ST1
ST2
VCC2 (V)
6
2
11
14
GND2
VCC2
420kHz
VCC = 5.5V
VCC = 5V
4
VCC = 4.5V
2
10µF
1
GND
4
0
0
1
1535 F10
50
100
TOTAL LOAD CURRENT, ILOAD (mA)
150
1535 F10a
LOGIC COMMON
FLOATING RS485 COMMON
1
2
** TRANSFORMER
COOPER (561) 241-7876
Figure 10
1535fa
9
LTC1535
U
W
U
U
APPLICATIO S I FOR ATIO
Driver Output and Slow Slew Rate Control
Monitoring Faults on RE
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 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 shortcircuited.
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.
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
DE
FAULT
FAULT
GND
BUFFER
POLL
FAULT
FAULT INDICATED WHEN RE IS THREE-STATED
1535 F11
Figure 11. Detecting Fault Conditions
1535fa
10
LTC1535
U
U
W
U
APPLICATIO S I FOR ATIO
Table 1. List of Transformers Designed for LTC1535
DC
ISOLATION
VOLTAGE PHONE
(1 Second) NUMBER
PART
NUMBER
MANUFACTURER
Cooper
CTX02-14659
Cooper
CTX02-14608
500V
Epcos AG (Germany) B78304-A1477-A3
(USA)
Midcom
500V
31160R
1.25kV
P1597
500V
Pulse FEE (France)
(561) 241-7876
3.75kVAC (561) 241-7876
(0 89) 636-2 80 00
(800) 888-7724
(605) 886-4385
(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
VCC > VUVH
VCC2 > VUVH
FUNCTION (PINS)
DC/DC Converter (2, 3)
RO (28)
VCC < VUVL
VCC2 > VUVH
VCC > VUVH
VCC2 < VUVL
VCC < VUVL
VCC2 > VUVL
THERMAL
SHUTDOWN
On
On
On
On
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
RO2 (17)
RE = Floating
Active
Active
Active
Active
Active
Driver Outputs
Y and Z (13,12)
Active
Hi-Z
Hi-Z
Hi-Z
Hi-Z
Communication 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
A
B
RE
DE
A–B
RO
RO2
X
1
1
1
0
0
X
≥ VTH(MAX)
1
1
X
1
0
0
1
0
X
≤ VTH(MIN)
0
0
X
0
X
Z
Z
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
Note: Z = high impedance, X = don’t care
1535fa
11
LTC1535
U
W
U
U
APPLICATIO S I FOR ATIO
High Voltage Considerations
The LTC1535 eliminates ground loops on data communication 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∅ 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 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 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 hyteresis 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.
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 Wiring Faults
1535fa
12
LTC1535
U
W
U
U
APPLICATIO S I FOR ATIO
DI
DI
Y–Z
Y–Z
1535 F13.tif
1535 F14.tif
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.tif
1535 F16.tif
Figure 15. Driver Output.
R = 27Ω, VCC2 = 5V, SLO = VCC2
Figure 16. Driver Output.
R = 27Ω, VCC2 = 5V, SLO = 0V
Y–Z
Y–Z
1535 F17.tif
Figure 17. Driver Differential Output.
R = 27Ω, VCC2 = 5V, SLO = VCC2
1535 F18.tif
Figure 18. Driver Differential Output.
R = 27Ω, VCC2 = 5V, SLO = 0V
1535fa
13
LTC1535
U
TYPICAL APPLICATIO S
3V
DE
Y
R
Z
R
CL1
DI
CL2
1535 F04
Figure 19. Fail-Safe Logic “0”
RO
RE
DE
DI
A
B
Y
Z
LTC1535
RO
RE
DE
DI
TTL INPUT
30k
A
B
Y
Z
LTC1535
TTL INPUT
30k
1535 TA05
(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
+
VCC
3
ST1
ST2
2
11
14
GND2
VCC2
420kHz
10µF
A
1
28
RO
RO
B
RO2
27
RE
1
VCC
26
DI
25
RE
Y
DE
Z
SLO
GND
4
15
17
13
120Ω
D
DI
16
120Ω
R
1
12
18
1535 TA02
LOGIC COMMON
FLOATING RS485 COMMON
1
2
** TRANSFORMER
COOPER (561) 241-7876
1535fa
14
LTC1535
U
PACKAGE DESCRIPTIO
SW Package
28-Lead Plastic Small Outline Isolation Barrier (Wide .300 Inch)
(Reference LTC DWG # 05-08-1690)
.697 – .712*
(17.70 – 18.08)
28
27
26
25
18
17
16
15
.394 – .419
(10.007 – 10.643)
NOTE 1
1
2
3
11
4
12
13
14
.291 – .299**
(7.391 – 7.595)
.005
(0.127)
RAD MIN
.037 – .045
(0.940 – 1.143)
.093 – .104
(2.362 – 2.642)
.010 – .029 × 45°
(0.254 – 0.737)
0° – 8° TYP
.009 – .013
(0.229 – 0.330)
.050
(1.270)
BSC
NOTE 1
.016 – .050
(0.406 – 1.270)
NOTE:
1. DIMENSIONS IN
.014 – .019
(0.356 – 0.482)
TYP
INCHES
(MILLIMETERS)
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
.004 – 0.012
(0.102 – 0.305)
SW28 (ISO) 0502
1535fa
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.
15
LTC1535
U
TYPICAL APPLICATIO
Complete, Isolated 24-Bit Data Acquisition System
1/2 BAT54C
LT1761-5
+
T1
10µF
16V
TANT
IN
OUT
SHDN
BYP
10µF
+
GND
1µF
10µF
10V
TANT
2
+
1/2 BAT54C
RO ST1
RE
DE
DI VCC1
“SDO”
“SCK”
LOGIC 5V
1
10µF
10V
TANT
+
ST2
VCC2
LTC1535
G1
G2
1
1
2
ISOLATION
BARRIER
1
A
B
Y
Z
= LOGIC COMMON
2
10µF
CERAMIC
10µF
10V
TANT
LTC2402
FO
SCK
SDO
CS
GND
1k
2
1535 TA03
= FLOATING COMMON
2
2
VCC
FSSET
CH1
CH0
ZSSET
2
T1 = COOPER CTX02-14659
RELATED PARTS
PART NUMBER
DESCRIPTION
COMMENTS
LT1424-5
Isolated Flyback Switching Regulator
±5% Accurate with No Optoisolator Required
LTC1485
High Speed RS485 Transceiver
10Mbps, Pin Compatible with LTC485
LTC1531
Self-Powered Isolated Comparator
2.5V Isolated Reference, 3000VRMS Isolation
LT1785/LT1791
±60V Fault Protected RS485 Transceiver, Half/Full-Duplex
±15kV ESD Protection, Industry Standard Pinout
LTC1690
Full-Duplex RS485 Transceiver
±15kV ESD Protection, Fail-Safe Receiver
1535fa
16
Linear Technology Corporation
LT/TP 1103 1K REV A • PRINTED IN USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
FAX: (408) 434-0507 ● www.linear.com
 LINEAR TECHNOLOGY CORPORATION 1999
Similar pages