LINER LTC1535 Isolated rs485 transceiver Datasheet

Final Electrical Specifications
LTC1535
Isolated RS485 Transceiver
August 1999
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DESCRIPTIO
FEATURES
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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 400kHz 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
Half- or Full-Duplex
Eliminates Ground Loops
350kBd Maximum Data Rate
Self-Powered with 400kHz Converter
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
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.
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APPLICATIO S
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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.
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TYPICAL APPLICATIO
**
CTX02-14659
1/2 BAT54C
+
10µF
2
1/2 BAT54C
2
VCC
1
+
VCC
3
ST1
ST2
2
11
14
GND2
VCC2
400kHz
10µF
A
1
28
RO
RO
R
B
RO2
RE
27
DE
26
25
DI
RE
Y
DE
D
DI
SLO
GND
4
Z
1
16
15
TWISTED-PAIR
CABLE
17
13
12
18
1535 TA01
LOGIC COMMON
FLOATING RS485 COMMON
1
2
** TRANSFORMER
COILTRONICS (561) 241-7876
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.
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LTC1535
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ABSOLUTE
AXI U RATI GS
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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 .............. 0°C ≤ TA ≤ 70°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
GND2 11
18 SLO
Z 12
17 RO2
Y 13
16 A
VCC2 14
15 B
SW PACKAGE
28-LEAD PLASTIC SO WIDE
TJMAX = 125°C, θJA = 125°C/W
Consult factory for Industrial and Military grade parts.
ELECTRICAL CHARACTERISTICS
The ● denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. 0°C ≤ TA ≤ 70°C, VCC = 5V, VCC2 = 5V unless otherwise noted.
SYMBOL
PARAMETER
VCC
VCC Supply Range
●
4.5
5.5
V
VCC2
VCC2 Supply Range
●
4.5
7.5
V
ICC
VCC Supply Current
No Load
●
13
18
mA
ICC2
VCC2 Supply Current
R = 27Ω, Figure 1
No Load
●
●
63
7
73
12
mA
mA
VOD1
Differential Driver Output
No Load
●
VOD2
Differential Driver Output
R = 50Ω (RS422) Note 2
R = 27Ω(RS485), Figure 1
●
●
2
1.5
2
Driver Short-Circuit Current
VOUT = HIGH
VOUT = LOW
–7V ≤ VCM ≤ 10V
–7V ≤ VCM ≤ 10V
●
●
75
75
100
100
VIH
Logic Input High Voltage
DE, DI, RE VCC = 4.5V
●
2
VIL
Logic Input Low Voltage
DE, DI, RE VCC = 4.5V
IIN
Input Current (A, B)
Note 3
IOSD1
CONDITIONS
MIN
TYP
MAX
5
UNITS
V
V
V
135
135
mA
mA
V
●
0.8
V
VIN = 12V
●
0.25
mA
VIN = – 7V
●
–0.20
mA
VTH
Receiver Input Threshold
–7V ≤ VCM ≤ 12V, Note 4
●
–200
–90
–10
mV
∆VTH
Receiver Input Hysteresis
–7V ≤ VCM ≤ 12V
●
10
30
70
mV
RIN
Receiver Input Impedance
●
50
68
85
kΩ
VOH
RO Output High Voltage
IRO = – 4mA, VCC = 4.5V
IRO = –10mA, VCC = 4.5V
●
3.7
4.0
3.4
VOL
RO Output Low Voltage
IRO = 4mA, VCC = 4.5V
IRO = 10mA, VCC = 4.5V
●
2
0.4
0.9
V
V
0.8
V
V
LTC1535
ELECTRICAL CHARACTERISTICS
The ● denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. 0°C ≤ TA ≤ 70°C, VCC = 5V, VCC2 = 5V unless otherwise noted.
SYMBOL
PARAMETER
IOZ
Driver Output Leakage
CONDITIONS
MIN
VOH2
RO2 Output High Voltage
IRO2 = – 4mA, VCC = 4.5V
IRO2 = –10mA, VCC = 4.5V
●
VOL2
RO2 Output Low Voltage
IRO2 = 4mA, VCC = 4.5V
IRO2 = 10mA, VCC = 4.5V
●
fSW
DC Converter Frequency
RSWH
DC Converter R High
VCC = 4.5V
●
RSWL
DC Converter R Low
VCC = 4.5V
●
IREL
RE Output Low Current
RE Sink Current, Fault = 0
●
IREH
RE Output High Current
RE Source Current, Fault = 1
●
VUVL
Undervoltage Low Threshold
RE Fault = 1, Note 5
●
VUVH
Undervoltage High Threshold
RE Fault = 0, Note 5
●
VISO
Isolation Voltage
1 Minute, Note 6
1 Second
●
3.7
TYP
MAX
UNITS
1
µA
3.9
3.4
V
V
0.4
0.9
0.8
V
V
420
520
kHz
4
6
Ω
2.5
5
Ω
– 40
– 50
– 80
µA
80
100
130
µA
3.90
4.00
4.25
V
4.05
4.20
4.40
290
2500
3000
V
VRMS
VRMS
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SWITCHI G CHARACTERISTICS
The ● denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. 0°C ≤ TA ≤ 70°C, VCC = 5V, VCC2 = 5V, R = 27Ω (RS485) unless
otherwise noted.
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
tSJ
Data Sample Jitter
Figure 8, Note 7
●
250
285
ns
fMAX
Max Baud Rate
Jitter = 10% Max, SLO = 1, Note 8
●
tPLH
Driver Input to Output
DE = 1, SLO = 1, Figure 3, Figure 5
DE = 1, SLO = 0, Figure 3, Figure 5
●
●
600
1300
855
1560
ns
ns
tPHL
Driver Input to Output
DE = 1, SLO = 1, Figure 3, Figure 5
DE = 1, SLO = 0, Figure 3, Figure 5
●
●
600
1300
855
1560
ns
ns
tr, tf
Driver Rise or Fall Time
DE = 1, SLO = 1, Figure 3, Figure 5
DE = 1, SLO = 0
●
●
20
500
50
1000
ns
ns
350
150
kBd
tZH
Driver Enable to Output
DI = 1, SLO = 1, Figure 4, Figure 6
●
1000
1400
ns
tZL
Driver Enable to Output
DI = 0, SLO = 1, Figure 4, Figure 6
●
1000
1400
ns
tLZ
Driver Disable to Output
DI = 0, SLO = 1, Figure 4, Figure 6
●
700
1000
ns
tHZ
Driver Disable to Output
DI = 1, SLO = 1, Figure 4, Figure 6
●
700
1000
ns
tPLH
Receiver Input to RO
RE = 0, Figure 2, Figure 7
●
600
855
ns
tPHL
Receiver Input to RO
RE = 0, Figure 2, Figure 7
●
600
855
ns
tPLH
Receiver Input to RO2
RE = 0, Figure 2, Figure 7
30
ns
tPHL
Receiver Input to RO2
RE = 0, Figure 2, Figure 7
30
ns
tr, tf
Receiver Rise or Fall Time
RE = 0, Figure 2, Figure 7
20
ns
tLZ
Receiver Disable to Output
Figure 2, Figure 8
30
ns
tHZ
Receiver Disable to Output
Figure 2, Figure 8
30
ns
tSTART
Initial Start-Up Time
Note 9
1200
ns
tTOF
Data Time-Out Fault
Note 9
1200
ns
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
VCC2 = GND2 ≤ 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 4.2V
and reenables when greater than 4.4V. 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 350kBd 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.
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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 GND 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.
RE (Pin 27): Receive Data Output Enable TTL Input. A low
level enables the receiver. This pin also provides a fault
output signal. (See Applications Information.)
RO (Pin 28): Receive Data TTL Output.
4
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.
LTC1535
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BLOCK DIAGRA
POWER SIDE
1
ISOLATED SIDE
1.3
+
2
3
ST1
ST2
11
14
GND2
VCC2
A
400kHz
1
28
DECODE
VCC
ENCODE
R
RO
B
EN
27
RE
EN
RO2
FAULT
Y
ENCODE
26
25
4
16
DECODE
D
DE
Z
EN
DI
SLO
EN
15
17
13
12
18
FAULT
GND
1535 BD
TEST CIRCUITS
Y
R
VOD
S1
TEST POINT
RECEIVER
OUTPUT
1k
VCC
VOC
1k
CRL
S2
R
1535 F02
Z
1535 F01
Figure 1. Driver DC Test Load
Figure 2. Receiver Timing Test Load
3V
DE
Y
R
Z
S1
CL1
DI
R
CL2
OUTPUT
UNDER TEST
VCC
500Ω
S2
CL
1535 F03
1535 F04
Figure 3. Driver Timing Test Circuit
Figure 4. Driver Timing Test Load
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LTC1535
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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 F05
tf
t SJ
Figure 5. 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 F06
t SJ
t SJ
Figure 6. 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 F07
Figure 7. 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
Figure 8. Receiver Enable and Disable Times
6
1535 F08
t SJ
LTC1535
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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.
The data is sampled and encoded before transmitting
across the isolation barrier, which will add sampling jitter
and delay to the signals. 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 isloated 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 9 shows the DC/DC converter in a configuration that can deliver up to to 100mA of
current to the isolated side using a Coiltronics CTX0214659 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 Coiltronics
CTX02-14608 transformer may be used.
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
400kHz
VCC = 5.5V
VCC = 5V
4
VCC = 4.5V
2
10µF
1
GND
4
0
0
1
1535 F09
50
100
TOTAL LOAD CURRENT, ILOAD (mA)
150
1535 F09a
LOGIC COMMON
FLOATING RS485 COMMON
1
2
** TRANSFORMER
COILTRONICS (561) 241-7876
Figure 9
7
LTC1535
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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 not be pulled too low
to prevent 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. 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 CMOS level SLO pin selects slow or fast slew rates on
the RS485 driver output. 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 the circuit in Figure 10 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.
VCC
RO
RE
VCC
RE
LTC1535
DI
POLL
DE
FAULT
FAULT
GND
BUFFER
POLL
FAULT
FAULT INDICATED WHEN RE IS THREE-STATED
Figure 10. Detecting Fault Conditions
8
1535 F10
LTC1535
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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␣ 11 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 and transients can be
limited by the addition of capacitor C1 and discharged
through R1–R4.
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 F11
Figure 11. Detecting Wiring Faults
9
LTC1535
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APPLICATIO S I FOR ATIO
DI
DI
Y–Z
Y–Z
Figure 12. Driver Propagation Delay
with Sample Jitter. SLO = VCC2
Figure 13. Driver Propagation Delay
with Sample Jitter. SLO = 0V
Z
Z
Y
Y
Figure 14. Driver Output. R = 27Ω, VCC2 = 5V, SLO = VCC2
Y–Z
Y–Z
Figure 16. Driver Differential Output.
R = 27Ω, VCC2 = 5V, SLO = VCC2
10
Figure 15. Driver Output. R = 27Ω, VCC2 = 5V, SLO = 0V
Figure 17. Driver Differential Output.
R = 27Ω, VCC2 = 5V, SLO = 0V
LTC1535
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PACKAGE DESCRIPTIO
Dimensions in inches (millimeters) unless otherwise noted.
SW Package
28-Lead Plastic Small Outline Isolation Barrier (Wide 0.300)
(LTC DWG # 05-08-1690)
0.697 – 0.712*
(17.70 – 18.08)
28
27
26
25
18
17
16
15
0.394 – 0.419
(10.007 – 10.643)
NOTE 1
1
2
3
11
4
12
13
14
0.291 – 0.299**
(7.391 – 7.595)
0.005
(0.127)
RAD MIN
0.037 – 0.045
(0.940 – 1.143)
0.093 – 0.104
(2.362 – 2.642)
0.010 – 0.029 × 45°
(0.254 – 0.737)
0° – 8° TYP
0.009 – 0.013
(0.229 – 0.330)
NOTE 1
0.050
(1.270)
BSC
0.016 – 0.050
(0.406 – 1.270)
NOTE:
1. 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.
0.014 – 0.019
(0.356 – 0.482)
TYP
0.004 – 0.012
(0.102 – 0.305)
SW28 (ISO) 1098
*DIMENSION DOES NOT INCLUDE MOLD FLASH. MOLD FLASH SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE
**DIMENSION DOES NOT INCLUDE INTERLEAD FLASH. INTERLEAD FLASH SHALL NOT EXCEED 0.010" (0.254mm) PER SIDE
11
LTC1535
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TYPICAL APPLICATIO
**
CTX02-14659
1/2 BAT54C
+
10µF
2
1/2 BAT54C
2
VCC
1
+
VCC
3
ST1
ST2
2
11
14
GND2
VCC2
400kHz
10µF
A
1
28
RO
RO
R
120Ω
B
RO2
RE
27
DE
26
25
DI
RE
Y
DE
D
DI
1
15
17
13
120Ω
Z
SLO
GND
4
16
12
18
1535 TA02
LOGIC COMMON
FLOATING RS485 COMMON
1
2
** TRANSFORMER
COILTRONICS (561) 241-7876
Figure 18. Full-Duplex Connection
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Full-Duplex RS485 Transceiver
±15kV ESD Protection, Fail-Safe Receiver
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Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408)432-1900 ● FAX: (408) 434-0507 ● www.linear-tech.com
1535i LT/TP 0899 4K • PRINTED IN USA
 LINEAR TECHNOLOGY CORPORATION 1999