1
LTC1535
Isolated RS485 Transceiver
August 1999
, LTC and LT are registered trademarks of Linear Technology Corporation.
Final Electrical Specifications
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 represen-
tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
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 com-
mon mode voltage ranges. An internal capacitive isolation
barrier provides 2500V
RMS
of isolation between the line
transceiver and the logic level interface. The powered side
contains a 400kHz push-pull converter to power the iso-
lated RS485 transceiver. Internal full-duplex communica-
tion 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 68k
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.
s
Isolated RS485 Receiver/Driver
s
RS485 with Large Common Mode Voltage
s
Breaking RS485 Ground Loops
s
Multiple Unterminated Line Taps
s
UL Rated Isolated RS485: 2500V
RMS
s
Half- or Full-Duplex
s
Eliminates Ground Loops
s
350kBd Maximum Data Rate
s
Self-Powered with 400kHz Converter
s
Fail-Safe Output High for Open or
Shorted Receiver Inputs
s
Short-Circuit Current Limit
s
Slow Slew Rate Control
s
68k
Input Impedance Allows Up to 128 Nodes
s
Thermal Shutdown
s
8kV ESD Protection On Driver Outputs and Receiver
Inputs
TYPICAL APPLICATIO
U
APPLICATIO S
U
FEATURES
DESCRIPTIO
U
**
D
Y
Z
SLO
2
1
1
R
A
B
RO2
1535 TA01
V
CC
RO
RE
DE
DI
GND
LOGIC COMMON
2
FLOATING RS485 COMMON
** TRANSFORMER
COILTRONICS (561) 241-7876
400kHz
28
27
26
25
4
17
15
16
18
12
13
14
11
1
+
+
GND2
1/2 BAT54C
1/2 BAT54C
V
CC2
ST1
ST2
3
2
V
CC
RO
RE
DE
DI
1
10
F
10
F
2
CTX02-14659
TWISTED-PAIR
CABLE
2
LTC1535
ORDER PART
NUMBER
LTC1535CSW
T
JMAX
= 125
C,
JA
= 125
C/W
Consult factory for Industrial and Military grade parts.
The
q
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25
C. 0
C
T
A
70
C, V
CC
= 5V, V
CC2
= 5V unless otherwise noted.
(Note 1)
V
CC
to GND ................................................................ 6V
V
CC2
to GND2 ............................................................ 8V
Control Input Voltage to GND ...... 0.3V to (V
CC
+ 0.3V)
Driver Input Voltage to GND ........ 0.3V to (V
CC
+ 0.3V)
Driver Output Voltage
(Driver Disabled) to GND2 .............. (V
CC2
13V) to 13V
Driver Output Voltage
(Driver Enabled) to GND2 ............... (V
CC2
13V) to 10V
Receiver Input Voltage to GND2 ............................
14V
Receiver Output Voltage .............. 0.3V to (V
CC
+ 0.3V)
Operating Temperature Range .............. 0
C
T
A
70
C
Lead Temperature (Soldering, 10 sec).................. 300
C
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
V
CC
V
CC
Supply Range
q
4.5
5.5
V
V
CC2
V
CC2
Supply Range
q
4.5
7.5
V
I
CC
V
CC
Supply Current
No Load
q
13
18
mA
I
CC2
V
CC2
Supply Current
R = 27
, Figure 1
q
63
73
mA
No Load
q
7
12
mA
V
OD1
Differential Driver Output
No Load
q
5
V
V
OD2
Differential Driver Output
R = 50
(RS422) Note 2
q
2
V
R = 27
(RS485), Figure 1
q
1.5
2
V
I
OSD1
Driver Short-Circuit Current
V
OUT
= HIGH
7V
V
CM
10V
q
75
100
135
mA
V
OUT
= LOW
7V
V
CM
10V
q
75
100
135
mA
V
IH
Logic Input High Voltage
DE, DI, RE V
CC
= 4.5V
q
2
V
V
IL
Logic Input Low Voltage
DE, DI, RE V
CC
= 4.5V
q
0.8
V
I
IN
Input Current (A, B)
Note 3
V
IN
= 12V
q
0.25
mA
V
IN
= 7V
q
0.20
mA
V
TH
Receiver Input Threshold
7V
V
CM
12V, Note 4
q
200
90
10
mV
V
TH
Receiver Input Hysteresis
7V
V
CM
12V
q
10
30
70
mV
R
IN
Receiver Input Impedance
q
50
68
85
k
V
OH
RO Output High Voltage
I
RO
= 4mA, V
CC
= 4.5V
q
3.7
4.0
V
I
RO
= 10mA, V
CC
= 4.5V
3.4
V
V
OL
RO Output Low Voltage
I
RO
= 4mA, V
CC
= 4.5V
q
0.4
0.8
V
I
RO
= 10mA, V
CC
= 4.5V
0.9
V
1
2
3
4
11
12
13
14
28
27
26
25
18
17
16
15
V
CC
ST1
ST2
GND
GND2
Z
Y
V
CC2
RO
RE
DE
DI
SLO
RO2
A
B
SW PACKAGE
28-LEAD PLASTIC SO WIDE
TOP VIEW
ABSOLUTE AXI U
RATI GS
W
W
W
U
PACKAGE/ORDER I FOR ATIO
U
U
W
ELECTRICAL CHARACTERISTICS
3
LTC1535
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
The
q
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25
C. 0
C
T
A
70
C, V
CC
= 5V, V
CC2
= 5V unless otherwise noted.
I
OZ
Driver Output Leakage
1
A
V
OH2
RO2 Output High Voltage
I
RO2
= 4mA, V
CC
= 4.5V
q
3.7
3.9
V
I
RO2
= 10mA, V
CC
= 4.5V
3.4
V
V
OL2
RO2 Output Low Voltage
I
RO2
= 4mA, V
CC
= 4.5V
q
0.4
0.8
V
I
RO2
= 10mA, V
CC
= 4.5V
0.9
V
f
SW
DC Converter Frequency
q
290
420
520
kHz
R
SWH
DC Converter R High
V
CC
= 4.5V
q
4
6
R
SWL
DC Converter R Low
V
CC
= 4.5V
q
2.5
5
I
REL
RE Output Low Current
RE Sink Current, Fault = 0
q
40
50
80
A
I
REH
RE Output High Current
RE Source Current, Fault = 1
q
80
100
130
A
V
UVL
Undervoltage Low Threshold
RE Fault = 1, Note 5
q
3.90
4.00
4.25
V
V
UVH
Undervoltage High Threshold
RE Fault = 0, Note 5
q
4.05
4.20
4.40
V
V
ISO
Isolation Voltage
1 Minute, Note 6
2500
V
RMS
1 Second
3000
V
RMS
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
t
SJ
Data Sample Jitter
Figure 8, Note 7
q
250
285
ns
f
MAX
Max Baud Rate
Jitter = 10% Max, SLO = 1, Note 8
q
350
kBd
t
PLH
Driver Input to Output
DE = 1, SLO = 1, Figure 3, Figure 5
q
600
855
ns
DE = 1, SLO = 0, Figure 3, Figure 5
q
1300
1560
ns
t
PHL
Driver Input to Output
DE = 1, SLO = 1, Figure 3, Figure 5
q
600
855
ns
DE = 1, SLO = 0, Figure 3, Figure 5
q
1300
1560
ns
t
r
, t
f
Driver Rise or Fall Time
DE = 1, SLO = 1, Figure 3, Figure 5
q
20
50
ns
DE = 1, SLO = 0
q
150
500
1000
ns
t
ZH
Driver Enable to Output
DI = 1, SLO = 1, Figure 4, Figure 6
q
1000
1400
ns
t
ZL
Driver Enable to Output
DI = 0, SLO = 1, Figure 4, Figure 6
q
1000
1400
ns
t
LZ
Driver Disable to Output
DI = 0, SLO = 1, Figure 4, Figure 6
q
700
1000
ns
t
HZ
Driver Disable to Output
DI = 1, SLO = 1, Figure 4, Figure 6
q
700
1000
ns
t
PLH
Receiver Input to RO
RE = 0, Figure 2, Figure 7
q
600
855
ns
t
PHL
Receiver Input to RO
RE = 0, Figure 2, Figure 7
q
600
855
ns
t
PLH
Receiver Input to RO2
RE = 0, Figure 2, Figure 7
30
ns
t
PHL
Receiver Input to RO2
RE = 0, Figure 2, Figure 7
30
ns
t
r
, t
f
Receiver Rise or Fall Time
RE = 0, Figure 2, Figure 7
20
ns
t
LZ
Receiver Disable to Output
Figure 2, Figure 8
30
ns
t
HZ
Receiver Disable to Output
Figure 2, Figure 8
30
ns
t
START
Initial Start-Up Time
Note 9
1200
ns
t
TOF
Data Time-Out Fault
Note 9
1200
ns
The
q
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25
C. 0
C
T
A
70
C, V
CC
= 5V, V
CC2
= 5V, R = 27
(RS485) unless
otherwise noted.
SWITCHI G CHARACTERISTICS
U
ELECTRICAL CHARACTERISTICS
4
LTC1535
POWER SIDE
V
CC
(Pin 1): 5V Supply. Bypass to GND with 10
F capaci-
tor.
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 Applications Information.)
RO (Pin 28): Receive Data TTL Output.
Note 1: Absolute Maximum Ratings are those values beyond which the
life of a device may be impaired.
Note 2: RS422 50
specification based on RS485 27
test.
Note 3: I
IN
is tested at V
CC2
= 5V, guaranteed by design from
V
CC2
= 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 V
CC2
or V
CC
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.
ISOLATED SIDE
GND2 (Pin 11): Isolated Side Power Ground.
Z (Pin 12): Differential Driver Inverting Output.
Y (Pin 13): Differential Driver Noninverting Output.
V
CC2
(Pin 14): 5V to 7.5V Supply from DC Transformer.
Bypass to GND with 10
F capacitor.
B (Pin 15): Differential Receiver Inverting Input.
A (Pin 16): Differential Receiver Noninverting Input.
RO2 (Pin 17): Isolated Side Receiver TTL Output.
SLO (Pin 18): Slow Slew Rate Control of RS485 Driver. A
low level forces the driver outputs into slow slew rate
mode.
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.
PI FU CTIO S
U
U
U
ELECTRICAL CHARACTERISTICS
5
LTC1535
BLOCK DIAGRA
W
1
1.3
POWER SIDE
ISOLATED SIDE
D
Y
Z
SLO
DECODE
EN
FAULT
R
A
B
RO2
1535 BD
V
CC
RO
RE
DE
DI
GND
ENCODE
EN
DECODE
EN
400kHz
FAULT
ENCODE
EN
28
27
26
25
4
17
15
16
18
12
13
14
11
1
+
GND2
V
CC2
ST1
ST2
3
2
TEST CIRCUITS
V
OD
Y
Z
R
R
V
OC
1535 F01
RECEIVER
OUTPUT
C
RL
1k
S1
S2
TEST POINT
V
CC
1k
1535 F02
3V
DE
Y
Z
DI
R
R
C
L1
C
L2
1535 F03
OUTPUT
UNDER TEST
C
L
S1
S2
V
CC
500
1535 F04
Figure 1. Driver DC Test Load
Figure 2. Receiver Timing Test Load
Figure 3. Driver Timing Test Circuit
Figure 4. Driver Timing Test Load
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