ltc1535csw-trpbf Linear Technology Corporation, ltc1535csw-trpbf Datasheet - Page 9

• Manufacturer Part Number: ltc1535csw-trpbf
• Description: Isolated Rs485 Transceiver
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
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 2500V
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 1 s to 2 s. 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
configuration, with SLO = V
is set by the internal sample rate. With SLO = 0V, the
frequency is reduced by the slower output rise and fall
times.
V
10 F
CC
+
1
1
1
4
1
LOGIC COMMON
V
GND
RMS
CC
U
ST1
of isolation. Communication between
2
U
3
2
FLOATING RS485 COMMON
CC2
ST2
CTX02-14659
, the oscillation frequency
420kHz
W
**
1/2 BAT54C
1/2 BAT54C
2
U
** TRANSFORMER
COOPER [phone]
2
Figure 10
GND2
+
11
I
LOAD
10 F
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 Cooper 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 V
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 over-
loaded, the supply voltage will trip the low voltage detec-
tion at 4.2V. For higher voltage stand-off, the Cooper
CTX02-14608 transformer may be used.
V
14
CC2
1535 F10
I
EXT
I
VCC2
8
6
4
2
0
0
V
CC
TOTAL LOAD CURRENT, I
= 5.5V
V
V
50
CC
CC2
= 5V
vs I
V
CC
LOAD
LTC1535
= 4.5V
100
LOAD
CC2
(mA)
1535 F10a
recom-
150
1535fa
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