Datasheet

LTC1535
10
1535fb
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 2500V
RMS
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 = V
CC2
, 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 V
CC2
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
V
CC
GND
LOGIC COMMON
2
FLOATING RS485 COMMON ** COOPER [phone]
420kHz
4
1411
1
+
+
GND2
1/2 BAT54C
1/2 BAT54C
I
LOAD
V
CC2
ST1 ST2
32
V
CC
1
10μF
10μF
2
CTX02-14659
I
EXT
I
VCC2
TOTAL LOAD CURRENT, I
LOAD
(mA)
0 50 100 150
V
CC2
(V)
1535 F10a
8
6
4
2
0
V
CC
= 5.5V
V
CC
= 5V
V
CC
= 4.5V
Figure 10
V
CC2
vs I
LOAD