7

LTC1535

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

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 approxi-

mately 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

µs to 2µs. A time-out fault will occur if

communication from the isloated side fails. Faults can be

monitored on the RE pin.

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 9 shows the DC/DC con-

verter in a configuration that can deliver up to to 100mA of

current to the isolated side using a Coiltronics CTX02-

14659 transformer.

Because the DC/DC converter is open-loop, care in choos-

ing low impedance parts is important for good regulation.

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 Coiltronics

CTX02-14608 transformer may be used.

Figure 9

**

2

1

1

1535 F09

GND

LOGIC COMMON

2

FLOATING RS485 COMMON

** TRANSFORMER

COILTRONICS (561) 241-7876

400kHz

4

14

11

1

+

+

GND2

1/2 BAT54C

1/2 BAT54C

ST1

ST2

3

2

1

10

µF

10

µF

2

CTX02-14659

0

50

100

150

1535 F09a

8

6

4

2

0