LTC1535

9

1535fa

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

(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

µs 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

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 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.

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.

Figure 10

**

2

1

1

1535 F10

GND

LOGIC COMMON

2

FLOATING RS485 COMMON

** TRANSFORMER

COOPER (561) 241-7876

420kHz

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 F10a

8

6

4

2

0