LTC1535CSW datasheet(10/18 Pages) LINER

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LTC1535

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 2500VRMS 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

conﬁguration, with SLO = VCC2, 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 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 choosing

low impedance parts is important for good regulation. Care

must also be taken to not exceed the VCC2 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 sufﬁcient 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.

1535 F10

VCC

GND

LOGIC COMMON

FLOATING RS485 COMMON

** COOPER (888) 414-2645

420kHz

GND2

1/2 BAT54C

ILOAD

VCC2

ST1

ST2

VCC

10μF

CTX02-14659

IEXT

IVCC2

TOTAL LOAD CURRENT, ILOAD (mA)

100

150

1535 F10a

VCC = 5.5V

VCC = 5V

VCC = 4.5V

Figure 10

VCC2 vs ILOAD

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