Transceivers with ±15kV ESD Protection

______________________________________________________________________________________

23

Machine Model

The Machine Model for ESD tests all pins using a

200pF storage capacitor and zero discharge resis-

tance. Its objective is to simulate the stress caused by

contact that occurs with handling and assembly during

manufacturing. All pins require this protection during

manufacturing, not just inputs and outputs. Therefore,

after PC board assembly, the Machine Model is less

relevant to I/O ports.

Skew

The self-oscillation circuit shown in Figure 5 is an excel-

lent way to get an approximate measure of the speed

of the MAX3535E/MXL1535E. An oscillation frequency

of 250kHz in this configuration implies a data rate of at

least 500kbps for the receiver and transmitter com-

bined. In practice, data can usually be sent and

received at a considerably higher data rate, normally

limited by the allowable jitter and data skew. If the sys-

tem can tolerate a 25% data skew, (the difference

specification implies a data rate of 877kbps. Lower

data rates result in less distortion and jitter (Figure 16).

Higher rates are possible but with more distortion and

jitter. The data rate should always be limited below

1.75Mbps for both receiver and driver to avoid interfer-

ence with the internal barrier communication.

Layout Considerations

The MAX3535E/MXL1535E pin configurations enable

optimal PC board layout by minimizing interconnection

lengths and crossovers:

• For maximum isolation, the isolation barrier should not

be breached except by the MAX3535E/MXL1535E and

the transformer. Connections and components from

one side of the barrier should not be located near those

of the other side of barrier.

•A shield trace connected to the ground on each side of

the barrier can help intercept capacitive currents that

might otherwise couple into the DI and SOL inputs. In a

double-sided or multilayer board, these shield traces

should be present on all conductor layers.

• Try to maximize the width of the isolation barrier

wherever possible. A clear space of at least 0.25in

between GND1 and GND2 is recommended.

90%

36.8%

TIME

CURRENT WAVEFORM

PEAK-TO-PEAK RINGING

(NOT DRAWN TO SCALE)

Ir

10%

0

0

AMPERES

Figure 15. Human Body Current Waveform

0

10

5

20

15

30

25

35

45

40

50

0

500 750

250

1000 1250 1500 1750 2000

DATA SKEW vs. DATA RATE

DATA RATE (kbps)

TYP SKEW

MAX SKEW

Figure 16. Data Skew vs. Data Rate Graph