ADM2687EBRIZ datasheet(19/24 Pages) AD

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ADM2682E/ADM2687E

Rev. 0 | Page 19 of 24

e unipolar ac or dc voltage cases. Any cross-insulation voltage

INSULATION LIFETIME

All insulation structures eventually break down when subjected to

voltage stress over a sufficiently long period. The rate of insulation

degradation is dependent on the characteristics of the voltage

waveform applied across the insulation. Analog Devices conducts

an extensive set of evaluations to determine the lifetime of the

insulation structure within the ADM2682E/ADM2687E.

Accelerated life testing is performed using voltage levels higher

than the rated continuous working voltage. Acceleration factors for

several operating conditions are determined, allowing calculation

of the time to failure at the working voltage of interest. The values

shown in Table 9 summarize the peak voltages for 50 years of

service life in several operating conditions. In many cases, the

working voltage approved by agency testing is higher than the

50-year service life voltage. Operation at working voltages higher

than the service life voltage listed leads to premature insulation

failure.

The insulation lifetime of the ADM2682E/ADM2687E depends

on the voltage waveform type imposed across the isolation barrier.

The iCoupler insulation structure degrades at different rates,

depending on whether the waveform is bipolar ac, unipolar ac,

or dc. Figure 42, Figure 43, and Figure 44 illustrate these different

isolation voltage waveforms.

Bipolar ac voltage is the most stringent environment. A 50-year

operating lifetime under the bipolar ac condition determines

the Analog Devices recommended maximum working voltage.

In the case of unipolar ac or dc voltage, the stress on the insulation

is significantly lower. This allows operation at higher working

voltages while still achieving a 50-year service life. The working

voltages listed in Table 9 can be applied while maintaining the

50-year minimum lifetime, provided the voltage conforms to either

waveform that does not conform to

Figure 43 or Figure 44 should

be treated as a bipolar ac waveform, and its peak voltage should

be limited to the 50-year lifetime voltage value listed in Table 9.

RATED PEAK VOLTAGE

Figure 42. Bipolar AC Waveform

RATED PEAK VOLTAGE

Figure 43. DC Waveform

RATED PEAK VOLTAGE

NOTES

1. THE VOLTAGE IS SHOWN AS SINUSODIAL FOR ILLUSTRATION

PURPOSES ONLY. IT IS MEANT TO REPRESENT ANY VOLTAGE

WAVEFORM VARYING BETWEEN 0 AND SOME LIMITING VALUE.

THE LIMITING VALUE CAN BE POSITIVE OR NEGATIVE, BUT THE

VOLTAGE CANNOT CROSS 0V.

Figure 44. Unipolar AC Waveform

ISOLATED SU

wer dc-to-dc

PPLY CONSIDERATIONS

The typical output voltage of the integrated isoPo

isolated supply is 3.3 V. The isolated supply in the ADM2682E/

ADM2687E is typically capable of supplying a current of 55 mA

when the junction temperature of the device is kept below 130°C.

This includes the current required by the internal RS-485 circuitry,

and typically, no additional current is available on VISOOUT for

external applications.

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