Data Sheet ADM2682E/ADM2687E
Rev. C | Page 19 of 24
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
the unipolar ac or dc voltage cases. Any cross-insulation voltage
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.
0V
RATED PEAK VOLTAGE
09927-021
Figure 42. Bipolar AC Waveform
0V
RATED PEAK VOLTAGE
09927-023
Figure 43. DC Waveform
0V
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.
09927-022
Figure 44. Unipolar AC Waveform
ISOLATED SUPPLY CONSIDERATIONS
The typical output voltage of the integrated isoPower dc-to-dc
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 V
ISOOUT
for
external applications.
