ADuM6402 datasheet(22/24 Pages) AD

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ADuM6400/ADuM6401/ADuM6402/ADuM6403/ADuM6404

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When the primary side is above the UVLO threshold, the data

input channels sample their inputs and begin sending encoded

pulses to the inactive secondary output channels. The outputs

on the primary side remain in their default low state because no

data comes from the secondary side inputs until secondary power

is established. The primary side oscillator also begins to operate,

transferring power to the secondary power circuits. The secondary

VISO voltage is below its UVLO limit at this point; the regulation

control signal from the secondary is not being generated. The

primary side power oscillator is allowed to free run in this

circumstance, supplying the maximum amount of power to

the secondary, until the secondary voltage rises to its regulation

setpoint. This creates a large inrush current transient at VDD1.

When the regulation point is reached, the regulation control

circuit produces the regulation control signal that modulates

the oscillator on the primary side. The VDD1 current is reduced

and is then proportional to the load current. The inrush current

is less than the short-circuit current shown in Figure 16. The

duration of the inrush depends on the VISO loading conditions

and the current available at the VDD1 pin.

Accelerated life testing is performed using voltage levels higher

than the rated continuous working voltage. Acceleration factors

for several operating conditions are determined, allowing calcu-

lation of the time to failure at the working voltage of interest.

The values shown in Table 20 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 ADuM640x 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 30,

Figure 31, and Figure 32 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 20 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 31 or Figure 32

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 20.

As the secondary side converter begins to accept power from

the primary, the VISO voltage starts to rise. When the secondary

side UVLO is reached, the secondary side outputs are initialized to

their default low state until data is received from the corresponding

primary side input. It can take up to 1 μs after the secondary side is

initialized for the state of the output to correlate with the primary

side input.

Secondary side inputs sample their state and transmit it to the

primary side. Outputs are valid about 1 μs after the secondary

side becomes active.

RATED PEAK VOLTAGE

Because the rate of charge of the secondary side power supply is

dependent on loading conditions, the input voltage, and the output

voltage level selected, take care with the design to allow the

converter sufficient time to stabilize before valid data is required.

Figure 30. Bipolar AC Waveform

When power is removed from VDD1, the primary side converter

and coupler shut down when the UVLO level is reached. The

secondary side stops receiving power and starts to discharge.

The outputs on the secondary side hold the last state that they

received from the primary side. Either the UVLO level is reached

and the outputs are placed in their high impedance state, or the

outputs detect a lack of activity from the primary side inputs

and the outputs are set to their default low value before the

secondary power reaches UVLO.

RATED PEAK VOLTAGE

Figure 31. DC Waveform

RATED PEAK VOLTAGE

NOTES

1. THE VOLTAGE IS SHOWN AS SINUSOIDAL 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.

INSULATION LIFETIME

All insulation structures eventually break down when subjected

to voltage stress over a sufficiently long period. The rate of insu-

lation 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 ADuM640x.

Figure 32. Unipolar AC Waveform

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