ADP1074ARWZ-R7 datasheet(26/32 Pages) AD

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ADP1074

Data Sheet

Rev. D | Page 26 of 32

VFB 1.2V

SS2 = 1.4V

DEPENDS ON

LOOP BANDWIDTH

TIME

VDD2

DEPENDS ON

LOOP BANDWIDTH

VDD2_UVLO

3.5V

Figure 21. Tracking with SS2 Pin

OCP COUNTER

During overload conditions, when the peak sensed currents

exceed the OCP threshold voltage of 120 mV on the CS pin, the

ADP1074 immediately terminates the remainder of the PWM

pulse. If the peak sense current continues to exceed the threshold

every switching period for 1.5 ms, the system enters hiccup mode,

by which it shuts down for approximately 40 ms and then soft

starts. During an exceeded overcurrent situation, such as a dead

short, it is likely that the programmed slope compensation is

not enough, and therefore, the system enters subharmonic

oscillation. If this is the case, the system cannot enter hiccup

mode because the OCP threshold is crossed every alternate

switching period, and the 1.5 ms hiccup counter resets.

To prevent this scenario, the ADP1074 latches the last known

state, whereby if an OCP condition registered as a 1 in one

switching period and as a 0 in the next switching period, it is

still counted as a 1. In this manner, the system can enter hiccup

mode even in subharmonic oscillation. Missing two OCP

thresholds consecutively resets the hiccup counter.

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 upon the characteristics of

the voltage waveform applied across the insulation. In addition

to the testing performed by the regulatory agencies, Analog

Devices carries out an extensive set of evaluations to determine

the lifetime of the insulation structure within the ADP1074.

Analog Devices performs accelerated life testing using voltage

levels higher than the rated continuous working voltage. Accel-

eration factors for several operating conditions are determined.

These factors allow calculation of the time to failure at the actual

working voltage.

The values shown in Table 10 summarize the peak voltage for

50 years of service life for a bipolar ac operating condition. In

many cases, the approved working voltage is higher than the

50 year service life voltage. Operation at these high working

voltages can lead to shortened insulation life in some cases.

The ADP1074 insulation lifetime depends on the voltage wave-

form 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 22,

Figure 23, and Figure 24 show these different isolation voltage

waveforms.

A bipolar ac voltage environment is the worst case for the iCoupler

products, yet meets the 50 year operating lifetime recommended

by Analog Devices for maximum working voltage. In the case

of unipolar ac or dc voltage, the stress on the insulation is

significantly lower. The low stress allows operation at higher

working voltages while still achieving a 50 year service life. Treat

any cross insulation voltage waveform that does not conform to

Figure 23 or Figure 24 as a bipolar ac waveform, and limit its peak

voltage to the 50 year lifetime voltage value listed in Table 10.

Note that the voltage presented in Figure 23 is shown as sinusoidal

for illustration purposes only. It is meant to represent any voltage

waveform varying between 0 V and some limiting value. The

limiting value can be positive or negative, but the voltage

cannot cross 0 V.

RATED PEAK VOLTAGE

Figure 22. Bipolar AC Waveform

RATED PEAK VOLTAGE

Figure 23. Unipolar AC Waveform

RATED PEAK VOLTAGE

Figure 24. DC Waveform

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