Data Sheet

ADP1071-1/ADP1071-2

Rev. B | Page 23 of 27

hiccup mode even in subharmonic oscillation. Missing two

OCP thresholds consecutively resets the hiccup counter.

8V LDO

ADP1071-1/ADP1071-2

1µA

3µA

LOGIC

VREF

1.2V

R1

R2

RH

VIN

VIN

EN

HYSTERESIS

GENERATOR

Figure 23. Precision EN with Adjustable Hysteresis

EXTERNAL START-UP CIRCUIT

For input voltages higher than 36 V, in which the power

dissipation in the internal 8 V LDO can be significant, the use

of an external start-up circuit is recommended. (See Figure 24

for an example.) In this case, the VIN and VREG1 pins are

shorted together and connect to the output of the start-up

circuit. Because the input pre-enable bias current, the VIN and

VREG1 start-up current, is approximately 160 μA, the output of

the start-up circuit must be able to provide this level of current

in order to soft start. The auxiliary winding then provides the

bias voltage, shutting off the start-up circuit after soft start

completes.

8V LDO

START-UP

CIRCUIT

ADP1071-1/ADP1071-2

1µA

4µA

LOGIC

VREF

1.2V

R1

R2

VIN

VAUX1

VREG_UVLO

VIN

VREG1

EN

HYSTERESIS

GENERATOR

Figure 24. Precision EN Circuit Connection with an External Start-Up Circuit

A fast start-up circuit is shown in Figure 25. This circuit requires

two components: a Zener diode, which sets up the start-up

voltage at the VIN and VREG1 pin, and a negative positive

negative (NPN) transistor, which sets up a fast current path for

charging up the start-up capacitor, C1. The start-up current

through R1 must be more than 160 μA, which is the minimum

specified start-up current, and the start-up voltage at VREG1

and VIN must be approximately 8 V to 13 V. The auxiliary

winding then provides the bias voltage, shutting off the NPN

transistor after the soft start completes.

ADP1071-1/

ADP1071-2

LDO

R2

C1

2.2µF

D1

8.7V TO 11V

<60mW

R1

VIN

Q1

R3

VIN

14V

FROM

AUXILIARY

WINDING

VREG1

EN

Figure 25. Fast Start-Up Circuit

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 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 ADP1071-1/

ADP1071-2.

Analog Devices performs accelerated life testing using voltage

levels higher than the rated continuous working voltage.

Acceleration 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 7 and Table 8 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 ADP1071-1/ADP1071-2 insulation lifetime depends on the

voltage waveform type imposed across the isolation barrier. The

i

Coupler insulation structure degrades at different rates

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

dc. Figure 26, Figure 27, and Figure 28 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 27 or Figure 28 as a bipolar ac waveform, and limit its peak

voltage to the 50-year lifetime voltage value listed in Table 7 and

Table 8.

Note that the voltage presented in Figure 27 is shown as

sinusoidal for illustration purposes only. It is meant to represent