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ADP1071-1 Datasheet(Fiches technique) 23 Page - Analog Devices

Numéro de pièce ADP1071-1
Description  Isolated Synchronous Flyback Controller
Télécharger  27 Pages
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Fabricant  AD [Analog Devices]
Site Internet  http://www.analog.com
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ADP1071-1 Datasheet(HTML) 23 Page - Analog Devices

 
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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
VAUX = 8V TO 13V
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


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