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ADP1071-1 Datasheet(Fiches technique) 19 Page - Analog Devices

Numéro de pièce ADP1071-1
Description  Isolated Synchronous Flyback Controller
Télécharger  27 Pages
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Data Sheet
ADP1071-1/ADP1071-2
Rev. B | Page 19 of 27
The root cause of the device exiting hiccup mode is due to the
effect that the OCP hiccup mode feature has on the SS2 pin.
During OCP recovery, the SS2 pin tracks the FB pin and
attempts a soft start from the precharge sequence. During this
time that SS2 tracks FB, the SS2 pin voltage can be less than the
FB pin voltage for a short interval, which causes the COMP pin
(output of the gm amplifier) to momentarily fall below the
maximum COMP pin clamp level. This event means that the
current limit required for the next few switching periods is less
than the maximum threshold and puts the device out of hiccup
mode because the ADP1071-1/ADP1071-2 fail to register
1.25 ms worth of consecutive overcurrent cycles and fails to
enter OCP hiccup mode.
The following scenarios guarantee OCP hiccup mode based on
the configuration of the VDD2 power supply:
When VDD2 is powered directly from the output voltage,
if a short circuit on the output terminals of the load occurs
after steady state regulation is achieved, the VDD2 pin
voltage is less than the UVLO threshold, and the device
enters hiccup mode for 200 ms, similar to the hiccup time
described in the Remote System Reset section.
When VDD2 is powered through auxiliary winding or
another configuration, when a short circuit occurs on the
output terminals, the auxiliary winding is not shorted and
maintains a positive voltage above the VDD2 UVLO
threshold. To enter hiccup mode, the following circuit is
recommended, as shown in Figure 19. The circuit operates
as follows: when the output voltage goes low due to a short
circuit, the D1 diode turns on, which pulls the base of the
bipolar junction transistor (BJT) low, shutting off VDD2.
The system then enters hiccup mode, as described in the
Remote System Reset section.
R3 is sized to bias the Zener diode and R4 is sized such that
(VZENER − 1)/R4 > IZENER, where VZENER is the voltage of the diode
and IZENER is the biasing current of the diode. This sizing ensures
that the impedance of the resistor is less than the impedance of
the diode, which causes the voltage of the diode to drop, and
allows VDD2 to enter UVLO.
If the output voltage is <5 V, the same procedure can be used to
size the R4 resistor. If a discrete LDO is not used, a simple
resistor and diode connector to the output voltage is sufficient.
In this case, the R4 resistor is sized to limit the current through
the D1 diode when the output voltage is 0 V during a short
circuit event. Because the bandwidth of the system is high, the
ADP1071-1/ADP1071-2 are able to maintain voltage regulation
at the proper voltage level, even if the auxiliary winding voltage
is higher than the output voltage. The soft start and soft start
from precharge conditions is met with the addition of this
circuit due to the bandwidth of the overall system.
R3
500Ω
R4
100Ω
VDD2
VOUT
AGND2
~6.3V
ZENER
D1
D1
VOUT
R4
100Ω
ALTERNATE OPTION
FROM AUXILIARY
WINDING
~10V
Figure 19. Recommended Circuit to Guarantee Hiccup Mode
TEMPERATURE SENSING
The ADP1071-1/ADP1071-2 have an internal temperature
sensor that shuts down the controller when the internal
temperature exceeds the OTP limit. At this time, the primary
and secondary MOSFET drivers (GATE and SR) are held low.
When the temperature drops below the OTP hysteresis level,
the ADP1071-1/ADP1071-2 restart with a soft start sequence.
FREQUENCY SETTING (RT PIN)
The switching frequency can be programmed in a range of
50 kHz to 600 kHz by connecting a resistor from RT to
AGND1. A small current flows out of the RT pin and the
voltage across it sets up the internal oscillator frequency. The
value of this pin is approximately 1.224 V in steady state. Use
the following equation to determine the resistor (in Ω) for a
particular switching frequency (in kHz):
12
11
(kHz)
1000
41.67 10
S
f
R
=
×
××
where:
fS is the switching frequency.
R is the resistor on the RT pin.
MAXIMUM DUTY CYCLE
To prevent the transformer core from saturating in the event of
high current or extreme load transient, a maximum duty cycle
clamp is internally set to 85%.
As an added protection feature to prevent open-loop conditions,
the maximum duty cycle is also applicable during soft start. If
the controller reaches the maximum duty cycle during soft start
for three consecutive switching periods, the 40 ms hiccup timer
is initiated.
FREQUENCY SYNCHRONIZATION
The switching frequency of the ADP1071-1/ADP1071-2 can be
synchronized to an external clock at the SYNC pin. When an
external clock rising edge is first detected, it takes approx-
imately seven to ten periods for the internal clock to lock in the
SYNC clock frequency. In between the time that the SYNC clock
is detected and the time that it is locked in, the controller
continues to operate with the internal oscillator frequency.
The SYNC frequency must be within ±10% of the internal oscil-
lator frequency set by the RT pin. Otherwise, synchronization does
not take place.




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