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ADP1071-1 Datasheet(Fiches technique) 17 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) 17 Page - Analog Devices

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Data Sheet
ADP1071-1/ADP1071-2
Rev. B | Page 17 of 27
SOFT START PROCEDURE
The following procedure assumes that the VDD2 pin is
powered directly from the output voltage of the power supply.
To ensure a smooth output voltage ramp during startup, the soft
start sequence is controlled by two soft start control circuits,
one in the primary (for open-loop soft start using the GATE
pin) and the other in the secondary (for closed-loop soft start,
using the SS2 pin). Proper handshaking between the primary side
and the secondary side is needed prior to the secondary side
taking control.
The open-loop soft start time is determined by the resistor on
the GATE pin prior to startup. The primary peak current is
increased gradually every switching period. The slew rate of the
increase in peak current is determined by selecting the GATE
resistor prior to startup. The current increases from a minimum
of 0 A to a maximum of 120 mV/RSENSE. This rate is the open-
loop soft start. Four speeds are available: 4 × 775, 16 × 775,
64 × 775, and 256 × 775 switching periods for resistors 100 kΩ,
10 kΩ, 22 kΩ, and 47 kΩ, respectively.
During this time, the ADP1071-1/ADP1071-2 start firing the
PWM pulses and the output voltage continues to build up
slowly if the average current on the secondary side exceeds the
load current. Because the ADP1071-1/ADP1071-2 are current
mode controllers, the output capacitor starts charging only
when the primary current limit exceeds the load current
requirement.
The handshaking process is as follows.
When VDD2 reaches the UVLO of approximately 3.5 V, the
internal circuitry on the secondary side is activated and the
ADP1071-1/ADP1071-2 initiate the following two processes:
1. The ADP1071-1/ADP1071-2 make the voltage on the
SS2 pin equal to the value on the FB pin, with an SS2 pin
current, at 10 times the nominal current source of 20 µA
on the SS2 pin.
2. Simultaneously, the current limit on the primary side is
transferred over to the secondary side and the voltage on
the COMP pin is made equal to the instantaneous current
limit of ±100 mV. There is a timeout for this process, which
is 1.5 ms after the VDD2 UVLO threshold is crossed.
When this process is satisfied, the transmission of the COMP signal
occurs from the secondary to the primary side. The ADP1071-1/
ADP1071-2 transmit the COMP signal by continuously sampling
the analog signal at the COMP pin. The sampled value is then
transmitted using a proprietary scheme to the primary side
where the instantaneous value of the CS pin is compared to the
COMP level to determine the falling edge of the GATE pulse.
The COMP signal is, therefore, a representation of the primary
current limit.
After COMP transmission begins, the primary side receives the
signal and control is completely handed over to the secondary
side when either the received level of COMP on the primary
side is within ±100 mV or up to 128 switching periods
(typically 8) have passed, starting from the first pulse being
transmitted to the primary side.
Then, after the control is handed over to the secondary side, the
closed-loop soft start begins, where the SS2 capacitor is charged
at a nominal rate of 20 µA. The output voltage then rises to the
regulation voltage based on the SS2 pin voltage. The voltage on
the SS2 pin continues to rise to 1.2 V, that is, the steady state
voltage on the FB pin. At this stage, the power supply is in
regulation, and the output voltage is at its target value.
At the end of the soft start process, the voltage on the SS2 pin
continues to rise to approximately 1.4 V. In steady state, the
FB pin (that is, the reference voltage) is 1.2 V.
The SR1 and SR2 synchronous drivers begin to pulse after
VDD2 crosses the UVLO threshold.
If the voltage at the VDD2 pin is greater than the UVLO voltage,
such as a soft start from the precharged output, or if the VDD2 pin
is powered by an external supply, the secondary side assumes
control from the moment the EN pin is enabled, and only SS2 is
used for the soft start procedure.
When initiating a soft start from the precharged output, the
SS2 pin tracks the FB pin and then initiates a soft start. This
process eliminates any glitches in the output voltage.
When soft starting into a precharged output, the SR gate is
prevented from turning on until the SS2 voltage reaches the
precharged voltage at the FB pin. This soft start scheme prevents
the output from being discharged, and it prevents reverse current.
Under abnormal situations, such as a shorted load or a transient
condition on the load during the soft start process, FB may not
be able to track SS2 accurately. If this condition occurs before the
VDD2 UVLO threshold is crossed, the open loop soft start is in
effect. If it occurs after the VDD2 UVLO threshold is crossed,
SS2 tracks the FB pin and then continues with the soft start
process until the regulation voltage is reached. In all conditions,
control is handed over to the secondary side if FB ≥ 1.2 V.
When the secondary VDD2 is directly powered by the output of
the converter, the minimum output voltage required is higher
than the secondary UVLO voltage. For output voltages less than
the secondary UVLO voltage, a third winding is needed to generate
an auxiliary voltage to power the secondary side circuitry. Alter-
nately, in most cases, a diode resistor capacitor combination from
the switch node can provide the voltage to VDD2.


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