IW1706 Datasheet, PDF (6/11 Pages) Dialog Semiconductor – Low-Power Off-Line Digital Green-Mode PWM Controller

iW1706
Low-Power Off-Line Digital Green-Mode PWM Controller
9.1 Pin Detail
Pin 1 – V_CC
Power supply for the controller during normal operation.
The controller will start up when V_CC reaches 11.0 V (typical)
and will shut-down when the V_CC voltage is 4.0 V (typical).
A decoupling capacitor of 0.1 μF or so should be connected
between the V_CC pin and GND.
Pin 2 – GND
Ground.
Pin 3 – V_SENSE
power adapter applications such as ADSL modems that
have large capacitive loads.
If at any time the Vcc voltage drops below the undervoltage
lockout (UVLO) threshold V_CC(UVL) then the iW1706 goes to
shutdown. At this time ENABLE signal becomes low and the
V_CC capacitor begins to charge up again towards the start-
up threshold to initiate a new soft-start process.
Start-up
Sequencing
V_CC(ST)
Sense signal input from auxiliary winding. This provides the
secondary voltage feedback used for output regulation.
V_CC
Pin 4 – I_SENSE
Primary current sense. Used for cycle-by-cycle peak current
control and limit.
Pin 5 – OUTPUT
ENABLE
Figure 9.1: Start-up Sequencing Diagram
Base drive for the external power BJT switch.
9.3 Understanding Primary Feedback
9.2 Adaptively Controlled Soft Start-Up
The iW1706 features an innovative proprietary soft-start
scheme to achieve fast yet smooth start-up with a wide
range of output loads, including capacitive loads typically
from 330 μF to 6000 μF, and for output voltage covering
typically from 5V to 12V. Prior to the start-up, the V_CC pin
is charged through startup resistors. When V_CC bypass
capacitor is fully charged to a voltage higher than the start-
up threshold V_CC(ST), the ENABLE signal becomes active
to enable the control logic, and the iW1706 commences
the soft-start function. During the soft-start process, the
primary-side peak current is limited cycle by cycle by the
I_PEAK comparator. The whole soft-start process can break
down into several stages based on the output voltage
levels, which is indirectly sensed by V_SENSE signal at the
primary side. At different stages, the iW1706 adaptively
controls the switching frequency and primary-side peak
current such that the output voltage can always build up
very fast at the early stages and smoothly transition to the
desired regulation voltage at the final stage, regardless of
any capacitive and resistive loads that the applications may
incur. With a lowest system cost, this adaptively controlled
soft start-up feature makes the iW1706 ideal in network
Figure 9.2 illustrates a simplified flyback converter. When
the switch Q1 conducts during t_ON(t), the current i_g(t) is
directly drawn from rectified sinusoid v_g(t). The energy E_g(t)
is stored in the magnetizing inductance L_M. The rectifying
diode D1 is reverse biased and the load current I_O is
supplied by the secondary capacitor C_O. When Q1 turns off,
D1 conducts and the stored energy E_g(t) is delivered to the
output.
i_in(t)
v_in(t)
+
i_g(t)
v_g(t)
–
T_S(t)
N:1
i_d(t)
V_O
D1
+
C_O
I_O
V_AUX
Q1
Figure 9.2: Simplified Flyback Converter
In order to tightly regulate the output voltage, the
information about the output voltage and load current need
to be accurately sensed. In the DCM flyback converter,
this information can be read via the auxiliary winding or
Rev. 1.1
iW1706
February 1, 2012

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