iW3620
Digital PWM Current-Mode Controller for AC/DC LED Driver
turns off so that the VCC capacitor can be charged up again
towards the start-up threshold. Figure 9.1 shows the start-
up sequence of the iW3620.
9.1 Pin Dꢀtꢁil
Pin 2 – VSENSE
Sense signal input from auxiliary winding. This provides the
secondary voltage feedback used for output regulation.
Start-up
Sequencing
Pin 3 – VIN
V
IN
Sense signal input from the rectified line voltage. VIN is used
for line regulation. The input line voltage is scaled down
using a resistor network. It is used for input undervoltage
and overvoltage protection. This pin also provides the supply
current to the IC during start-up.
V
CC(ST)
V
CC
Pin 4 – SD
External shutdown control. If the shutdown control is not
used, this pin should be connected to GND via a resistor.
ENABLE
Pin 5 – GND
Ground.
Figure 9.1 : Start-up Sequencing Diagram
9.3 Undꢀꢃstꢁndinꢅ Pꢃimꢁꢃꢆ Fꢀꢀdꢄꢁck
Figure 9.2 illustrates a simplified flyback converter. When the
switch Q1 conducts during tON(t), the current ig(t) is directly
drawn from rectified sinusoid vg(t). The energy Eg(t) is stored
in the magnetizing inductance LM. The rectifying diode D1 is
reverse biased and the load current IO is supplied by the
secondary capacitor CO. When Q1 turns off, D1 conducts
and the stored energy Eg(t) is delivered to the output.
Pin 6 – ISENSE
Primary current sense. Used for cycle by cycle peak current
control.
Pin 7 – OUTPUT
Gate drive for the external MOSFET switch.
Pin 8 – VCC
i (t)
i (t)
i (t)
in
g
d
N:1
V
+
O
+
D1
I
O
C
O
Power supply for the controller during normal operation. The
controller will start up when VCC reaches 12 V (typical) and
will shut-down when the VCC voltage is below 6 V (typical). A
decoupling capacitor should be connected between the VCC
pin and GND.
v (t)
g
V
v (t)
AUX
in
–
V
AUX
Q1
T (t)
S
9.2 Stꢁꢃt-ꢂp
Figure 9.2 : Simplified Flyback Converter
Prior to start-up the VIN pin charges up the VCC capacitor
through the diode between VIN and VCC (see Figure 8.1).
In order to tightly regulate the output voltage, the information
about the output voltage and load current needs to be
accurately sensed. In the DCM flyback converter, this
information can be read via the auxiliary winding. During the
Q1 on-time, the load current is supplied from the output filter
capacitor CO. The voltage across LM is vg(t), assuming the
voltage dropped across Q1 is zero. The current in Q1 ramps
up linearly at a rate of:
When VCC is fully charged to a voltage higher than the start-
up threshold VCC(ST), the ENABLE signal becomes active and
enables the control logic; the VIN switch turns on, and the
analog-to-digital converter begins to sense the input voltage.
Once the voltage on the VIN pin is above VINSTLOW, the iW3620
commences soft start function. An adaptive soft-start control
algorithm is applied at startup state, during which the initial
output pulses will be small and gradually get larger until the
full pulse width is achieved. The peak current is limited cycle
by cycle by IPEAK comparator.
dig (t) vg (t)
=
(9.1)
dt
LM
At the end of on-time, the current has ramped up to:
If at any time the VCC voltage drops below VCC(UVL) threshold
then all the digital logic is reset. At this time VIN switch
Rev. 1.9
iW3620
February 4, 2022
Page 8
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