IL44608N
OPERATING DESCRIPTION
The pin 3 senses the feedback current provided by the
opto coupler. During the switching phase the switch S2 is
closed and the shunt regulator is accessible by the pin 3.
The shunt regulator voltage is typically 5.0 V. The dy-
namic resistance of the shunt regulator represented by the
zener diode is 20 Ω. The gain of the Control input is given
on Figure 11/, which shows the duty cycle as a function of
the current injected into the pin 3.
Current Sense
The inductor current is converted to a positive voltage
by inserting a ground reference sense resistor RSense in se-
ries with the power switch.
The maximum current sense threshold is fixed at 1.0 V.
The peak current is given by the following equation:
A 4.0 kHz filter network is inserted between the shunt
regulator and the PWM comparator to cancel the high fre-
quency residual noise.
The switch S3 is closed in Stand–by mode during the
Latched Off Phase while the switch S2 remains open. (See
section PULSED MODE DUTY CYCLE CONTROL).
The resistor Rdpulsed (Rduty cycle burst) has no effect on
the regulation process. This resistor is used to determine
the burst duty cycle described in the chapter “Pulsed Duty
Cycle Control”
1
Ipkmax
=
(A)
RSENSE(Ω)
In stand–by mode, this current can be lowered as due
activation of a 200 µA current source:
1 - (R (k Ω ) × 0,2)
cs
Ipk max
−
stby
=
(A)
R
(Ω )
SENSE
PWM Latch
The current sense input consists of a filter (6.0 k Ω, 4.0
pF) and of a leading edge blanking. Thanks to that, this pin
is not sensitive to the power switch turn on noise and
spikes and practically in most applications, no filtering
network is required to sense the current.
The IL44608 works in voltage mode. The on–time is
controlled by the PWM comparator that compares the os-
cillator sawtooth with the regulation block output (refer to
the block diagram on page 2).
The PWM latch is initialized by the oscillator and is re-
set by the PWM comparator or by the current sense com-
parator in case of an over current. This configuration en-
sures that only a single pulse appears at the circuit output
during an oscillator cycle.
Finally, this pin is used:
– as a protection against over currents (Isense > I)
– as a reduction of the peak current during a Pulsed
Mode switching phase.
The overcurrent propagation delay is reduced by produc-
ing a sharp output turn off (high slew rate). This results in
an abrupt output turn off in the event of an over current
and in the majority of the pulsed mode switching sequence.
5
IKSEMICON [ IK SEMICON CO., LTD ]
