IL44608 Datasheet, PDF (5/13 Pages) IK Semicon Co., Ltd – External Components Reliable and Flexible SMPS Controller

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 dynam-
ic 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.
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”
IL44608N
Current Sense
The inductor current is converted to a positive voltage
by inserting a ground reference sense resistor R_Sense 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:
1
Ipk _max = _{R SENSE (Ω)} (A)
In stand–by mode, this current can be lowered as due ac-
tivation of a 200 μA current source:
Ipk max− stby = ^{1 - (R cs (kΩ) × 0,2)} (A)
R _SENSE (Ω)
PWM Latch
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.
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.
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 producing
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.
2011, February, Rev. 01