IL44608N
Demagnetization Section
The IL44608N demagnetization detection consists of a
comparator designed to compare the VCC winding voltage
to a reference that is typically equal to 50 mV.
This reference is chosen low to increase effectiveness
of the demagnetization detection even during start–up.
A latch is incorporated to turn the demagnetization
block output into a low level as soon as a voltage less than
50 mV is detected, and to keep it in this state until a new
pulse is generated on the output. This avoids any ringing
on the input signal which may alter the demagnetization
detection.
For a higher safety, the demagnetization block output
is also directly connected to the output, which is disabled
during the demagnetization phase.
The demagnetization pin is also used for the quick, pro-
grammable OVP. In fact, the demagnetization input cur-
rent is sensed so that the circuit output is latched off when
this current is detected as higher than 120 µA.
The complete demagnetization status DMG is used to
inhibit the recharge of the CT capacitor. Thus in case of
incomplete transformer demagnetization the next switch-
ing cycle is postpone until the DMG signal appears. The
oscillator remains at 2.4 V corresponding to the sawtooth
valley voltage. In this way the SMPS is working in the so
called SOPS mode (Self Oscillating Power Supply). In that
case the effective switching frequency is variable and no
longer depends on the oscillator timing but on the external
working conditions (Refer to DMG signal in the Figure 6)
This function can be inhibited by grounding it but in
this case, the quick and programmable OVP is also dis-
abled.
The OSC and Clock signals are provided according
to the Figure 6. The Clock signals correspond to the CT
capacitor discharge. The bottom curve represents the cur-
rent flowing in the sense resistor Rcs. It starts from zero
and stops when the sawtooth value is equal to the control
voltage Vcont. In this way the SMPS is regulated with a
voltage mode control.
Oscillator
The IL44608 contains a fixed frequency oscillator. It
is built around a fixed value capacitor CT succes-
sively charged and discharged by two distinct current
sources ICH and IDCH. The window comparator
senses the CT voltage value and activates the sources
when the voltage is reaching the 2.4 V/4.0 V levels.
6
IKSEMICON [ IK SEMICON CO., LTD ]
