LG MW-60SZ12 Service Manual page 19

(4) Control mode (PWM Latch / Feed Back)
Control mode of IC uses MIX Mode which is switched to the
voltage control mode in lightload and and current control
mode in heavyload linearly according to load.
MIX control mode operates PWM control comparing standard
triangle shaped wave with current waveform piled up on DC
FB voltage. So input voltage difference of PWM Comparator
in waveform cross becomes bigger and as a result, stable
PWM signal is gained.
In MIX control mode, enough noise margin can be obtained
with changing voltage PWM of standard triangle shaped
wave. Also,even in continuation inductor mode in which On
Duty is over 50%, Sub-harmonic phenomenon is suppressed
and every essential part of internal oscillator becomes opti-
mum.
(5) Protect circuit, Protect circuit against over-voltage,
Protect circuit against overheat (OLP/OVP/TSD)
1) Protect circuit against overload
This IC has protect circuit against over-current which
detects over-current every pulse.
In over-current, FB system cuts Photo Coupler for FB off
because output voltage doesn't reach set voltage.
If detecting "Over-current Detector" signal and the "FB
Current Mirror Detector" signal, internal timer count starts
operating 655ms later after detection.
Just after overflow, the "Count Out" signal is output and at
the same time previous operation is stopped. In this state,
main switch is fixed to off and latch circuit starts operating.
Also, in latch operation, latch circuit needs 400uA (Max)
for maintenance and this current is influenced by the elec-
tric potential of Vcc terminal.
If the supplied voltage in Vcc terminal goes below power
voltage for releasing latch circuit, 8.7V (TYP), latch opera-
tion can be released. When this latch operation is
released, this IC begins start operation from Soft Start.
2) Protect circuit against over-voltage
If the supplied voltage in subsidiary power terminal (Vcc)
of this IC exceedes 22.2V (TYP), oscillation operation is
stopped by the protect circuit against over-voltage, and in
this state, main switch is fixed to off and latch operation
begins.
* power voltage Vcc(ovp) for OVP operation = 22.2V (TYP)
3) Protect circuit against overheat
When the temperature of internal chip reaches 150°C
(TYP), oscillation operation stops. In this state, main
switch is fixed to off and latch operation begins.
(6) Soft Start, Protect circuit against over-current
1) Protect circuit against over-current
In main switch's on, drain current is monitored. If drain cur-
rent over standard value is detected, main switch is
switched to off immediately. This drain current detection is
performed every time main switch is on. The standard volt-
age in OCP terminal is 0.75V (TYP).
2) Soft Start function
Aa this IC has Soft Start function which is Timer way,
external condensor for Soft Start is unnecessary.
Soft Start function is performed by increasing standard
voltage for detecting over-current step by step and by con-
trolling current in main switch.
There are 4 steps and voltage level and continuant time of
each steps are fixed inside. Table SS(1) ~ SS(4) shows
this.
Symbol
Vss(1)
Vss(2)
Vss(3)
Vss(4)
- 19 -
MIN TYP
0.16 - -
0.35 - -
0.51 - -
0.63 - -
MAX
0.35
0.54
0.68
0.75