FA5558 Fuji Electric holdings CO.,Ltd, FA5558 Datasheet - Page 18

• Manufacturer Part Number: FA5558
• Description: Green Mode Pwm Ic
• Manufacturer: Fuji Electric holdings CO.,Ltd

In normal operation, the output voltage of the power supply
is maintained constant by changing the threshold voltage
of the current comparator via the FB terminal voltage.
When the output voltage decreases, the feedback circuit
increases the FB voltage to allow the threshold voltage of
the current comparator to scale out to Low, thus increasing
the MOSFET current.
The maximum input threshold voltage of the IS terminal,
VthIS1 (-0.95V or -0.45V typ) controls the maximum
current of the MOSFET. If the FB terminal voltage
increases under overload, the output of the reverse
amplifier scales out to Low, decreasing down to lower than
VthIS1. The threshold voltage of the IS terminal is thus
controlled not to exceed VthIS1.
The oscillator outputs pulses for determining the maximum
duty cycle. Using these pulses, the maximum duty cycle
has been set to 80% (typ).
(4) One shot circuit (minimum ON width)
When the MOSFET is turned on, a surge current is
generated due to discharge corresponding to the
capacitance of the main circuit and gate drive current. If
this surge current reaches the IS terminal threshold voltage,
the
consequently normal pulses may not be generated from
the OUT terminal.
To avoid this phenomenon, a minimum ON width of OUT
terminal output is set within the one-shot circuit block of the
IC.
If a trigger signal having the switching frequency is input
from the oscillator, a pulse having a specific width is output
as a PWM latch (F.F.) set signal.
Since the set signal has priority over the input signal of the
PWM latch, the output of the PWM latch (F.F.) is not
reversed while the set signal from the one-shot circuit is
being input, even if a reset signal is input from the current
comparator (IS comp.) (See Fig.5)
As a result, the input to the IS terminal is kept invalid for the
specified period of time immediately after the output pulse
is generated from the OUT terminal (minimum ON width),
and made not to respond to the surge current at turn-on.
(See Fig.8)
This minimum ON width function eliminates the need of a
noise filter for the IS terminal in principle.
The minimum ON width is usually set to 1250ns (typ) in
normal operations, and to 280ns (typ) at startup or
rebooting to prevent the transient MOSFET drain voltage
from surging.
In addition, an exclusive comparator is integrated to keep
the output pulse at zero under no load. (See Fig.9)
This comparator reverses its output when the FB terminal
voltage decreases down to 400mV (typ), preventing a set
pulse to be input to the PWM latch (F.F.). The output is thus
maintained in Low state and switching is stopped.
AN-025E Rev.0.1
December-2008
current
comparator
output
is
reversed,
FA5558/87/88/89
FA5581/82/83/84
and
18
(set pulse)
FB
OUT pin
voltage
output
IS pin
voltage
1shot
2
Fig.9 Output shutdown function of FB pin
OSC
OFF(FB)
400mV
Fig.7 Current comparator
Fig.8 Minimum ON width
Minimum ON width
Fuji Electric Device Technology
Output pulse does not stop here
because of this minimum ON width.
CLR
1 shot
S
R
F.F.
Q
IS pin
threshold
voltage
5
OUT