FA5626 FUJI [Fuji Electric], FA5626 Datasheet - Page 17

• Manufacturer Part Number: FA5626
• Description: Fuji Switching Power Supply Control IC
• Manufacturer: FUJI [Fuji Electric]

The reverse amplifier output or the maximum IS pin input
threshold voltage, VthIS1, whichever is higher, is given
precedence as the IS pin threshold voltage.
(Example: When the output of the reverse amplifier is -0.2V
in a product whose maximum threshold voltage of the IS pin,
VthIS1, is -0.5V, the output of the reverse amplifier is given
precedence and thus the current comparator is reversed
when the IS pin voltage reaches -0.2V.)
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 pin 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 pin, VthIS1
(-0.5V typ) controls the maximum current of the MOSFET. If
the FB pin 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 pin 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 85% (typ).
Reduction of dependency of OCP on input voltage
This IC has an improved OCP function which changes the
maximum input threshold voltage (the current limit
threshold voltage) so that OCP dependency on the input
voltage will be compensated.
The maximum input threshold voltage is lowers by 5%
when VH pin voltage (V
if VH pin is connected to AC line.
VthIS1 = –0.525V typ. when VVH < VHVTH2
VthIS1 = –0.500V typ. when VVH ≥ VHVTH2
(See p. 24 "9-(7) Reduction of dependency of overload
detection level on input voltage".)
(4) One shot circuit (minimum ON width)
generated
capacitance of the main circuit and gate drive current. If this
surge current reaches the IS pin threshold voltage, the
current comparator output is reversed, and consequently
normal pulses may not be generated from the OUT pin.
To avoid this phenomenon, a minimum ON width of OUT
pin 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 pin is kept invalid for the
specified period of time immediately after the output pulse
is generated from the OUT pin (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
AN-071E Rev.1.2
April-2011
When the MOSFET is turned on, a surge current is
Fuji Electric Co., Ltd.
due
to
discharge
VH
) is over 148Vdc (approx. 105Vac)
corresponding
to
FA5626
the
noise filter for the IS pin in principle.
The minimum ON width is usually set to 1250ns or 1700ns
(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 pin
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.
17
FB
Fig.9 Output shutdown function of FB pin
2
5V
Fig.7 Current comparator
Fig.8 Minimum ON width
A=1/AvIS
AMP
VthIS1
INV
A
4
http://www.fujielectric.co.jp/products/semiconductor/
GND
IS comp.
3
IS