FA5695 Datasheet, PDF (13/21 Pages) Fuji Electric – Fuji Switching Power Supply Control IC

FA5695
9. Description of each circuit block
(1) Error amplifier
The error amplifier is to make the output voltage constant with
feedback control. For this IC, a transconductance type is used for the
error amplifier.
The non-inverting input terminal is connected to internal reference
voltage of 2.5V (typ.).
The inverting input terminal is fed with output voltage of the
power-factor correction converter, and normally use divided voltage
with resistors. To the inverting input, internal constant current source
of 1.8μA is connected for FB open detection function.
The output of the error amplifier (COMP) is connected to the PWM
comparator and controls the on time of the OUT output.
The output voltage of PFC contains much of ripple of frequency 2
times AC power line (50 or 60Hz). When this ripple component
becomes largely appears in the output of the error amplifier, the
power-factor correction converter does not stably operate. In order to
obtain the stable operation, connect capacitors and a resistor at Pin
No. 2 (COMP) as shown in Fig.5.
Vout
1.8uA
R1
FB
1
R2
C3
RAMP
OSC
ERRAMP
VREF(2.5V)
OUT
F/F
PWM
Comp
COMP
2
R3
C4
C5
Maximum
最_o大_nオ_tiン_m幅_e
Fig.5 Error amplifier circuit
(2) Soft start circuit
FA5695 is equipped with soft start function to suppress rushing
startup and overshoot of output voltage when starting.
The soft start circuit works after UVLO and standby is released
and before the soft start cancellation voltage is exceeded. In the
meantime, the soft start function restricts the startup speed of the
output voltage by limiting the maximum on time to about 80% when
the FB terminal voltage is lower than the reference voltage. (Fig. 6)
The on time limited by the soft start is cancelled when the FB terminal
voltage becomes higher than the soft start cancellation voltage.
(3) Overvoltage protection circuit (OVP)
This circuit is to limit the voltage when the output voltage of the
power-factor correction converter exceeds the set value.
When this IC starts up or load changes sharply, the output voltage
of the converter may exceed the set value. In such a case, this
protection circuit works to control the output voltage.
FA5695 has 2 of OVP function as shown below.
It controls the ON width linearly when the output exceeds the
reference voltage.
Dynamic OVP - - - Built-in FB pin
It stops the output pulse when the output exceeds 1.08 times of
reference voltage.
Static OVP - - - Built-in FB pin and OVP pin
The operation of FB pin which has two functions above is
described below.
FB pin voltage is usually 2.5V as same as the reference voltage.
When the startup or a sudden change of load, FB pin voltage rises
and will exceed 2.5V. In this case, a function which limits ON width
depending on FB pin voltage becomes active. (Dynamic OVP) If FB
pin voltage rises more and exceeds a reference voltage of
comparator (Vfb*1.08), another function becomes active and stops
the output pulse during exceeding the reference. (Fig.7)
When FB pin voltage decreases to 1.040 times of reference
voltage or lower, IC outputs pulses again.
100%
80%
VF voltage
FB電圧
Vfb*0.95 Vfb
(2.5V)
Fig.6 maximum on time at soft start
On time
100%
70%
VF Voltage
Vfb Vfb*1.04 Vfb*1.08
(2.5V)
Fig.7 ON time at overvoltage
Fuji Electric Co., Ltd.
AN-073E Rev.1.1
April-2011
http://www.fujielectric.co.jp/products/semiconductor/
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