FA5527 FUJI [Fuji Electric], FA5527 Datasheet - Page 16

• Manufacturer Part Number: FA5527
• Description: Fuji Switching Power Control IC
• Manufacturer: FUJI [Fuji Electric]

(2) Oscillator
(3) Current comparator and PWM latch
steady operation at heavy load, the oscillating frequency
is set at 130kHz for FA5526/36, 100kHz for FA5527/37
or 60kHz for FA5528/38 inside the IC.
decrease oscillating frequency at light load to reduce
standby power dissipation. When FB pin voltage
becomes 1.05V or less at light load, the frequency starts
decreasing.
decreases almost linearly to the minimum operating
frequency (Fig.4). The minimum operating frequency,
Fmin, is set at 1.1kHz.
determining the switching frequency, a pulse signal for
determining the maximum duty cycle and a ramp signal
for slope compensation.
comparators. Fig.5 shows a block diagram for basic
operation and Fig.6 a timing chart.
to the PWM latch (F.F.) as a set signal through a blanking
circuit, increasing PMW latch output and also OUT pin
voltage.
monitors a MOSFET current and generates a reset
signal when OUT pin voltage reaches the threshold
voltage. Then, PWM latch (F.F.) output and OUT pin
voltage go into low state
threshold voltage due to a feedback signal.
are level-shifted and input to the current comparator (IS
comp.) as threshold voltage. In addition, the reference
voltage of 0.52V is input to the IC to determine IS pin
maximum threshold voltage.
AN-087E Rev.1.0
April-2011
On the other hand, the current comparator (IS comp.)
The oscillator determines switching frequency. For
In addition, the IC has a function to automatically
At light load, as FB pin voltage drops, the frequency
The
FA5526/27/28/36/37/38
A trigger signal is generated by the oscillator and input
The output is controlled through varying IS comparator
As shown in Fig.7, FB pin voltage and CS pin voltage
The lowest of the three inputs is given a high priority.
Fuji Electric Co., Ltd.
oscillator
generates
have
a
trigger
FA5526/5527/5528/5536/5537/5538
current
signal
mode
for
( OUT pin signal )
Flip Flop ( F.F. )
Drain Current of
MOSFET “Q1”
proportional to
IS pin Voltage
Fig. 6 Timing chart for current-mode basic operation
“Q” output
Output Signal
Fig.5 Current-mode basic operation circuit block
(reset pulse)
(set pulse)
16
IS comp.
Blanking
Output
OSC
Fig.4 Oscillating frequency
3
IS comp.
Blanking
IS
http://www.fujielectric.co.jp/products/semiconductor/
R
S
F.F.
Q
IS comp.
Minimum Voltage
Among inverting
input
C1
5
OUT
Rs