FA5528 데이터 시트보기 (PDF) - Fuji Electric

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FA5528

(FA5526 thru FA5538 Series) Fuji Switching Power Control IC

Fuji Electric 

FA5526/5527/5528/5536/5537/5538

(2) Oscillator

The oscillator determines switching frequency. For

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.

In addition, the IC has a function to automatically

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.

At light load, as FB pin voltage drops, the frequency

decreases almost linearly to the minimum operating

frequency (Fig.4). The minimum operating frequency,

Fmin, is set at 1.1kHz.

Fig.4 Oscillating frequency

The oscillator generates a trigger signal for

determining the switching frequency, a pulse signal for

determining the maximum duty cycle and a ramp signal

for slope compensation.

(3) Current comparator and PWM latch

FA5526/27/28/36/37/38 have current mode

comparators. Fig.5 shows a block diagram for basic

operation and Fig.6 a timing chart.

A trigger signal is generated by the oscillator and input

to the PWM latch (F.F.) as a set signal through a blanking

circuit, increasing PMW latch output and also OUT pin

voltage.

On the other hand, the current comparator (IS comp.)

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

C1

OSC Blanking

Rs

IS comp.

SQ

R

F.F.

5

OUT

3

IS

Fig.5 Current-mode basic operation circuit block

Blanking

Output Signal

(set pulse)

The output is controlled through varying IS comparator

threshold voltage due to a feedback signal.

Flip Flop ( F.F. )

“Q” output

( OUT pin signal )

As shown in Fig.7, FB pin voltage and CS pin voltage

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.

The lowest of the three inputs is given a high priority.

IS pin Voltage

proportional to

Drain Current of

MOSFET “Q1”

IS comp.

Output

(reset pulse)

IS comp.

Minimum Voltage

Among inverting

input

Fuji Electric Co., Ltd.

AN-087E Rev.1.0

April-2011

Fig. 6 Timing chart for current-mode basic operation

16

http://www.fujielectric.co.jp/products/semiconductor/

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