FIP4F8S datasheet(16/33 Pages) FUJI

16 / 33 page

AN-087E Rev.1.0

April-2011

FA5526/5527/5528/5536/5537/5538

Fuji Electric Co., Ltd.

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

(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.

The

oscillator

generates

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

The output is controlled through varying IS comparator

threshold voltage due to a feedback 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.

Fig.4

Oscillating frequency

OSC

Blanking

IS comp.

F.F.

OUT

Fig.5

Current-mode basic operation circuit block

Blanking

Output Signal

(set pulse)

Flip Flop ( F.F. )

“Q” output

( OUT pin signal )

IS comp.

Output

(reset pulse)

IS pin Voltage

proportional to

Drain Current of

MOSFET “Q1”

IS comp.

Minimum Voltage

Among inverting

input

Fig. 6

Timing chart for current-mode basic operation

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