FA5502 데이터 시트보기 (PDF) - Fuji Electric

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FA5502

Power Supply Control IC

Fuji Electric 

FA5502P/M

8. Description of each circuit

(1) Oscillator section

The oscillator generates sawtooth waveform

between 0.3V and 3.4V by charging and discharging

capacitor. Fig.1 shows the connection. The oscillation

frequency is determined by CT and RT.

(see characteristics curve).

The oscillator waveform is input to the PWM

comparator. The oscillator is also used for determining

the maximum duty cycle of output pulses. Concretely,

a signal is sent to the output circuit section and the

OUT pin is forced to be Low level during the CT

discharge period (fall time of CT pin voltage).

REF

13

3.4V

RT

CT

15

OSC

CT

0

0.3V

Dmax

t

Dead time

Fig.1 Oscillator circuit

SYNC pin (pin 14) is a synchronizing signal input pin.

It is usable for synchronized operation. When it is

desired to adopt synchronized operation, the

free-running frequency (determined by CT and RT)

must be set about 10% lower than that of external

synchronizing signal.

The input resistance in SYNC pin is approximately

16kΩ. Usually, a square-wave synchronizing signal is

differentiated by R and C, and the voltage input to

SYNC pin is so arranged to be below 1V within CT

discharge period. Concretely, the waveform must

satisfy the condition in Fig.3.

Depending on the amplitude, etc. of square-wave

signal used as external synchronizing signal, the RC

differentiating circuit shown in Fig.2 could not generate

a waveform in Fig.3. In such a case, add a resistor

between SYNC pin and GND so as to clear the

condition in Fig.3.

Fig.5 shows timing chart of synchronized operation.

Note that diode D2 in Fig.2 is required so that no

negative voltage will be applied to SYNC pin while in

the discharge period of capacitor Csy in the

differentiating circuit. Considering the rated voltage of

SYNC pin, use a Schottky diode of a low forward

voltage.

Unless the external synchronization function is used,

connect SYNC pin to GND pin to avoid a malfunction.

Quality is our message

Rsy Csy

SYNC

14

synchronizing

signal

SYNC pin

signal

16k

D2

Fig.2 SYNC pin circuit (1)

OSC

VSYNC

=1.5V(typ)

less SYNC pin voltage waveform

than

5V

2V

more than 50ns

1V

less than 200ns

0

t

Fig.3 Condition for SYNC pin signal

Rsy Csy

synchronizing R14

signal

SYNC

14

16k

D2

Fig.4 SYNC pin circuit (2)

OSC

CT pin voltage

t

Synchronizing

signal

t

SYNC pin

voltage

t

OUT pin

voltage

t

Fig.5 Timing chart of synchronized operation

13

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