FA5502PSLASHM datasheet(13/23 Pages) FUJI

13 / 23 page

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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

OSC

3.4V

0.3V

Dmax

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

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

16k

Ω. 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.

SYNC

OSC

16k

synchronizing

signal

Csy

Rsy

SYNC pin

signal

VSYNC

=1.5V(typ)

Fig.2 SYNC pin circuit (1)

less than 200ns

more than 50ns

less

than

SYNC pin voltage waveform

Fig.3 Condition for SYNC pin signal

SYNC

OSC

16k

synchronizing

signal

Rsy

Csy

R14

Fig.4 SYNC pin circuit (2)

CT pin voltage

Synchronizing

signal

SYNC pin

voltage

OUT pin

voltage

Fig.5 Timing chart of synchronized operation

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