FA5502P/M
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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).
OSC
Rsy
Csy
SYNC
14
synchronizing
signal
16k
V
SYNC
=1.5V(typ)
D2
SYNC pin
signal
Fig.2 SYNC pin circuit (1)
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).
SYNC pin voltage waveform
less
than
5V
REF
13
3.4V
2V
1V
more than 50ns
less than 200ns
RT
CT
CT
15
OSC
0.3V
0
t
Dmax
Dead time
Fig.1 Oscillator circuit
0
t
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.
Fig.3 Condition for SYNC pin signal
OSC
Rsy Csy
SYNC
14
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.
synchronizing
R14
16k
signal
D2
Fig.4 SYNC pin circuit (2)
CT pin voltage
t
t
t
t
Synchronizing
signal
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.
SYNC pin
voltage
OUT pin
voltage
Unless the external synchronization function is used,
connect SYNC pin to GND pin to avoid a malfunction.
Fig.5 Timing chart of synchronized operation
13
FUJI [ FUJI ELECTRIC ]
