參數(shù)資料
型號: FA5502
廠商: FUJI ELECTRIC HOLDINGS CO., LTD.
英文描述: POWER SUPPLY CONTROL IC
中文描述: 電源控制IC
文件頁數(shù): 13/23頁
文件大?。?/td> 215K
代理商: FA5502
8. Description of each circuit
(1) Oscillator section
The
oscillator
between 0.3V and 3.4V by charging and discharging
capacitor. Fig.1 shows the connection. The oscillation
frequency is determined by CT and R
T
.
(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 C
T
discharge period (fall time of CT pin voltage).
13
Quality is our message
FA5502P/M
generates
sawtooth
waveform
13
15
REF
CT
OSC
R
T
C
T
t
3.4V
0.3V
C
0
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
to
adopt
synchronized
free-running frequency (determined by C
T
and R
T
)
must be set about 10% lower than that of external
synchronizing signal.
The input resistance in SYNC pin is approximately
16k
. 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.
operation,
the
14
SYNC
OSC
16k
synchronizing
signal
D2
Csy
Rsy
SYNC pin
signal
V
SYNC
=1.5V(typ)
Fig.2 SYNC pin circuit (1)
less than 200ns
more than 50ns
2V
1V
less
than
5V
t
S
0
SYNC pin voltage waveform
Fig.3 Condition for SYNC pin signal
14
SYNC
OSC
16k
synchronizing
signal
Rsy
Csy
R14
D2
Fig.4 SYNC pin circuit (2)
CT pin voltage
Synchronizing
signal
SYNC pin
voltage
OUT pin
voltage
t
t
t
t
Fig.5 Timing chart of synchronized operation
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