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參數資料
| 型號： | FA5305APS |
| 廠商： | FUJI ELECTRIC HOLDINGS CO., LTD. |
| 元件分類： | 基準電壓源/電流源 |
| 英文描述： | Bipolar IC For Switching Power Supply Control |
| 中文描述： | 雙極集成電路開關電源控制 |
| 文件頁數： | 5/17頁 |
| 文件大小： | 280K |
| 代理商： | FA5305APS |
5
FA5304AP(S)/FA5305AP(S)
Fig. 1 Configuration with error amplifier
Fig. 2 Configuration with optocoupler (FB pin input)
Fig. 3 PWM comparator
Fig. 4 PWM comparator timing chart
I
Description of each circuit
1. Oscillator
(See block diagram on page 8.)
The oscillator generates a triangular waveform by charging
and discharging a capacitor. CT pin voltage oscillates
between an upper limit of approx. 3.0V and a lower limit of
approx. 1.0V. The oscillation frequency is determined by a
external capacitance C
T
connected to CT pin, and
approximately given by the following equation:
..................(1)
The recommended oscillation range is between 5k and
600kHz.
The oscillator output is connected to a PWM comparator.
2. Feedback circuit
Figure 1 gives an example of connection in which built-in error
amplifier is used to couple the feedback signal to IN(-) pin. Let n
2
be the number of turns of secondary winding L
2
and n
3
be the
number of turns of tertiary winding L
3
. V
CC
and Vout are given by
Vcc= 2(V)(R
1
+R
2
)/R
2
....................................(2)
V
OUT
Q
(n
2
/n
3
)(Vcc+V
D3
)–V
D2
........................(3)
(where V
D2
and V
D3
are the forward voltage drops across diodes D
2
and D
3
respectively).
Here, the following equation must be satisfied to prevent from
the malfunction of OUT pin at shutdown.
(R1R2)/ (R1+R2)
L
11k
...............................(4)
Figure 2 gives an example of connection in which an
optocoupler is used to couple the feedback signal to the FB
pin. If this circuit causes power supply instability, the frequency
gain can be decreased by connecting R
4
and C
4
as shown in
figure 2. R
4
should be between several tens of ohms to
several kiloohms and C
4
should be between several thousand
picofarads to one microfarads.
3. PWM comparator
The PWM comparator has four inputs as shown in Figure 3.
Oscillator output
x
is compared with CS pin voltage
, FB pin
, and DT voltage
{
. The lowest of three inputs
,
, and
{
is compared with output
x
. If it is lower than the oscillator
output, the PWM comparator output is high, and if it is higher
than the oscillator output, the PWM comparator output is low
(see Fig. 4).
The IC output voltage is high during when the comparator
output is low, and the IC output voltage is low during when the
comparator output is high.
When the IC is powered up, CS pin voltage
controls soft
start operation. The output pulse then begins to widen
gradually. During normal operation, the output pulse width is
determined within the maximum duty cycle (FA5304A,
FA5305A: 45%) set by DT voltage
{
under the condition set
by feedback signal
, to stabilize the output voltage.
C
T
(pF)
f (kH
Z
) =
4.8 10
4
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