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參數(shù)資料
| 型號(hào)： | AN8038S |
| 廠商： | PANASONIC CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | AC-DC switching power supply control IC with RCC local resonance circuit for improved conformance with energy conservation laws |
| 中文描述： | 2 A SWITCHING CONTROLLER, PDSO8 |
| 封裝： | 0.225 INCH, PLASTIC, SOP-8 |
| 文件頁數(shù)： | 17/22頁 |
| 文件大?。?/td> | 179K |
| 代理商： | AN8038S |
Voltage Regulators
AN8038, AN8038S
17
I
Application Notes (continued)
[2] Operation descriptions (continued)
4. Power supply output control system (IFB: feedback) (continued)
Soft start
When a power supply is started, it starts up in an overloaded state due to the capacitor connected to the power
supply output. Since the power supply output voltage is low in this state, the normal constant-voltage control
would rise the power supply to its maximum duty ratio. Although the pulse-by-pulse overcurrent protection
circuit (CLM) would limit the current, due to filter and other delays, it cannot decrease the pulse width to
zero, and thus large currents could flow in both the main switch (the power MOSFET) and the secondary side
diode. This could result in destruction of these components in the worst case. To prevent this, soft start is used
to suppress surge currents at power supply startup.
Soft start is installed by inserting R3 and C4 between the IFB pin (pin 4) and the GND pin (pin 6) as shown
in figure 8. When the IC supply voltage reaches the startup voltage, and the start circuit operates, an open bias
(about 6.4 V) is output to the IFB pin. A charging current (I
IFB
) flows from the IFB pin into C4 due to this
voltage. As a result, since startup begins at relatively high VFB, output control is started from short t
ON
. Since
the voltage across C4 rises according to the time constant determined by R3 and C4, I
IFB
becomes smaller with
time, and the t
ON
time increases gradually.
Due to the above operation, the current that flows in the power MOSFET at power-on increases gradually.
As a result, surge currents are suppressed.
However, this reduces the transient response of the feedback loop, so care is required in designing this
circuit.
G
V
CC
IFB
Out
CLM
C8
Startup resistor
R1
Rectified AC
Figure 8
To AC(
)
PC1
R3
C4
R8
R7
C6
5. Output block
This IC adopts a totem pole (push-pull) structure output circuit in which NPN transistors as shown in figure 9
sinks and sources current to rapidly drive the power MOSFET which is a capacitive load.
This circuit provides maximum sink and source currents of
0.1 A and
+
0.2 A (DC), and peak currents of
1 A
and
+
2 A. Furthermore, this circuit has a sink capability of 1 mA (typical) even when the supply voltage has fallen
under the stop voltage, and thus can turn off the power MOSFET reliably.
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