an8038 Panasonic Corporation of North America, an8038 Datasheet - Page 17

• Manufacturer Part Number: an8038
• Description: Ac-dc Switching Power Supply Control Ic With Rcc Local Resonance Circuit For Improved Conformance With Energy Conservation Laws
• Manufacturer: Panasonic Corporation of North America

Voltage Regulators
[2] Operation descriptions (continued)
4. Power supply output control system (IFB: feedback) (continued)
5. Output block
Application Notes (continued)
sinks and sources current to rapidly drive the power MOSFET which is a capacitive load.
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.
Soft start
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.
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
voltage. As a result, since startup begins at relatively high VFB, output control is started from short t
the voltage across C4 rises according to the time constant determined by R3 and C4, I
time, and the t
As a result, surge currents are suppressed.
circuit.
This IC adopts a totem pole (push-pull) structure output circuit in which NPN transistors as shown in figure 9
This circuit provides maximum sink and source currents of 0.1 A and 0.2 A (DC), and peak currents of 1 A
When a power supply is started, it starts up in an overloaded state due to the capacitor connected to the power
Soft start is installed by inserting R3 and C4 between the IFB pin (pin 4) and the GND pin (pin 6) as shown
Due to the above operation, the current that flows in the power MOSFET at power-on increases gradually.
However, this reduces the transient response of the feedback loop, so care is required in designing this
ON
time increases gradually.
To AC( )
Rectified AC
CLM
V
IFB
Out
CC
Figure 8
R3
C4
R8
C8
C6
PC1
Startup resistor
R1
IFB
) flows from the IFB pin into C4 due to this
R7
AN8038, AN8038S
IFB
becomes smaller with
ON
. Since
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