an8027 Panasonic Corporation of North America, an8027 Datasheet - Page 14

• Manufacturer Part Number: an8027
• Description: Ac-dc Switching Power Supply Control Ic With Standby Mode
• Manufacturer: Panasonic Corporation of North America

AN8027, AN8037
14
[1] Operation descriptions (continued)
Application Notes (continued)
Soft start (continued)
FB pin (Pin 9) and GND pin (Pin 5) so that the switching
regulator will be in soft start operation. When the voltage
supplied to the IC reaches the start voltage and the opera-
tion of the startup circuit begins, an open bias voltage of
approximately 6.4 V is output to the FB pin. As a result,
the charge current I
the switching regulator starts up at high V
forming output control while the T
voltage difference between both edges of C4 rises accord-
ing to the time constant determined by R3 and C4, thus
decreasing I
T
the power MOSFET gradually after the switching regula-
tor is turned on, thus suppressing the inrush current. How-
ever, this reduces the transient response of the feedback
loop, so care is required in designing this circuit.
ON
(Pin 6)
(Pin 3)
V
As shown in figure 6, connect R3 and C4 between the
The above operation increases the flow of current to
Notes on selecting R3
low resistance value for R3 is selected, the slope of the triangular wave on the CF pin immediately after the
startup may become extremely steep, leading to malfunction of the oscillation circuit. Accordingly, we rec-
ommend that you adjust R3 so that the drive pulse width t
(Refer to figure 7.)
pin (Pin 3). Figure 8 indicates recommended values of R3 corresponding to the capacitance value C
OUT
CF
period gradually.
The oscillation circuit of this IC is designed under the condition that the T
Note that the drive pulse width t
(In actual mounting conditions, the correlation shown in figure 8 may deviate slightly.)
FB
Start
with the lapse of time and increasing the
FB
flows into C4 from the FB pin. Then
t
1
Figure 7
ON
period is short. The
1
depends on the capacitance value of the C
FB
, thus per-
1
Rectified AC
immediately after the startup is 0.5 ms or greater.
AN8027, AN8037
3 000
2 000
1 000
To AC
0
10
ON
GND
CLM
Startup resistor
V
12
Out
period is definite. If excessively
FB
CC
CF
Voltage Regulators
that is connected to the CF
R3
C4
14
Figure 6
R8
Figure 8
R3 (k )
R1
16
C6
C8
PC1
18
V
C4 0.033 F
CC
R7
CF
20
16 V
.
22