an8022l Panasonic Corporation of North America, an8022l Datasheet - Page 11

• Manufacturer Part Number: an8022l
• Description: Ac-dc Switching Power Supply Control Ic
• Manufacturer: Panasonic Corporation of North America

Voltage Regulators
[2] Operation descriptions (continued)
4. Overcurrent protection circuit (OVP) (continued)
5. Soft start
Application Notes (continued)
CLM ( )
Terminal
voltage
V
Terminal
voltage
Power
MOSFET
current
OUT
rise under overload condition. Under this condition, the power supply output is low. For the normal PWM control,
attempt is made to limit the current by the pulse-by-pulse over current protection so that the power supply output
could rise at maximum duty. However, pulses can not be made down to zero due to circuit delay. As a result, large
current flows in the mains switch (the power MOSFET) or in the diode in the secondary side, and in the worst case
these parts are damaged.
with gradually widening duty from the minimum one (0%) at the power supply start is adopted.
life of parts and raise the reliability of the power supply.
PWM, three voltages are input: the voltage to which the current feedback level is converted (hereinafter referred
to as V
inside the IC), and the triangular wave oscillation voltage (hereinafter referred to as V
input in the non-reverse input ( ) of the PWM comparator and V
signals of the non-reverse input, the lowest one is selected for input to the PWM comparator.
this capacitor is set to be sufficiently discharged by the transistor inside the IC.
the constant current source inside the IC. Therefore V
At start of the power supply, the capacitor connected to the power supply output causes the power supply to
For this reason, soft start function in which the power supply output does not rise with maximum duty but rise
The use of this function requires more rise time of power supply output. However, it can extend the service
The soft start (SS) terminal is connected to the PWM input (hereinafter its voltage is referred to as V
The external capacitor (hereinafter referred to as C
When the supply voltage exceeds the start voltage to start the IC operation, charging is started in the C
0
0
0
FB
), the voltage determining the maximum duty (hereinafter referred to as V
V
Figure 6. Pulse-by-pulse overcurrent detector operation waveform
TH
( 200 mV typ.)
SS
SS
) is connected to the SS terminal. In the pre-start condition,
gradually rises from 0 V.
Overshoot
due to
delay
CT
is input in the reverse input ( ). Among the three
Pulse width can not be made
shorter than this width due to delay
AN8022L, AN8022SB
DTC
). This voltage is determined
CT
). V
SS
, V
Time
Time
Time
FB
and V
SS
). In the
DTC
SS
are
11
by