an8022l Panasonic Corporation of North America, an8022l Datasheet - Page 12

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

AN8022L, AN8022SB
12
[2] Operation descriptions (continued)
5. Soft start (continued)
6. Timer latch
Application Notes (continued)
at the medium voltage of the triangular wave oscillation waveform as constant voltage. Therefore, at operation
starting, the V
oscillation waveform.
V
The soft start function works only up to that point and after that the normal control comes.
overcurrent protection is not sufficient for protection of the transformer, Fast Recovery Diode (FRD), Schottky
Diode in the secondary side and the power MOSFET. For this reason, the timer latch function is employed, which
stops the power supply by hitting the OVP, when the overcurrent condition continues for a certain period.
(at this time the pulse-by-pulse overcurrent protector is in the operating condition). The decrease of the power
supply output is detected as the decrease of current amount from the current feedback terminal of the normal
PWM control. When the decrease amount of this current exceeds a certain value, the comparator inside the IC
reverses to flow the constant current to the TIM/OVP terminal.
to rise the OVP terminal voltage. When the OVP operating threshold voltage (6 V typical) is reached, the OVP starts
operation to stop the IC and keeps this stop condition. (Refer to Figure 8.)
operation (hereinafter referred to as timer period) should be longer than the rise time of the power supply. Since
at operation start the IC is in the same condition as the overload or output short-circuit condition, if the timer
period is shorter, the power supply works latch and can not start.
the external capacitor connected to the TIM/OVP terminal. However, particular care should be taken, because too
large value of this capacitance may cause the breakdown of the power supply.
Timer period
SS
On the other hand, the V
As the result, the output of the IC generates the pulse of duty which gradually becomes large with the rise of
However, when the V
When the short-circuit or overload of the power supply output continues for a certain period, the pulse-by-pulse
The short-circuit or overload of the power supply output is monitored as the decrease of the power supply output
The external capacitor is connected to the TIM/OVP terminal. Electric charges are accumulated in this capacitor
The period from the time when an error of the power supply output is detected to the time when the OVP starts
Therefore, the IC is designed so that the timer period can be set to any desired value with capacitance value of
from the minimum duty. (Refer to figure 7.)
SS
is input to the PWM comparator as the lowest voltage and is compared with the triangular wave
V
V
V
V
V
CT
SS
FB
DTC
OUT
0 V
0 V
SS
FB
exceeds the V
has high voltage because the power supply output is low. And, the V
Figure 7. Soft start operation waveform
V
V
FB
CT
FB
or V
DTC
, the duty of the output pulse depends on the V
V
V
V
DTC
FB
SS
Voltage Regulators
DTC
is positioned
FB
or V
DTC
.