an8021l Panasonic Corporation of North America, an8021l Datasheet - Page 13

• Manufacturer Part Number: an8021l
• Description: Ac-dc Switching Power Supply Control Ic
• Manufacturer: Panasonic Corporation of North America

Voltage Regulators
[2] Operation descriptions (continued)
6. Timer latch
Application Notes (continued)
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 and the constant current flows 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
Power supply
output voltage
Power MOSFET
current
TIM/OVP
terminal
voltage
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
V
OVP
V
I
DS
O
0
0
0
Figure 8. Timer latch basic operation
AN8021L, AN8021SB
Power supply stop
Power supply stop
Power supply stop
Time
Time
Time
OVP
V
TH
6 V (typ.)
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