an8021l Panasonic Corporation of North America, an8021l Datasheet - Page 11

• Manufacturer Part Number: an8021l
• 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)
Application Notes (continued)
in the primary side (power MOSFET). Taking advantage of the above fact, by regulating the upper limit of the pulse
current flowing in the main switch, the circuit protects the parts which are easily damaged by the overcurrent.
ends of the low resistance, which is connected between the source of power MOSFET and the power supply
GND. When the power MOSFET is turned on and the threshold voltage of CLM (current limit) is detected, the
overcurrent protection circuit controls so that current can not flow further by turning off the output to turn off the
power MOSFET. The threshold voltage of CLM is approximately 200 mV (typical) under T
to GND of the IC. This control is repeated for each cycle. Once the overcurrent is detected, the off condition is kept
during that cycle, and it can not be turned on until the next cycle. The overcurrent detection method described above
is called pulse-by-pulse overcurrent detection. (Refer to figure 6.)
functions to remove the noise generated by the parasitic capacitance
which is equivalently formed at turning-on of the power MOSFET.
protection. Therefore, if this detection level fluctuates with temperature or dispersion, the operating current level
of the power supply overcurrent protection also fluctuates. Since such level fluctuation increases the necessity of
withstand capability for the parts to be used and in the worst case it means the cause of destruction, the accuracy
of detection level is raised as much as possible for these ICs, the AN8021L and AN8021SB.
Notes on the detection level precision
V
Terminal
voltage
Power
MOSFET
current
The overcurrent of the power supply output is proportional to the value of current flowing in the main switch
For the current flowing in the main switch, the current detection is achieved by monitoring the voltage in both
The R4, R5 and C3 in figure 5 construct the filter circuit, which
This overcurrent detection level is reflected on the operating current level of the power supply overcurrent
CLM ( )
Terminal
voltage
OUT
0
0
V
Figure 6. Pulse-by-pulse overcurrent detector operation waveform
TH
( 200 mV typ.)
Overshoot
due to
delay
GND
CLM
AN8021L, AN8021SB
Pulse width can not be
made shorter than this
width due to delay
R4
Figure
a
25 C with respect
5
C3
R5
Time
Time
Time
R3
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