AN8021L 查看數據表（PDF） - Panasonic Corporation
零件编号
产品描述 (功能)
生产厂家
AN8021L Datasheet PDF : 16 Pages
| |||
Voltage Regulators
AN8021L, AN8021SB
s Application Notes (continued)
[2] Operation descriptions (continued)
4. Overcurrent protection circuit (OVP)
The overcurrent of the power supply output is proportional to the value of current flowing in the main switch
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.
For the current flowing in the main switch, the current detection is achieved by monitoring the voltage in both
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 Ta = 25°C with respect
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.)
The R4, R5 and C3 in figure 5 construct the filter circuit, which
functions to remove the noise generated by the parasitic capacitance
which is equivalently formed at turning-on of the power MOSFET.
GND R4
R5
CLM
C3
R3
• Notes on the detection level precision
Figure 5
This overcurrent detection level is reflected on the operating current level of the power supply overcurrent
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.
CLM (−)
Terminal
voltage
VTH (−200 mV typ.)
Overshoot
due to
delay
Time
Pulse width can not be
made shorter than this
width due to delay
VOUT
Terminal
voltage
0
Time
Power
MOSFET
current
0
Figure 6. Pulse-by-pulse overcurrent detector operation waveform
Time
11
Share Link: 