AN8014 Ver la hoja de datos (PDF) - Panasonic Corporation
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AN8014 Datasheet PDF : 18 Pages
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Voltage regulators
AN8014S
s Application Notes (continued)
[1] Function descriptions (continued)
8. Overcurrent protection block
Utilizing that the overcurrent of power output is proportional to the current value which flows in the main switch
(power MOSFET), the block regulates the upper limit of the current flowing in the main switch, thus protects the
parts such as main switch device, a flywheel diode and a choke coil from the damage caused by the overcurrent.
The current detection are done by monitoring, at CLM pin (pin 10), the voltage drop in resistor which is placed
between the main switch device and VCC pin.
When the main switch device (power MOSFET) is switched on and the voltage of CLM pin reaches "VCC −
95 mV", threshold level for overcurrent detection, the output drive transistor is cut off so that no more current
flows in the main switch device. This control is repeated at each cycle. When overcurrent is detected once, the
transistor remains off during the same cycle, and is switched on in the next cycle.
Such an overcurrent detection method is called "Pulse-by-pulse overcurrent detection."
(3) Output Off
Triangular wave (CT)
Error amplifier output (FB)
(5) Turned on in the next cycle
1.32 V
0.44 V
Output waveform (Out)
High
Low
Overcurrent protection input (CLM)
(1) Overcurrent detection
(2) Latch set
Latch circuit set signal
tDLY : Delay time
Latch circuit reset signal
(4) Latch reset
Figure 5. Waveforms of the pulse-by-pulse overcurrent protection operation
VCC
VCC − 95 mV
High
Low
High
Low
R2 and C1 shown in figure 6 constitute a low-pass
filter to eliminate noise due to parasitic capacitance when
the power MOSFET is turned on.
The cut-off frequency of the filter is obtained from
the following.
1
fC = 2πC1R2 [Hz]
C1
R2
In
Out
R1
V-Out CLM
Figure 6. CLM noise filter circuit
11
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