AN8021L 查看數據表（PDF） - Panasonic Corporation
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AN8021L Datasheet PDF : 16 Pages
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AN8021L, AN8021SB
Voltage Regulators
s Application Notes (continued)
[2] Operation descriptions (continued)
5. Soft start
At start of the power supply, the capacitor connected to the power supply output causes the power supply to
rise under overload condition. Under this condition, the power supply output is low. For the normal PWM control,
attempt is made to limit the current by the pulse-by-pulse overcurrent protection so that the power supply output
can rise at maximum duty. However, pulses can not be made down to zero due to circuit delay. As a result, large
current flows in the mains switch (the power MOSFET) or in the diode in the secondary side, and in the worst case
these parts are damaged.
For this reason, soft start function in which the power supply output does not rise with maximum duty but
rises with gradually widening duty from the minimum one (0%) at the power supply start is adopted.
The use of this function requires more rise time of power supply output. However, it can extend the service
life of parts and raise the reliability of the power supply.
The soft start (SS) terminal is connected to the PWM input (hereinafter its voltage is referred to as VSS). In the
PWM, three voltages are input; the voltage to which the current feedback level is converted (hereinafter referred
to as VFB), the voltage determining the maximum duty (hereinafter referred to as VDTC). This voltage is determined
inside the IC), and the triangular wave oscillation voltage (hereinafter referred to as VCT). VSS , VFB and VDTC are
input in the non-reverse input (+) of the PWM comparator and VCT is input in the reverse input (−). Among the three
signals of the non-reverse input, the lowest one is selected for input to the PWM comparator.
The external capacitor (hereinafter referred to as CSS) is connected to the SS terminal. In the pre-start condition,
this capacitor is set to be sufficiently discharged by the transistor inside the IC.
When the supply voltage exceeds the start voltage to start the IC operation, charging is started in the CSS by
the constant current source inside the IC. Therefore VSS gradually rises from 0 V.
On the other hand, the VFB has high voltage because the power supply output is low. And, the VDTC is positioned
at the medium voltage of the triangular wave oscillation waveform as constant voltage. Therefore, at operation
starting, the VSS is input to the PWM comparator as the lowest voltage and is compared with the triangular wave
oscillation waveform.
As the result, the output of the IC generates the pulse of duty which gradually becomes large with the rise of
VSS from the minimum duty. (Refer to figure 7.)
However, when the VSS exceeds the VFB or VDTC , the duty of the output pulse depends on the VFB or VDTC .
The soft start function works only up to that point and after that the normal control comes.
VFB
VCT
VCT
VSS
VFB
VDTC
0V
VSS
VDTC
VFB
VOUT
0V
Figure 7. Soft start operation waveform
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