AN8022L 查看數據表（PDF） - Panasonic Corporation

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AN8022L

AC-DC switching power supply control IC

Panasonic Corporation 

AN8022L, AN8022SB

Voltage Regulators

s Application Notes (continued)

[2] Operation descriptions (continued)

2. Oscillation circuit

The PWM is an abbreviation of Pulse Width Modulation. In this IC, a smaller voltage between the voltage level

which is converted from the current input to IFB terminal and dead-time control level which is fixed internally is

compared with the internal triangular oscillation level through PWM comparator, and optimal duty is determined,

and then it is output via output driving stage.

• Triangular wave oscillation

The triangular waveform oscillation is performed through constant current charge/constant current

discharge to/from the external capacitor connected to the CT. The ratio of the charge current to the discharge

current is set inside, and the current value is determined by the external resistor connected to the RT terminal.

The RT terminal voltage is determined by the level which is a resistor-divided voltage of the internal

reference voltage (which is determined by Zener diode and VBE of NPN transistor, and temperature-

compensated). For this reason, the effect of fluctuation with temperature and dispersion is small. By the use

of a temperature-compensated external resistor, the effect of the fluctuation with temperature and dispersion

on the charge and discharge current value will be reduced further.

Moreover, since the upper/lower voltage level of the triangular wave oscillation is given by the resistor-

divider internal reference voltage, the effect of fluctuation with temperature and dispersion has been

suppressed.

Moreover, since the upper/lower voltage level of the triangular wave oscillation is given by the resistor-

division of internal reference voltage, the effect of fluctuation with temperature and dispersion has been

suppressed.

As described above, the sufficient consideration has been given to the effect of fluctuation with

temperature and dispersion in the design of the triangular wave oscillation frequency.

(Reference calculation of oscillation frequency)

5

fOSC = 6 × CT × RT [Hz]

3. Overvoltage protection circuit (OVP)

OVP is an abbreviation of Over Voltage Protection. It refers to a self-diagnosis function, which stops the power

supply to protect the load when the power supply output generates abnormal voltage higher than the normal

output voltage due to failure of the control system or an abnormal voltage applied from the outside (figure 2 and

figure 3).

Basically, it is set to monitor the voltage of supply voltage VCC terminal of the IC. Normally, the VCC voltage

is supplied from the transformer drive coil. Since this voltage is proportional to the secondary side output voltage,

it still operates even when the secondary side output has over voltage.

1) When the voltage input to the OVP terminal exceeds the threshold voltage (6.0 V typical) as the result of power

supply output abnormality, the protective circuit shuts down the internal reference voltage of the IC to stop all

of the controls and keeps this stop condition.

2) The OVP is released (reset) under the following conditions:

• Decreasing the supply voltage (VCC < 8.4 V typical: OVP release supply voltage)

The discharge circuit is incorporated so that the electric charge which is charged in the capacitor

connected to the OVP terminal can be discharged with the constant current of 5 µA (typical) for the next re-start.

Vth(OUT) =

Secondary side output voltage under normal operation VOUT

VCC terminal voltage under normal operation

× V7

V7 = Vth(OVP) + VZ

Vth(OUT): Secondary side output overvoltage threshold

Vth(OVP): OVP operation threshold

VZ : Zener voltage (external parts of OVP terminal)

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