AN8021L データシートの表示（PDF） - Panasonic Corporation

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AN8021L

AC-DC switching power supply control IC

Panasonic Corporation 

AN8021L, AN8021SB

Voltage Regulators

s Application Notes (continued)

[2] Operation descriptions

1. Start/stop circuit block

After AC rectification

Start resistance

R1

• Start mechanism

When AC voltage is applied and the sup-

ply voltage reaches the start voltage through

the current from the start resistor, the IC starts

operation. Then the power MOSFET driving

VCC

VOUT

C1

GND

starts. Thereby, bias is generated in the trans-

former and the supply voltage is given from the

Before start

Start

bias coil to the IC. (This is point a in figure 1.)

During the period from the time when the

Start voltage

a

Voltage supplied

from bias coil

start voltage is reached and the voltage is gen-

erated in the bias coil to the time when the IC

is provided with a sufficient supply voltage,

Stop voltage

Start condition

b

c Start failure

the supply voltage of the IC is supplied by the

capacitor (C1) connected to VCC .

Figure 1

Since the supply voltage continuously decreases during the above period (area b in figure 1), the power supply

is not able to start (state c in figure 1), if the stop voltage of the IC is reached before the sufficient supply voltage

is supplied from the bias coil.

• Function

The start/stop circuit block is provided with the function to monitor the VCC voltage, and to start the operation

of IC when VCC voltage reaches the start voltage (14.2 V typical), and to stop when it decreases under the stop

voltage (9.2 V typical). A large voltage difference is set between start and stop (5.0 V typical), so that it is easier

to select the start resistor and the capacitor to be connected to VCC .

Note) To start up the IC operation, the startup current which is a pre-start current plus a circuit drive current is necessary.

Set the resistance value so as to supply a startup current of 450 µA.

2. Oscillation circuit

The PWM is an abbreviation of pulse width modulation. In this IC, the 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-

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 tempera-

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

(Reference calculation of oscillation frequency)

5

fOSC = 6 × CT × RT [Hz]

8

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