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AN8028

Self-excited RCC pseudo-resonance type AC-DC switching power supply control IC

Panasonic Corporation 

Voltage Regulators

s Terminal Equivalent Circuits (continued)

Pin No.

Equivalent circuit

8

VCC

7.3 V

Comp.

5 µA

8

9

VCC

TON

9

AN8028

Description

I/O

TIM/OVP:

I

Terminal for use both OVP (overvoltage protec-

tion circuit) and timer latch.

[OVP]

When overvoltage signal of the power supply is

detected and high is inputted to the terminal, it

turns off the internal circuit. At the same time, it

holds that condition (latch). To reset the OVP

latch, the VCC should be decreased to a voltage

lower than the release voltage.

[Timer latch]

It detects the output voltage fall due to the

overcurrent condition of the power supply out-

put through the current level inputted to IFB.

When the IIFB decreases under the current of

certain value, the charge current flows in the

capacitor which is connected to this terminal.

Then, when the capacitor is charged up to the

threshold voltage of the OVP, the OVP works so

that the IC could keep the operation stop condi-

tion.

FB:

I

Terminal for connecting the photocoupler for

error voltage feedback of the power supply out-

put.

It is possible to cancel about 180 µA of the dark

current of photocoupler.

s Application Notes

[1] Operation descriptions

1. Start/stop circuit block

• Start mechanism

When AC voltage is applied and the supply

voltage reaches the start voltage through the

current from start resistor, the IC starts opera-

tion. Then the power MOSFET driving starts.

Thereby, bias is generated in the transformer

and the supply voltage is given from the bias

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

During the period from the time when the

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,

the supply voltage of the IC is supplied by the

capacitor (C8) connected to VCC .

After AC rectification

Start resistance

R1

VCC

C8

VOUT

GND

Before start

Start

a

voltage

Stop

voltage

b

Figure 1

Start

Voltage supplied

from bias coil

Start condition

c Start failure

5

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