7

Voltage Regulators

AN8027, AN8037

s Application Notes

[1] Operation descriptions

1. Start/stop circuit block

• Startup mechanism

After the AC voltage is applied and the

supply voltage due to the current in the startup

resistor reaches the startup voltage and the IC

begins to operate, drive of the power MOSFET

begins. This causes a bias in the transformer, and

the supply voltage is provided to the IC from

the bias winding. (This is point a in figure 1.)

During the period between the point when the

startup voltage is reached, and the point when

the bias winding can generate a voltage enough

to supply the IC, the IC supply voltage is pro-

Since the supply voltage falls during this period

(area b in figure 1), if the supply voltage falls

below the IC stop-voltage before an adequate

supply voltage can be provided by the bias wind-

ing, it will not be possible to start the power sup-

ply. (This is the state at point c in figure 1.)

• Functions

voltage (14.9 V typical), and stops operation when the voltage falls below the stop voltage (8.6 V typical). Since

a large voltage difference (6.3 V typical) is taken between the start and stop voltages, it is easy to select values for

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 400

µA.

2. Oscillation circuit

to the CF pin (pin 3) to determine the switching timing of the power MOSFET.

The IC is in constant voltage control by changing the on-period of the power MOSFET without making off-

period change while the IC is in normal (RCC continuous) operation mode. At that time, the on-period is con-

trolled by directly changing the output pulse width of the oscillation circuit, and the maximum on-period can be

adjusted with input voltage compensation by detecting the input voltage with the flow of current from the TR pin

(TR source current). Refer to figure 2. When the IC is in standby mode (for light loads), the stable, efficient

a decrease in frequency. Refer to figure 3.

The following provides information on how to set on- and off-period.

• Setting the on-period

An approximate on-period of the power MOSFET is obtained from the following formula. Refer to figure 2.

GND

C8

b

Figure 1

Startup failure

Startup state

Voltage supplied

from bias winding

Startup

Standby

Startup

voltage

Stop

voltage

a

c

Rectified AC

Startup resistor

R1