10

FA531X series

(2) Startup circuit connected to rectified line

This method is not suitable for FA531X, especially concerned

with re-startup operation just after power-off or startup which

AC input voltage goes up slowly. Fig. 21 shows a startup

directly.

following equations.

dVcc/dt(V/s)=

1.8/(Cs(

µF))..........................................(8)

2 • (AC input effective voltage)

This operation is repeated several times.

voltage is equal to Vccon (=ON threshold) because capacitor

is out of the condition required by equation (4). After that, re-

startup by power-on can not be guaranteed even when

equation (8) is satisfied.

The image of that the startup is impossible is shown in Fig. 22.

It is necessary to startup IC that supply current Icc(startup) to

VCC is over 4mA in the condition of Tj < 100

°C during Vcc is

kept at Vccon( 16V, balance state at Vccon after the repeated

operation.

Icc(start-up) > 4mA

at Vcc=Vccon, Tj<100

°C, after power-off

This balance state that startup is impossible tends to occur at

higher temperature. If power-on is done when Vcc is not kept

at Vccon (for example:power-off is done and after enough time

that C1 is discharged until Vcc can not be pulled up to Vccon),

the IC can startup in the condition given by equation(8).

In some cases, such as when the load current of power supply

is changed rapidly, you may want to prolong the hold time of

the power supply output by means of maintaining Vcc over the

off threshold.

For this purpose, connect diode D4 and electrolytic capacitor

C4 as shown in Fig. 23. This prolongs the hold time of the

power supply voltage Vcc regardless of the period from power-

on to startup.

Fig. 21 Startup circuit example(2)

Fig. 22 A image of waveform when re-startup is impossible

Fig. 23 Startup circuit example(3)

Startup is impossible

Power ON

Power OFF

Vccon

Vccoff

Startup is impossible (dVcc/dt <1.8/Cs

just before Vcc reaches Vccon).

Icc>4mA is necessary for startup at

Tj <100

°C and dVcc/dt=0.