A5317P 查看數據表（PDF） - Fuji Electric

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A5317P

Bipolar IC For Switching Power Supply Control

Fuji Electric 

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

circuit that a startup resistor RA is connected to rectified line

directly.

The period from power-on to startup is determined by RA, RB

and CA. RA, RB and CA must be designed to satisfy the

following equations.

dVcc/dt(V/s)=

(1/CA)•{( VIN –Vccon )/RA– Vccon/RB–Iccst } >

1.8/(Cs(µF))..........................................(8)

RA(kΩ) < ( VIN– 9.2(V) )/{ 0.4(mA) + ( 9.2(V)/RB(kΩ) ) }....(9)

VIN : ǰ2 • (AC input effective voltage)

After power-off, once VCC falls down below OFF threshold

voltage, VCC rises up again and re-startup occurs while the

capacitor C1 is discharged until approximately zero because

VCC voltage rises up by the current flowing RA.

This operation is repeated several times.

After the repeated operation, IC stops in the condition that VCC

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

C1 is discharged gradually and the decreased VCC inclination

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(Լ16V, 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)

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.

Power OFF

Vccon

Vccoff

Power ON

Startup is impossible

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

Fig. 23 Startup circuit example(3)

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