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.
Fig. 21 Startup circuit example(2)
This operation is repeated several times.
Startup is impossible (dVcc/dt <1.8/Cs
just before Vcc reaches Vccon).
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.
Icc>4mA is necessary for startup at
Tj <100°C and dVcc/dt=0.
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.
Power OFF
Power ON
Vccon
Startup is impossible
Icc(start-up) > 4mA
at Vcc=Vccon, Tj<100°C, after power-off
Vccoff
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).
Fig. 22 A image of waveform when re-startup is impossible
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. 23 Startup circuit example(3)
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FUJI [ FUJI ELECTRIC ]
