fa5311bs Fuji Electric holdings CO.,Ltd, fa5311bs Datasheet - Page 10

• Manufacturer Part Number: fa5311bs
• Description: Bipolar Ic For Switching Power Supply Control
• Manufacturer: Fuji Electric holdings CO.,Ltd

(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 R
directly.
The period from power-on to startup is determined by R
and C
following equations.
dVcc/dt(V/s)=
(1/C
1.8/(Cs( F))..........................................(8)
R
V
After power-off, once V
voltage, V
capacitor C
V
This operation is repeated several times.
After the repeated operation, IC stops in the condition that V
voltage is equal to Vccon (=ON threshold) because capacitor
C
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.
A
IN
CC
1
(k ) < ( V
is discharged gradually and the decreased V
:
A
voltage rises up by the current flowing R
)•{( V
A
2 • (AC input effective voltage)
. R
CC
A
IN
1
, R
IN
–Vccon )/R
rises up again and re-startup occurs while the
is discharged until approximately zero because
– 9.2(V) )/{ 0.4(mA) + ( 9.2(V)/R
B
and C
A
CC
must be designed to satisfy the
A
– Vccon/R
falls down below OFF threshold
A
is connected to rectified line
B
–Iccst } >
A
.
B
(k ) ) }....(9)
CC
inclination
A
, R
B
CC
Vccon
Vccoff
Fig. 22 A image of waveform when re-startup is impossible
Power OFF
Fig. 23 Startup circuit example(3)
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 ON
Startup is impossible
FA531X series
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