FA551X
16
3. The startup period
The start up period from the time the power is on to the time the
IC is turn to on is approximately given by:
To shorten the start-up period, the capacitor C2 or resistor R1
should be decreased. But in some case, such as when the
load current of the power supply is changed rapidly, you may
want to prolong the hold time of the Vcc voltage over the off
threshold. In this case the capacitor C2 cannot be decreased
and the resistor R1 should be decreased. But loss of the
resistor R1 increases.
In such case, the circuit shown in Fig. 23 is effective to shorten
start-up period without increasing the loss of the resistor R1.
The capacitor C2 is decreased to shorten the start-up period
and, after the IC starts up, Vcc voltage supplied from C3 to
prolong the hold time of the Vcc voltage.
The start-up period of this circuit also is approximately given by
the expression in (11)
4. Setting soft start period and OFF latch delay
independently
Figure 24 shows a circuit for setting the soft start period and
OFF latch delay independently. In this circuit, capacitance CS
determines the soft start period, and capacitance CL
determines the OFF latch delay. If the overload shutdown or
overvoltage shutdown functions raise the CS pin voltage to
around 5V, zener diode Zn becomes conductive to charge
capacitor CL. The OFF latch delay can be thus prolonged by
capacitance CL.
5. Overvoltage protection using the VCC pin
These ICs contain an overvoltage protection function detecting
the Vcc voltage using internal ZD (See item 5 in “Description of
each circuit”). If Vcc voltage exceed 31.8V, the current of 14mA
flows through the internal ZD and the overvoltage protection
function operates.
After this protection function operates, the IC continues to
consume the large current if high voltage continues to be
applied to the Vcc pin. Mind that total IC loss does not exceed
the rating. If the voltage source applied to Vcc pin has relatively
high impedance and cannot supply the current of 14mA,
overvoltage protection function does not operate. But the
internal ZD maintains the Vcc voltage 32V or less and protects
the IC.
Where:
R1: Startup resistor [
Ω]
C2: Capacitor between VCC and GND pin [F]
Vac: Effective value of AC input voltage [V]
Fig. 22
Startup circuit (3)
Fig. 23
Startup circuit (4)
VCC
FA551X
C2
C3
D1
D2
R1
6
VCC
FA551X
C2
D1
R1
6
Fig. 24
Independent setting of soft start period and OFF latch
deley
FA551X
GND
CS
Cs
Zn
CL
5V
8
4
V1=
2
Vac ..... (Connecting a startup resistor before rectification)
π
2
Vac ....... (Connecting a startup resistor after rectification)
()
tstart-up = –C2
R1
In 1 –
16.5
V1
................................. (11)
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