FSAR001B Fairchild Semiconductor, FSAR001B Datasheet - Page 9

• Manufacturer Part Number: FSAR001B
• Manufacturer: Fairchild Semiconductor

© 2010 Fairchild Semiconductor Corporation
FSAR001 • Rev. 1.0.0
Functional Description
VDD Pin Over-Voltage Protection
After startup, FSAR001 turns on the OVP function.
During conduction angle interval, the V
two kinds of behavior. One is that if the V
recharges to OVP trigger point (8.5V for 5V version),
the FSAR001 turns off the power MOSFET to limit the
maximum V
V
conduction interval, the power MOSFET is turned off by
AC synchronous signal and continuously recharges V
capacitor at next duration. Using this technique, energy
is drawn from the AC mains only during the low-voltage
portions of each positive half cycle, reducing the
V
V
DD
DET
DET
V
V
IN
IN
peak
DET
V
cannot recharge to the OVP level during the
,
,
IN
I
max
min
IN
t0
=
=
t1
DD
Figure 18. Typical Waveforms
R
R
3
3
voltage. The other is that if the voltage of
R
R
+
+
4
4
R
R
t2
4
4
Ts/2
×
×
Vac
Vac
a.
b.
Ts
(Continued)
DD
DD
voltage has
capacitor
EQ 1
t
t
DD
9
dissipation in the first stage. During the power MOSFET
turn-on, the current provided by the commutated AC
voltage is used to supply the loads and to charge the
V
switches off, the loads receive the required currents by
the capacitor discharge. For this reason, it is important
to properly set the conduction angle.
For the V
interval; the energy is drawn from commutated AC bus,
which not only provides the output requirement but also
recharges the V
conduction angle, the V
system requirement. The V
by maximum loading power. The capacitor is evaluated
by Equation 2; the selection chart is shown in Figure 19.
C
DD
VDD
Figure 19.
capacitor. In this way, when the power MOSFET
600
500
400
300
200
100
=
0
I
DD
LOAD
0
capacitor selection, during conduction angle
1
×
10
DD
V
10
m
DD
capacitor to OVP level. Outside of
( ,
Capacitor vs. Output Current
unit
Load Current (mA)
DD
capacitor supplies the whole
20
DD
=
F
capacitor can be reduced
)
30
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40
EQ 2
50