Data Sheet

© 2006 Fairchild Semiconductor Corporation www.fairchildsemi.com
FSQ-Series • Rev. 1.1.3 17
FSCQ-Series — Green Mode Fairchild Power Switch (FPS™)
5. Soft Start: The FSCQ series has an internal soft-start
circuit that increases PWM comparator’s inverting input
voltage together with the SenseFET current slowly after
it starts up. The typical soft start time is 20 ms. The
pulse width to the power switching device is
progressively increased to establish the correct working
conditions for transformers, inductors, and capacitors.
Increasing the pulse width to the power switching device
also helps prevent transformer saturation and reduces
the stress on the secondary diode during startup. For a
fast build up of the output voltage, an offset is
introduced in the soft-start reference current.
6. Burst Operation: To minimize the power
consumption in the standby mode, the FSCQ series
employs burst operation. Once FSCQ series enters
burst mode, FSCQ series allows all output voltages and
effective switching frequency to be reduced. Figure 36
shows the typical feedback circuit for C-TV applications.
In normal operation, the picture on signal is applied and
the transistor Q
1
is turned on, which decouples R
3
, D
Z
and D
1
from the feedback network. Therefore, only V
O1
is regulated by the feedback circuit in normal operation
and determined by R
1
and R
2
as:










2
21
1
5.2
R
RR
V
NORM
O
(6)
In standby mode, the picture ON signal is disabled and
the transistor Q
1
is turned off, which couples R
3
, D
Z
, and
D
1
to the reference pin of KA431. Then, V
O2
is
determined by the Zener diode breakdown voltage.
Assuming that the forward voltage drop of D
1
is 0.7V,
V
O2
in standby mode is approximately given by:
5.27.0
2

Z
STBY
O
VV
(7)
Picture ON
Micom
Linear
Regulator
VO2
V
O1
(B+)
KA431
R
2
R
1
R
3
R
bias
R
D
R
F
C
F
D
1
Q1
A
C
R
Dz
Figure 36. Typical Feedback Circuit to Drop
Output Voltage in Standby Mode
Figure 38 shows the burst mode operation waveforms.
When the picture ON signal is disabled, Q
1
is turned off
and R
3
and D
z
are connected to the reference pin of
KA431 through D
1
. Before V
o2
drops to V
o2
stby
, the
voltage on the reference pin of KA431 is higher than
2.5 V, which increases the current through the opto
LED. This pulls down the feedback voltage (V
FB
) of
FSCQ series and forces FSCQ series to stop switching.
If the switching is disabled longer than 1.4 ms, FSCQ
series enters into burst operation and the operating
current is reduced from I
OP
to 0.25 mA (I
OB
). Since there
is no switching, V
o2
decreases until it reaches V
o2
stby
. As
V
o2
reaches V
o2
stby
, the current through the opto LED
decreases allowing the feedback voltage to rise. When
the feedback voltage reaches 0.4 V, FSCQ series
resumes switching with a predetermined peak drain
current of 0.9 A. After burst switching for 1.4 ms, FSCQ
series stops switching and checks the feedback voltage.
If the feedback voltage is below 0.4 V, FSCQ series
stops switching until the feedback voltage increases to
0.4 V. If the feedback voltage is above 0.4 V, FSCQ
series goes back to the normal operation. The output
voltage drop circuit can be implemented alternatively, as
shown in Figure 37. In the circuit, the FSCQ series goes
into burst mode, when picture off signal is applied to Q
1
.
Then, V
o2
is determined by the Zener diode breakdown
voltage. Assuming that the forward voltage drop of opto
LED is 1 V, the approximate value of V
o2
in standby
mode is given by:
1
2

Z
STBY
O
VV
(8)
Picture OFF
Micom
Linear
Regulator
VO2
V
O1
(B+)
KA431
R
2
R
1
R
bias
R
D
R
F
C
F
A
C
R
Dz
Q
1
Figure 37. Feedback Circuit to Drop Output
Voltage in Standby Mode