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FSCQ-Series Rev. 1.1.2
FSCQ-Series Green Mode Fairchild Power Switch (FPS™)
output, the output voltage may exceed the rated voltage
before the over load protection triggers, resulting in the
breakdown of the devices in the secondary side. In order
to prevent this situation, an over voltage protection
(OVP) circuit is employed. In general, the peak voltage of
the sync signal is propor tional to the output voltage and
the FSCQ-Series uses a sync signal instead of directly
monitoring the output voltage. If the sync signal exceeds
12V, an OVP is triggered resulting in a shutdown of
SMPS. In order to av oid undesired triggering of O VP dur-
ing normal operation, the peak voltage of the sync signal
should be designed to be below 12V. This protection is
implemented in the auto restart mode.
4.4 Thermal Shutdown (TSD): The SenseFET and the
control IC are built in one pac kage . This mak es it easy for
the control IC to detect abnormal over temperature of the
SenseFET. When the temperature exceeds approxi-
mately 150°C, the thermal shutdown triggers. This pro-
tection is implemented in the latch mode.
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 star ts up. The typical soft start time is 20ms. 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 pre-
vent transformer saturation and reduces the stress on
the secondar y 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: In order to minimize the power con-
sumption in the standby mode, the FSCQ-Series
employs burst operation. Once FSCQ-Series enters into
the burst mode, FSCQ-Series allows all output voltages
and effective switching frequency to be reduced. Figure
15 shows the typical feedback circuit for C-TV applica-
tions. In normal operation, the picture on signal is
applied and the transistor Q1 is turned on, which decou-
ples R3, Dz and D1 from the feedback network. There-
fore, only Vo1 is regulated by the feedback circuit in
normal operation and determined by R1 and R2 as:
In the standby mode, the picture ON signal is disabled
and the transistor Q1 is turned off, which couples R3, Dz,
and D1 to the reference pin of KA431. Then, Vo2 is deter-
mined by the zener diode breakdown voltage. Assuming
that the f orward voltage drop of D1 is 0.7V, Vo2 in standb y
mode is approximately given by:
Figure 15. Typical Feedback Circuit to Drop
Output Voltage in Standby Mode
Figure 17 shows the burst mode operation waveforms.
When the picture ON signal is disabled, Q1 is turned off
and R3 and Dz are connected to the reference pin of
KA431 through D1. Before Vo2 drops to Vo2stby, the volt-
age on the reference pin of KA431 is higher than 2.5V,
which increases the current through the opto LED. This
pulls down the feedback voltage (VFB) of FSCQ-Series
and forces FSCQ-Series to stop switching. If the switch-
ing is disabled longer than 1.4ms, FSCQ-Series enters
into burst operation and the operating current is reduced
from IOP to 0.25mA (IOB). Since there is no s witching, Vo2
decreases until it reaches Vo2stby. As V o2 reaches Vo2stby,
the current through the opto LED decreases allowing the
feedback voltage to rise. When the feedback voltage
reaches 0.4V, FSCQ-Series resumes switching with a
predetermined peak drain current of 0.9A. After burst
switching for 1.4ms, FSCQ-Series stops switching and
checks the feedback voltage. If the feedback voltage is
below 0.4V, FSCQ-Ser ies stops switching until the feed-
back voltage increases to 0.4V. If the feedback voltage is
above 0.4V, FSCQ-Series goes back to the normal oper-
ation.
The output voltage drop circuit can be implemented
alternatively as shown in Figure 16. In the circuit of Fig-
ure 16, the FSCQ-Series goes into burst mode, when
picture off signal is applied to Q1. Then, Vo2 is deter-
mined by the zener diode breakdown voltage. Assuming
that the forward voltage drop of opto LED is 1V, the
approximate value of Vo2 in standby mode is given by:
Vo1
norm 2.5 R1R2
+
R2
--------------------
•=
Vo2stby VZ0.7 2.5++=
Picture ON
Micom
VO2 Linear
Regulator
VO1
(B+)
KA431 R2
R1R3
Rbias
RD
RF
CFD1Q1
A
CR
DZ
Vo2stby VZ1+=