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FSCQ1565RP Datasheet(PDF) 17 Page - Fairchild Semiconductor

Part No. FSCQ1565RP
Description  Green Mode Fairchild Power Switch (FPS) for Quasi-Resonant Switching Converter
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Maker  FAIRCHILD [Fairchild Semiconductor]
Homepage  http://www.fairchildsemi.com
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FSCQ1565RP Datasheet(HTML) 17 Page - Fairchild Semiconductor

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FSCQ1565RP
17
forcing the preset maximum current to be supplied to the
SMPS until the over load protection triggers. Because more
energy than required is provided to the 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 proportional to
the output voltage and the FSCQ1565RP uses sync signal
instead of directly monitoring the output voltage. If sync
signal exceeds 12V, an OVP is triggered resulting in a
shutdown of SMPS. In order to avoid undesired triggering of
OVP during normal operation, the peak voltage of sync
signal should be designed to be below 12V. This protection
is implemented in auto restart mode.
4.4 Thermal Shutdown (TSD) : The SenseFET and the
control IC are built in one package. This makes it easy for
the control IC to detect the abnormal over temperature of the
SenseFET. When the temperature exceeds approximately
150
°C, the thermal shutdown triggers. This protection is
implemented in latch mode.
5. Soft Start : The FSCQ1565RP has an internal soft start
circuit that increases PWM comparator inverting input
voltage together with the SenseFET current slowly after it
starts up. The typical soft start time is 20msec. The pulse
width to the power switching device is progressively
increased to establish the correct working conditions for
transformers, inductors, and capacitors. It also helps to
prevent transformer saturation and reduce 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 : In order to minimize the power
consumption in the standby mode, FSCQ1565RP employs
burst operation. Once FSCQ1565RP enters into burt mode,
FSCQ1565RP allows all output voltages and effective
switching frequency to be reduced. Figure 15 shows the
typical feedback circuit for C-TV applications. In normal
operation, the picture on signal is applied and the transistor
Q1 is turned on, which de-couples R3, Dz and D1 from the
feedback network. Therefore, only Vo1 is regulated by the
feedback circuit in normal operation and determined by R1
and R2 as
In 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 determined by the
zener diode breakdown voltage. Assuming that the forward
voltage drop of D1 is 0.7V, Vo2 in standby mode is approxi-
mately given by
Figure 12. Typical feedback circuit to drop output voltage
in standby mode
Figure 16 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 Vo2
stby, the voltage 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 FSCQ1565RP and forces FSCQ1565RP to
stop switching. If the switching is disabled longer than
1.4ms, FSCQ1565RP enters into burst operation and the
operating current is reduced from 4mA (IOP) to 0.35mA
(IOB). Since there is no switching, Vo2 decrease until it
reaches Vo2
stby. As Vo2 reaches Vo2stby, the current through
the opto LED decreases allowing the feedback voltage to
rise. When the feedback voltage reaches 0.4V, FSCQ1565RP
resumes switching with a predetermined peak drain current
of 0.9A. After burst switching for 1.4ms, FSCQ1565RP
stops switching and checks the feedback voltage. If the
feedback voltage is below 0.4V, FSCQ1565RP stops
switching until the feedback voltage increases to 0.4V. If the
feedback voltage is above 0.4V, FSCQ1565RP goes back to
the normal operation.
V
o1
norm
2.5
R
1
R
2
+
R
2
---------------------


=
V
o2
stby
V
Z
0.7
2.5
++
=
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


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