Data Sheet
© 2007 Fairchild Semiconductor Corporation www.fairchildsemi.com
FSQ100 Rev. 1.0.2 9
FSQ100 — Green Mode Fairchild Power Switch (FPS
TM
)
4.1 Overload Protection (OLP): Overload is defined as
the load current exceeding a pre-set level due to an
unexpected event. In this situation, the protection circuit
should be activated to protect the SMPS. However,
even when the SMPS is operating normally, the over
load protection (OLP) circuit can be activated during the
load transition. To avoid this undesired operation, the
OLP circuit is designed to be activated after a specified
time to determine whether it is a transient situation or a
true overload situation. If the output consumes more
than the maximum power determined by I
LIM
, the output
voltage (V
O
) decreases below its rating voltage. This
reduces the current through the opto-coupler LED,
which also reduces the opto-coupler transistor current,
thus increasing the feedback voltage (V
FB
). If V
FB
exceeds 3V, the feedback input diode is blocked and the
5µA current source (I
DELAY
) starts to charge C
FB
slowly
up to V
CC
. In this condition, V
FB
increases until it reaches
4.5V, when the switching operation is terminated, as
shown in Figure 18. The shutdown delay time is the time
required to charge C
FB
from 3V to 4.5V with a 5µA
current source.
V
FB
t
3V
4.5V
Overload Protection
t
12
=Cfb×(V(
t
2
)-V( t
1
)) /
I
DELAY
t
1
t
2
VtVVtVAI
I
tVtV
C
fb
t
DELAY
DELAY
5.4)(,3)(,5;
)()(
21
12
12
===
−
=µ
Figure 18. Overload Protection (OLP)
4.2 Thermal Shutdown (TSD): The SenseFET and the
control IC are integrated, making it easier for the control
IC to detect the temperature of the SenseFET. When
the temperature exceeds approximately 145C, thermal
shutdown is activated.
5. Soft-Start: The FPS has an internal soft-start circuit
that slowly increases the feedback voltage, together with
the SenseFET current, right after it starts. The typical
soft-start time is 15ms, as shown in Figure 19, where
progressive increment of the SenseFET current is
allowed during the startup phase. Soft-start circuit
progressively increases current limits to establish proper
working conditions for transformers, inductors,
capacitors, and switching devices. It also helps to
prevent transformer saturation and reduces the stress
on the secondary diode.
2.14ms
7steps
0.31A
0.55A
t
Drain current
Figure 19. Internal Soft-Start
6. Burst Operation: To minimize the power dissipation
in standby mode, the FSQ100 enters burst-mode
operation. As the load decreases, the feedback voltage
decreases. The device automatically enters burst mode
when the feedback voltage drops below V
BURL
(0.55V).
At this point, switching stops and the output voltages
start to drop. This causes the feedback voltage to rise.
Once is passes V
BURH
(0.70V), switching starts again.
The feedback voltage falls and the process repeats.
Burst-mode operation alternately enables and disables
switching of the power MOSFET to reduce the switching
loss in standby mode.
OSC
4
V
fb
S
R
Q
GATE
DRIVER
5µA 400µA
0.7 0 V
/0.55V
on /off
Burst Operation Block
Figure 20. Burst Operation Block
V
FB
V
ds
0.55V
0. 70V
I
ds
V
O
V
O
set
t
Figure 21. Burst Operation Function