FSQ510_09 FAIRCHILD [Fairchild Semiconductor], FSQ510_09 Datasheet - Page 10

• Manufacturer Part Number: FSQ510_09
• Description: Green Mode Fairchild Power Switch
• Manufacturer: FAIRCHILD [Fairchild Semiconductor]

© 2009 Fairchild Semiconductor Corporation
FSQ510, FSQ510H, and FSQ510M • Rev. 1.3.0
4. Protection Circuits: The FSQ510 (H or M) has two
self-protective functions, overload protection (OLP) and
thermal
implemented as auto-restart mode. Once the fault
condition is detected, switching is terminated and the
SenseFET remains off. This causes V
V
voltage of 6.7V, the protection is reset and the startup
circuit charges the V
the start voltage of 8.7V, the FSQ510 (H or M) resumes
normal operation. If the fault condition is not removed,
the SenseFET remains off and V
voltage again. In this manner, the auto-restart can
alternately enable and disable the switching of the
power SenseFET until the fault condition is eliminated.
Because these protection circuits are fully integrated
into the IC without external components, reliability is
improved without increasing cost.
4.1 Overload Protection (OLP): Overload is defined as
the load current exceeding its normal level due to an
unexpected event. In this situation, the protection circuit
should trigger to protect the SMPS. However, even
when the SMPS is in the normal operation, the overload
protection circuit can be triggered during the load
transition. To avoid this undesired operation, the
overload protection circuit is designed to trigger only
after a specified time to determine whether it is a
transient situation or a true overload situation. Because
of the pulse-by-pulse current limit capability, the
maximum peak current through the SenseFET is limited
and, therefore, the maximum input power is restricted
with a given input voltage. If the output consumes more
than this maximum power, the output voltage (V
decreases below the set voltage. This reduces the
current through the opto-coupler LED, which also
reduces the opto-coupler transistor current, increasing
the feedback voltage (V
blocked and the 5µA current source starts to charge C
slowly up
until it reaches 4.7V, when the switching operation is
terminated, as shown in Figure 17. The delay time for
shutdown is the time required to charge C
4.7V with 5µA. A 20 ~ 50ms delay time is typical for
6.7V
8.7V
V
V
CC
B
ds
CC
Figure 16. Auto Restart Protection Waveforms
B
falls down to the under-voltage lockout (UVLO) stop
Power
.
on
shutdown
In this condition, V
operation
Normal
CC
occurs
(TSD).
Fault
FB
capacitor. When V
). If V
situation
Fault
FB
The
FB
removed
continues increasing
exceeds 2.8V, D1 is
Fault
CC
protections
CC
drops to stop
B
to fall. When
operation
from 2.8V to
Normal
CC
reaches
are
t
o
B
)
most applications. This protection is implemented in
auto-restart mode.
4.2 Thermal Shutdown (TSD): The SenseFET and the
control IC on a die in one package make it easy for the
control IC to detect the abnormal over temperature of
the
approximately 140°C, the thermal shutdown triggers
and the FPS stops operation. The FPS operates in
auto-restart mode until the temperature decreases to
around 80°C, when normal operation resumes.
5. Soft-Start: The FPS has an internal soft-start circuit
that increases the VS-PWM comparator inverting input
voltage, together with the SenseFET current, slowly
after it starts up. The typical soft-start time is 5ms. The
pulse
progressively increased to establish the correct working
conditions for transformers, inductors, and capacitors.
The voltage on the output capacitors is progressively
increased with the intention of smoothly establishing the
required output voltage. This helps prevent transformer
saturation and reduces stress on the secondary diode
during startup.
6.
dissipation in standby mode, the FPS enters burst-
mode operation. As the load decreases, the feedback
voltage decreases. As shown in Figure 18, the device
automatically enters burst mode when the feedback
voltage drops below V
switching stops and the output voltages start to drop at
a rate dependent on standby current load. This causes
the feedback voltage to rise. Once it passes V
(850mV), switching resumes. The feedback voltage
then falls and the process repeats. Burst mode
alternately enables and disables switching of the
SenseFET, reducing switching loss in standby mode.
4.7V
2.8V
10
V
FB
Burst-Mode
SenseFET.
width
Figure 17. Overload Protection
t
1
to
Overload Protection
Operation:
the
If
t
12
= C
power
BURL
the
B
•(4.7-2.8)/I
(750mV). At this point,
temperature
To
switching
delay
minimize
www.fairchildsemi.com
device
exceeds
t
2
power
BURH
t
is