FSQ0365_11 Datasheet, PDF (13/21 Pages) Fairchild Semiconductor – Green Mode Fairchild Power Switch for Valley Switching Converter – Low EMI and High Efficiency

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, the reliability is improved without
increasing cost.
V_DS
Power
on
Fault
occurs
Fault
removed
V_CC
12V
8V
t
Normal
FSQ0365RN Rev. 00 operation
Fault
situation
Normal
operation
Figure 23. Auto-Restart Protection Waveforms
4.1 Overload Protection (OLP): Overload is defined as
the load current exceeding its normal level due to an
unexpected abnormal 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 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_O) decreases below the set 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 2.8V, D1 is blocked and the 5µA current source
starts to charge CB slowly up to V_CC. In this condition,
V_FB continues increasing until it reaches 6V, when the
switching operation is terminated, as shown in Figure
24. The delay for shutdown is the time required to
charge CB from 2.8V to 6V with 5µA. A 20 ~ 50ms delay
is typical for most applications.
V_FB
6.0V
FSQ0365RN Rev.00
Overload protection
2.8V
t₁₂= C_FB*(6.0-2.8)/I_delay
t₁
t₂ t
Figure 24. Overload Protection
4.2 Abnormal Over-Current Protection (AOCP): When
the secondary rectifier diodes or the transformer pins
are shorted, a steep current with extremely high-di/dt
can flow through the SenseFET during the LEB time.
Even though the FSQ-series has Overload Protection
(OLP), it is not enough to protect the FSQ-series in that
abnormal case, since severe current stress is imposed
on the SenseFET until OLP triggers. The FSQ-series
has an internal Abnormal Over-Current Protection
(AOCP) circuit as shown in Figure 25. When the gate
turn-on signal is applied to the power SenseFET, the
AOCP block is enabled and monitors the current
through the sensing resistor. The voltage across the
resistor is compared with a preset AOCP level. If the
sensing resistor voltage is greater than the AOCP level,
the set signal is applied to the latch, resulting in the
shutdown of the SMPS.
3R
OSC
PWM
LEB
200ns
SQ
RQ
R
Gate
driver
AOCP
FSQ0365RN Rev.00
R_sense
V_OCP
1
GND
Figure 25. Abnormal Over-Current Protection
© 2007 Fairchild Semiconductor Corporation
FSQ0365, FSQ0265, FSQ0165, FSQ321 • Rev. 1.0.6
13
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