PRELIMINARY DATASHEET
feedback voltage (Vfb). If Vfb exceeds 2.5V, D1 is
4. Protection Circuits: The FSQ-series has several
self-protective functions, such as Overload Protection
(OLP), Abnormal Over-Current Protection (AOCP), and
Thermal Shutdown (TSD). All the protections are
implemented as auto-restart mode. Once the fault
condition is detected, switching is terminated and the
SenseFET remains off. This causes VCC to fall. When
VCC falls down to the Under-Voltage Lockout (UVLO)
stop voltage of 6.7V, the protection is reset and the
start-up circuit charges the VCC capacitor. When VCC
reaches the start voltage of 8.7V, the FSQ-series
resumes normal operation. If the fault condition is not
removed, the SenseFET remains off and VCC drops to
stop 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, the reliability is
improved without increasing cost.
blocked and the 5µA current source starts to charge
CB slowly up to VCC. In this condition, Vfb continues
increasing until it reaches 4.5V, when the switching
operation is terminated, as shown in Figure 8. The
delay time for shutdown is the time required to charge
CB from 2.5V to 4.5V with 5µA. A 20 ~ 50ms delay
time is typical for most applications. This protection is
implemented in auto restart mode.
VFB
Overload protection
4.5V
2.8V
Fault
occurs
Fault
removed
Power
on
T12= CB*(6.0-2.8)/Idelay
Vds
T1
Figure 8. Overload Protection
T2
t
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
OLP (Overload Protection), it is not enough to protect
the FSQ-series in this abnormal case, since severe
current stress is imposed on the SenseFET until OLP
triggers. The FSQ-series has an internal AOCP
circuit, as shown in Figure 9. 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.
Vcc
8.7V
6.7V
t
Normal
operation
Fault
situation
Normal
operation
Figure 7. 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 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 (Vo) 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
6R
OSC
S
Q
Q
PWM
Gate
driver
R
R
LEB
Rsense
+
2
AOCP
GND
-
Vaocp
Figure 9. Abnormal Over-Current Protection
© 2006 Fairchild Semiconductor Corporation
FSQ510, FSQ510H Rev. 0.0.3
www.fairchildsemi.com
8
FAIRCHILD [ FAIRCHILD SEMICONDUCTOR ]
