FSQ0765RQ Datasheet, PDF (13/19 Pages) Fairchild Semiconductor – Green-Mode Fairchild Power Switch

The switching frequency is the combination of blank time
(t_B) and detection time window (t_W). In case of a heavy
load, the sync voltage remains flat after t_B and waits for
valley detection during t_W. This leads to a low switching
frequency not suitable for heavy loads. To correct this
drawback, additional timing is used. The timing
conditions are described in Figures 25, 26, and 27. When
the V_sync remains flat higher than 4.4V at the end of t_B
that is t_X, the next switching cycle starts after internal
delay time from t_X. In the second case, the next switching
occurs on the valley when the V_sync goes below 4.4V
within t_B. Once V_sync detects the first valley within t_B, the
other switching cycle follows classical QRC operation.
t_B=15μs
t_X
t_B=15μs
t_X
I_DS
I_DS
V_DS
ingnore
4.4V
V_sync
1.2V
1.0V
internal delay
FSQ0765R Rev. 00
Figure 27. After V_sync Finds First Valley
I_DS
I_DS
V_DS
V_sync
4.4V
1.2V
1.0V
internal delay
FSQ0765R Rev. 00
Figure 25. V_sync > 4.4V at t_X
t_B=15μs
t_X
I_DS
I_DS
4. Protection Circuits: The FSQ-series has several
self-protective functions, such as Overload Protection
(OLP), Abnormal Over-Current Protection (AOCP),
Over-Voltage Protection (OVP), 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 V_CC to fall. When V_CC falls down to the
Under-Voltage Lockout (UVLO) stop voltage of 8V, the
protection is reset and the startup circuit charges the
V_CC capacitor. When the V_CC reaches the start voltage
of 12V, normal operation resumes. If the fault condition is
not removed, the SenseFET remains off and V_CC 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.
V_DS
Power
on
Fault
occurs
Fault
removed
V_DS
V_sync
4.4V
1.2V
1.0V
internal delay
FSQ0765R Rev. 00
Figure 26. V_sync < 4.4V at t_X
V_CC
12V
8V
Normal
Fault
FSQ0765R Rev. 00 _{operation situation}
Normal ^t
operation
Figure 28. Auto Restart Protection Waveforms
© 2008 Fairchild Semiconductor Corporation
FSQ0765RQ Rev. 1.0.1
13
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