FSQ0765RS Fairchild Semiconductor, FSQ0765RS Datasheet - Page 13

• Manufacturer Part Number: FSQ0765RS
• Description: A Quasi-Resonant Converter (QRC) generally shows lower EMI and higher power conversion efficiency than a conventional hard-switched converter with a fixed switching frequency
• Manufacturer: Fairchild Semiconductor

FSQ0765RS • Rev. 1.0.2
© 2008 Fairchild Semiconductor Corporation
The switching frequency is the combination of blank time
(t
load, the sync voltage remains flat after t
valley detection during t
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
which is instant t
internal delay time from t
switching occurs on the valley when the V
4.4V within t
other switching cycle follows classical QRC operation.
B
) and detection time window (t
sync
remains flat higher than 4.4V at the end of t
B
. Once V
Figure 34. V
Figure 35. V
I
I
DS
DS
X
t
t
B
B
, the next switching cycle starts after
=15
=15
internal delay
internal delay
sync
μ
μ
V
V
s
s
W
sync
sync
X
. This leads to a low switching
detects the first valley in t
V
V
. In the second case, the next
sync
sync
DS
DS
< 4.4V at t
> 4.4V at t
W
t
t
X
X
). In case of a heavy
FSQ0765R Rev. 00
FSQ0765R Rev. 00
sync
B
X
X
I
DS
and waits for
I
DS
4.4V
1.2V
1.0V
4.4V
1.2V
1.0V
goes below
B
, the
B
13
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
Under-Voltage Lockout (UVLO) stop voltage of 8V, the
protection is reset and the startup circuit charges the
V
of 12V, normal operation resumes. If the fault condition is
not removed, the SenseFET remains off and V
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
CC
V
12V
FSQ0765R Rev. 00
8V
CC
DS
Figure 37. Auto-Restart Protection Waveforms
capacitor. When the V
Figure 36. After V
Power
on
I
DS
CC
operation
Normal
t
B
=15
to fall. When V
V
V
sync
DS
μ
occurs
s
internal delay
Fault
sync
situation
ingnore
CC
Fault
Finds First Valley
reaches the start voltage
t
X
removed
CC
Fault
falls down to the
FSQ0765R Rev. 00
I
DS
operation
www.fairchildsemi.com
4.4V
1.2V
1.0V
Normal
CC
drops
t