FSQ0565R_08 FAIRCHILD [Fairchild Semiconductor], FSQ0565R_08 Datasheet - Page 16

• Manufacturer Part Number: FSQ0565R_08
• Manufacturer: FAIRCHILD [Fairchild Semiconductor]

FSQ0565R, FSQ0765R Rev. 1.0.1
© 2008 Fairchild Semiconductor Corporation
If no valley is found during t
forced to turn on at the end of t
the devices have a minimum switching frequency of
48kHz and a maximum switching frequency of 67kHz.
One-shot
Figure 34. QRC Operation with Limited Frequency
GateX2
FSQ0765R Rev. 00
V
AVS
V
gate
DS
I
DS
t
GateX2: Counting V
B
=15
triggering
t
I
B
DS
I
DS
=15
fixed
t
B
=15
μ
I
DS
s
μ
t
s
t
s
max
B
t
μ
s
t
B
max
s
1st or 2nd is depend on GateX2
=21
=15
t
t
t
s
s
s
=21
gate
μ
μ
s
s
μ
every 2 pulses independent on other signals.
s
W
, the internal SenseFET is
V
gate
W
I
fixed
DS
Figure 36. Alternating Valley Switching (AVS)
continued 2 pulses
I
(Case D). Therefore,
DS
I
DS
1st valley switching
t
I
W
DS
FSQ0765R Rev. 00
=6
de-triggering
μ
t
B
s
D
A
B
C
fixed
triggering
V
gate
continued another 2 pulses
t
B
16
2nd valley switching
8. AVS (Alternating Valley Switching): Due to the
quasi-resonant operation with limited frequency, the
switching frequency varies depending on input voltage,
load transition, and so on. At high input voltage, the
switching on time is relatively small compared to low
input voltage. The input voltage variance is small and the
switching frequency modulation width becomes small. To
improve the EMI performance, AVS is enabled when
input voltage is high and the switching on time is small.
Internally, quasi-resonant operation is divided into two
categories; one is first valley switching and the other is
second-valley switching after blanking time. In AVS, two
successive occurrences of first-valley switching and the
other two successive occurrences of second-valley
switching is alternatively selected to maximize frequency
modulation. As depicted in Figure 35, the switching
frequency hops when the input voltage is high. The
internal timing diagram of AVS is described in Figure 36.
67kHz
59kHz
53kHz
48kHz
f
fixed
s
frequency
Figure 35. Switching Frequency Range
Modulation frequency is approximately 17kHz.
Constant
FSQ0765 R Rev.00
1st valley- 2nd valley frequency modulation.
CCM
D
t
B
1st or 2nd is dependent on GateX2
Assume the resonant period is 2
C
Variable frequency within limited range
fixed
1st valley switching
AVS trigger point
t
B
B
DCM
V
gate
fixed
continued 2 pulses
AVS region
μ s
A
t
B
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17
19
21
V
15
1
1
μ
1
μ
in
μ
1
s
μ
s
s
s