ncp1927 ON Semiconductor, ncp1927 Datasheet - Page 15

• Manufacturer Part Number: ncp1927
• Description: Combination Power Factor Correction Controller And Flyback Controller For Flat Panel Tvs
• Manufacturer: ON Semiconductor

sense signal to an internal ramp generated by the Soft−Skip
timer instead of the feedback voltage. Since the LEB of the
FCS Pin prevents operation at nearly 0% duty ratio, the
controller instead compares the soft−skip ramp to an internal
sawtooth signal generated by the oscillator (not subjected to
LEB). This causes the controller to operate briefly in voltage
Figure 15. Skip Cycle with Soft−Skip Timing Diagram
V
CS Setpoint
FSKIP(HYS)
V
FDRV
FSKIP
V
V
fold
FFB
FFB
FCS
Soft−Skip
Enters
Soft−Skip
K
V
V
t
LEB
FSKIP
FSKIP
FFB
Figure 14. Skip Cycle with Soft−Skip Architecture
Soft−Skip
Figure 16. Soft−Skip Timing Diagram
Reset
Soft−skip ramp
−
+
Soft−Skip
Time
Time
Time
Exits
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t
SSKIP
15
mode instead of current mode. Once the CS signal reaches
the feedback voltage, the controller resumes normal
operation in current mode. The skip mode block diagram is
shown in Figure 14. The ramp timing and overall timing
diagrams are shown in Figures 15 and 16.
goes above V
allowing the controller to operate in current mode.
drops if a load transient occurs while the controller is in skip
mode.
During the Soft−Skip duration if the feedback voltage
This transient load detection feature avoids large output
+
−
Sawtooth
Oscillator
D
MAX
fold
, the Soft−Skip ends instantaneously
S
R
Q
FDRV stage