ncp1927 ON Semiconductor, ncp1927 Datasheet - Page 14

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

necessary to express the internal ramp signal in terms of its
slope (dI
current sense resistor (R
Equation 4.
where V
is the turns ratio, and L
calculated using Equation 5.
where a is the percentage of dV
Overload Protection with Fault Timer
supply, the feedback loop asks for more power than the
controller can deliver, and the current limit threshold
reaches V
(t
PFC is disabled, and a V
reaches V
power−on sequence. If the overload is still present, the fault
timer continues to run and the cycle repeats when it expires.
The fault timer is reset if the current limit threshold goes
back below V
prevent the fault timer from resetting due to noise. This
autorecovery operation is depicted in Figure 12.
FOVLD
When calculating the proper value for R
The inductor downslope (dV
Using the results from Equations 3 and 4, R
When an overload occurs on the output of the power
When the timer expires, FDRV pulses are stopped, the
) is enabled.
D
OSC
CC(on)
is the forward drop of the output rectifier, N
ILIM
dV
/dt). This is done using Equation 3.
ILIM
dt
P(off)
. When this event occurs, a fault timer
, the controller starts according to the initial
dl
. A short delay, t
OSC
dt
R
+ R
SCOMP
+
P
sense
is the primary inductance.
I
ramp(MAX)
sense
+
CC
@
a @
P(off)
) is then calculated using
D
hiccup occurs. When V
V
P(off)
dI
MAX
out
dV
OSC
dt
delay(FOVLD)
/dt) projected across the
@ f
P(off)
) V
dt
/dt to be injected.
L
OSC
P
D
@
N
N
SCOMP
S
P
SCOMP
, is added to
(eq. 3)
(eq. 4)
(eq. 5)
http://onsemi.com
, it is
can be
S
/N
CC
P
14
Frequency Foldback
the frequency of the internal oscillator is linearly reduced
from its nominal value down to f
26 kHz). The frequency foldback starts when the voltage on
the FFB pin goes below V
reaches V
while the oscillator frequency decreases. This is shown in
Figure 13.
Skip Cycle Mode with Soft−Skip
decreasing, skip mode is activated and the driver stops
switching. While the driver is disabled, V
As soon as V
driver starts to switch again, but the duty ratio is gradually
increased from nearly 0% over a short Soft−Skip duration
(t
Oscillator Frequency
SSKIP
Figure 13. Switching Frequency as V
In order to improve the efficiency at light load conditions,
When the feedback voltage reaches V
Output Load
f
Max Load
Fault Timer
OSC(MIN)
Fault Flag
V
80 ms
V
f
). This is accomplished by comparing the current
CC(on)
CC(off)
OSC
DRV
V
CC
Figure 12. Operation During Overload
FSKIP
Overcurrent
applied
Skip
FFB
. The current−mode control remains active
V
t
FOVLD
FSKIP
rises above V
starts
Fault
timer
Controller
fold
V
cycle if Fault
still present)
(new burst
stops
At V
fold
Restart
Fault is
CC(on)
reset
, and is completed before V
FSKIP
OSC(MIN)
+ V
FFB
FFB
FSKIP(HYS)
disappears
begins to rise.
Decreases
FSKIP
Fault
(typically
FFB
time
time
time
time
time
while
, the
FFB