ncp1927 ON Semiconductor, ncp1927 Datasheet - Page 12

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

THERMAL SHUTDOWN
minimum), a temperature sensing circuit disables the gate
drives and a V
reaches V
temperature drops below T
CLAMPED DRIVERS
capable of sourcing 500 mA and sinking 800 mA each.
Since V
is internally clamped to 16 V (maximum) to allow the use of
20 V MOSFETs.
FLYBACK CONTROLLER
current mode architecture where the switch−off event is
dictated by the peak current setpoint.
Oscillator with Maximum Duty Ratio and Frequency
Jittering
that sets the switching frequency with an accuracy of
$7.7%. The maximum duty ratio of the FDRV pin is 80%
(typical).
frequency jitters at f
value, with a triangle−wave shape and at a frequency of f
When the junction temperature exceeds T
The NCP1927 includes two powerful MOSFET drivers
The NCP1927 flyback stage implements a standard
The NCP1927 flyback controller includes an oscillator
In order to improve the EMI signature, the switching
Figure 6. V
CC
CC(on)
is rated at 30 V (maximum), each driver output
CC
CC
, the cycle repeats unless the junction
hiccup occurs (see Figure 5). When V
Behavior During Shutdown Mode
MOD
($6% typical) around its nominal
TSHDN
.
TSHDN
(140°C
http://onsemi.com
jitter
CC
12
(125 Hz typical). The frequency jittering is fully disabled
during soft−start and frequency foldback. Figure 7 depicts
the jittering operation.
Current Sensing
the feedback voltage sets the peak current flowing in the
transformer and the MOSFET. This is done through the
PWM comparator. The switch current is sensed across a
resistor and the resulting voltage is applied to the FCS pin.
It is then applied to one input of the PWM comparator
through a 250 ns leading edge blanking (LEB) block. On the
other input, the feedback voltage divided by K
5) sets the current limit threshold. When the current reaches
this threshold, the output driver is turned off. A dedicated
comparator monitors the current sense voltage, and if it
reaches the maximum value, V
output driver is turned off immediately. This occurs even if
the limit imposed by the feedback voltage is higher than
V
circuit.
FDRV
ILIM
NCP1927 is a current−mode controller, which means that
Nominal f
f
f
OSC
OSC
. Figure 8 shows the schematic of the current sense
V
Figure 8. Current Sense Block Schematic
CC
+ f
- f
MOD
MOD
OSC
f
OSC
Q
Figure 7. Frequency Jittering
R
S
f
(OSC)
(125 Hz)
8 ms
ILIM
(typically 0.7 V), the
R
V
FFB
V
FFB(open)
ILIM
FFB
÷
t
(typically
K
LEB
Time
FFB
FFB
FCS