ncp1251 ON Semiconductor, ncp1251 Datasheet - Page 18

• Manufacturer Part Number: ncp1251
• Description: Ncp1251 Current-mode Pwm Controller For Off-line Power Supplies
• Manufacturer: ON Semiconductor

Slope Compensation
signal. This is the buffered oscillator clock delivered only
during the on time. Its amplitude is around 2.5 V at the
maximum duty−cycle. Ramp compensation is a known
means used to cure sub harmonic oscillations in Continuous
Conduction
2.5 V swing. If the clock operates at a 65 kHz frequency,
then the available oscillator slope corresponds to:
The NCP1251 includes an internal ramp compensation
In the NCP1251 controller, the oscillator ramp features a
Figure 46. Inserting a Resistor in Series with the Current Sense Information Brings Ramp Compensation and
Mode
Figure 45. An Auto−Recovery Hiccup Mode is Activated for Faults Longer than 100 ms
(CCM)
reset
latch
−2.72
−2.72
−11.5
−11.5
23.6
23.6
14.8
14.8
6.05
6.05
operated
−2.12
−2.12
−8.13
−8.13
15.9
15.9
9.90
9.90
3.89
3.89
445m
445m
Stabilizes the Converter in CCM Operation.
4.32
4.32
3.35
3.35
2.38
2.38
1.41
1.41
1
1
+
−
vcc
vcc
from FB
setpoint
500u
500u
current−mode
2
2
vdrv
vdrv
ON
3
3
ilprim
ilprim
LEB
http://onsemi.com
1.50m
1.50m
18
20k
time in seconds
time in seconds
converters. These oscillations take place at half the
switching frequency and occur only during CCM with a
duty−cycle greater than 50%. To lower the current loop gain,
one usually injects between 50% and 100% of the inductor
downslope. Figure 46 depicts how internally the ramp is
generated. Please note that the ramp signal will be
disconnected from the CS pin, during the off time.
2.50m
2.50m
S
SS
SS
CS
ramp
0V
2.5 V
+
+ 133 kV s or 133 mV ms
Rcomp
3.50m
3.50m
V
ramp,peak
V
V
V (t)
V
DRV
DRV
cc
cc
I
I
L
L
T
p
p
sw
(t)
(t)
D
4.50m
4.50m
max
Rsense
+
1
1
2
2
3
3
2.5
15m
0.8
(eq. 10)