NCP1271 ON Semiconductor, NCP1271 Datasheet - Page 12
![no-image](/images/manufacturer_photos/0/4/495/on_semiconductor_sml.jpg)
NCP1271
Manufacturer Part Number
NCP1271
Description
Soft?skip? Mode Standby Pwm Controller With Adjustable Skip Level And External Latch
Manufacturer
ON Semiconductor
Datasheet
1.NCP1271.pdf
(20 pages)
Available stocks
Company
Part Number
Manufacturer
Quantity
Price
Part Number:
NCP1271A
Manufacturer:
ON/安森美
Quantity:
20 000
Company:
Part Number:
NCP1271ADR2G
Manufacturer:
ST
Quantity:
21 000
Part Number:
NCP1271ADR2G
Manufacturer:
ON/安森美
Quantity:
20 000
Part Number:
NCP1271BDR2G
Manufacturer:
ON/安森美
Quantity:
20 000
Company:
Part Number:
NCP1271D100R2G
Manufacturer:
ONSemiconduc
Quantity:
2 802
Part Number:
NCP1271D100R2G
Manufacturer:
ON/安森美
Quantity:
20 000
Company:
Part Number:
NCP1271D65R2G
Manufacturer:
ON
Quantity:
22 500
Part Number:
NCP1271D65R2G
Manufacturer:
ON/安森美
Quantity:
20 000
Company:
Part Number:
NCP1271P100G
Manufacturer:
ON Semiconductor
Quantity:
2
Soft-Start Operation
included
comparator. When the NCP1271 starts up, a soft-start
voltage V
to 1.0 V in 4.0 ms and stays at 1.0 V afterward. This voltage
V
voltage (V
modulation voltage V
generation. Initially, (V
output voltage is low. As a result, V
soft start function and slowly ramps up the duty cycle (and
therefore the primary current) for the initial 4.0 ms. This
provides a greatly reduced stress on the power devices
during startup.
SS
Figures 28 and 29 show how the soft-start feature is
Figure 28. V
Figure 29. Soft-Start (Time = 0 at V
is compared with the divided-by-3 feedback pin
4 ms
4 ms
4 ms
Soft-start voltage, V
Feedback pin voltage divided-by-3, V
Pulse Width Modulation voltage, V
Drain Current, I
time must be less than130 ms
to prevent fault condition
SS
FB
in
begins at 0 V. V
V
/3). The lesser of V
FB
V
the
/ 3
SS
PWM
D
0
is the lesser of V
pulse-width
SS
FB
PWM
/3) is above 1.0 V because the
1
SS
+
-
increases gradually from 0 V
in the PWM duty cycle
SS
and (V
PWM
modulation
PWM
1 V
V
SS
FB
FB
PWM
CC
/3
is limited by the
and (V
/3) becomes the
= V
1 V
CC(on)
FB
(PWM)
/3)
http://onsemi.com
time
time
time
time
1 V
)
NCP1271
12
Current-Mode Pulse-Width Modulation
PWM with internal ramp compensation. A pair of current
sense resistors R
current I
inductor and current sense resistor, a corresponding voltage
ramp is placed on the CS pin (pin 3). This voltage ranges
from very low to as high as the modulation voltage V
(maximum of 1.0 V) before turning the drive off. If the
internal current ramp is ignored (i.e., R
maximum possible drain current I
Equation 2. This sets the primary current limit on a cycle
by cycle basis.
internal clock turns the Drive Output (Pin 5) high in each
switching cycle. The Drive Output goes low when the CS
(Pin 3) voltage V
V
maximum duty cycle is limited to 80% (typically) in the
output RS latch.
PWM
Output
PWM
The NCP1271 uses a current-mode fixed-frequency
The timing diagram of the PWM is in Figure 31. An
. This generates the pulse width (or duty cycle). The
Figure 31. Current-Mode Timing Diagram
Figure 30. Current-Mode Implementation
D
. As the drain current ramps up through the
80%
max duty
Q
R
S
CS
Clock
CS
intersects with the modulation voltage
I D(max) + 1 V
and R
+
-
(1V max. signal)
V
PWM
ramp
180ns
LEB
I
ramp
R CS
sense the flyback drain
V
1
CS
D(max)
0
ramp
3
PWM
Output
V
clock
CS
CS
≈ 0) then the
is shown in
V
R
PWM
ramp
V
(eq. 2)
bulk
R
PWM
I D
CS