ncp1230a ON Semiconductor, ncp1230a Datasheet - Page 13

• Manufacturer Part Number: ncp1230a
• Description: Low?standby Power Soft Skip Mode Controller
• Manufacturer: ON Semiconductor

Feedback
directly to the open−collector output of an optocoupler. The
pin is pulled−up through a 20 kW resistor to the internal Vdd
supply (6.5 volts nominal). The feedback input signal is
divided down, by a factor of three, and connected to the
negative (−) input of the PWM comparator. The positive (+)
input to the PWM comparator is the current sense signal
(Figure 30).
the feedback signal is proportional to the output power. At
the beginning of the cycle, the power switch is turns−on and
the current begins to increase in the primary of the
transformer, when the peak current crosses the feedback
voltage level, the PWM comparators switches from a logic
level low, to a logic level high, resetting the PWM latching
Flip−Flop, turning off the power switch until the next
oscillator clock cycle begins.
ESD protection.
Soft Skip Mode
cycle logic (Figure 32). When the feedback voltage drops
below 25% of the maximum peak current (1.0 V/Rsense) the
IC prevents the current from decreasing any further and
starts to blank the output pulses. This is called the soft skip
mode. While the controller is in the soft skip mode the power
transfer now depends upon the duty cycle of the pulse burst
width which reduces the average input power demand.
required to meet the output power demands, the NCP1230A
goes through a soft−start cycle ramping up the current to the
output. (Refer to Figure 31)
The feedback pin has been designed to be connected
The NCP1230A is a peak current mode controller, where
2
3
The feedback pin input is clamped to a nominal 10 volt for
The feedback input is connected in parallel with the skip
When the controller determines that more energy is
FB
2.3 Vpp
Ramp
18k
20k
Vdd
Figure 30.
10 V
55k
LEB
25k
+
−
PWM
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output power has been reached, the NCP1230A
immediately turns off the skip mode (it does not ramp down
the output current). This Soft Skip Mode is ideal for
applications where a low standby power is required and a
minimization of any audible noise that may be associated
with magnetics and snubber components is needed.
where:
where:
where:
the skip mode
When the feedback signal determines that the desired
V
I
R
3 = Feedback divider ratio.
P
L
f = NCP1230A controller frequency
pk
in
p
s
c
= Current sense resistor,
= Primary inductance
= control voltage (Feedback pin input),
= Peak primary current,
= is the power level where the NCP1230A will go into
SkipLevel + 3V @ 25% + 0.75V
P in +
V c + I pk @ R s @ 3
I pk +
I pk @ R s + 1V
I pk + 0.75
Figure 31.
P in + P out
L p @ f @ I pk 2
R s @ 3
2 @ P in
L p @ f
Eff
2