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NCP1650
Comparator monitors the output voltage and will shutdown
operation of the PWM circuit if the output voltage exceeds
8% above the normal regulation level. The Overshoot
Comparator signal is fed into the second Set input to the
latch.
AC Reference Buffer
The AC reference buffer converts the voltage generated
by the AC error amplifier to be converted into a current to
be summed with the ramp compensation signal and the
instantaneous current signal.
The buffer amplifier, converts the input voltage to a
current by creating a current equal to the voltage difference
between the AC error amplifier output and the 2.9 volt
reference dropped across the 14 k Ω resistor. The bipolar
transistor level shifts the voltage and maintains the proper
current into the current mirror. The current mirror has a 1:2
ratio and delivers the output current to the PWM input. This
current is summed with the currents of the ramp
compensation signal and the instantaneous current signal to
determine the turn--off point in the switching cycle.
Soft--Start Circuit
The AC error amplifier has been configured such that a
AC
Comp
3
AC
ERROR
AMP
2.9 V
+
--
+
--
CURRENT
MIRROR
i 1
14 k
2 X i 1
16 k
PWM,
Ramp
Comp
Current
Sense
Amp
low output level will cause the output duty cycle to go to
zero. This will have the effect of soft--starting the unit at
turn--on, since the output is coupled to ground through a
capacitor.
There will be an initial offset of the output voltage due to
the output current and the resistor at pin 3. For example, if
the output is saturated in the high state at turn on, it will
source 50 m A. If pin 3 is terminated with a 2.2 k Ω resistor
and a 0.01 F capacitor, the initial step will be:
50 m A × 2.2 k = 0.11 volts
and the rate of rise will be:
Unity Gain Amplifier
Figure 39. AC Reference Buffer Schematic
The buffer ’s transfer function is:
50 m A ∕ 0.01 m F = 5 mv ∕ m s
or, 560 m s until the output is at 2.9 volts, which corresponds
to full duty cycle.
An external soft--start circuit can be added, as shown in
Figure 29, if additional time is desired.
iout = (2.9 V ? Vac) ∕ 7 k
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