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NCP1653, NCP1653A
R S I S
NCP1653
V S
becomes its maximum value and generates zero on time t 1 .
Then, V out decreases and the minimum can be V out = V in in
a boost converter. Going down to V in , V out automatically
enters the previous two regions (i.e., follower boost region
or constant output voltage region) and hence output voltage
V out cannot reach input voltage V in as long as the NCP1653
provides a duty ratio for the operation of the boost
converter.
In conclusion, the NCP1653 circuit operates in one of the
following conditions:
Constant output voltage mode: The output voltage is
regulated around the range between 96% and 100% of R FB
× I ref . The output voltage is described in (eq.16). Its
behavior is similar to a follower boost.
Follower boost mode: The output voltage is regulated
under 96% of R FB × I ref and I control = I control(max) = I ref /2 =
100 m A. The output voltage is described in (eq.15).
Overvoltage Protection (OVP)
When the feedback current I FB is higher than 107% of the
reference current I ref (i.e., V out > 107% R FB × I ref ), the
Drive Output (Pin 7) of the device goes low for protection.
The circuit automatically resumes operation when the
feedback current becomes lower than 107% of the
reference current I ref .
The maximum OVP threshold is limited to 230 m A which
corresponds to 230 m A × 1.92 M W + 2.5 V = 444.1 V when
R FB = 1.92 M W (680 k W + 680 k W + 560 k W ) and
V FB1 = 2.5 V (for the worst case referring to Figure 11).
Hence, it is generally recommended to use 450 V rating
output capacitor to allow some design margin.
Undervoltage Protection (UVP)
I CC
to enable the NCP1653 to operate. Hence, UVP happens
when the output voltage is abnormally undervoltage, the
FB pin (Pin 1) is opened, or the FB pin (Pin 1) is manually
pulled low.
Soft ? Start
The device provides no output (or no duty ratio) when the
V control (Pin 2) voltage is zero (i.e., V control = 0 V). An
external capacitor C control connected to the V control pin
provides a gradually increment of the V control voltage (or
the duty ratio) in the startup and hence provides a soft ? start
feature.
Current Sense
The device senses the inductor current I L by the current
sense scheme in Figure 36. The device maintains the
voltage at the CS pin (Pin 4) to be zero voltage (i.e.,
V S ≈ 0 V) so that (eq.10) can be formulated.
I L
CS
+
Gnd
R CS I L ?
Figure 36. Current Sensing
This scheme has the advantage of the minimum number
of components for current sensing and the inrush current
limitation by the resistor R CS . Hence, the sense current I S
represents the inductor current I L and will be used in the
PFC duty modulation to generate the multiplier voltage
V M , Overpower Limitation (OPL), and overcurrent
protection.
I CC2
Shutdown
Operating
Overcurrent Protection (OCP)
Overcurrent protection is reached when I S is larger than
I S(OCP) (200 m A typical). The offset voltage of the CS pin
is typical 10 mV and it is neglected in the calculation.
12% I ref
FB
ref
RSIS(OCP) RS
IL(OCP) +
200 m A
+
I stdn
8% I I
Figure 35. Undervoltage Protection
When the feedback current I FB is less than 8% of the
reference current I ref (i.e., the output voltage V out is less
than 8% of its nominal value), the device is shut down and
consumes less than 50 m A. The device automatically starts
operation when the output voltage goes above 12% of the
nominal regulation level. In normal situation of boost
converter configuration, the output voltage V out is always
greater than the input voltage V in and the feedback current
I FB is always greater than 8% and 12% of the nominal level
Hence, the maximum OCP inductor current threshold
I L(OCP) is obtained in (eq.15).
(eq.18)
RCS RCS
When overcurrent protection threshold is reached, the
Drive Output (Pin 7) of the device goes low. The device
automatically resumes operation when the inductor current
goes below the threshold.
Input Voltage Sense
The device senses the RMS input voltage V ac by the
sensing scheme in Figure 37. The internal current mirror is
with a typical 4 V offset voltage at its input so that the
current I vac can be derived in (eq.9). An external capacitor
C vac is to maintain the In pin (Pin 3) voltage in the
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