ncp1608 ON Semiconductor, ncp1608 Datasheet - Page 12

• Manufacturer Part Number: ncp1608
• Description: Critical Conduction Mode Pfc Controller Utilizing A Transconductance Error Amplifier
• Manufacturer: ON Semiconductor

linearly to the peak value. When the switch opens, the
inductor current linearly decreases to zero. When the
inductor current decreases to zero, the drain voltage of the
switch (V
switching cycle does not begin, then V
V
the result that high power factor in CrM operation is
achieved when the on time (t
during an ac cycle and is calculated using Equation 1.
Where P
is the efficiency, and Vac is the rms input voltage.
illustrated in Figure 28. The on time is constant, but the off
time varies and is dependent on the instantaneous line
voltage. The constant on time causes the peak inductor
current (I
NCP1608 represents an ideal method to implement a
constant on time CrM control in a cost−effective and robust
solution by incorporating an accurate regulation circuit, a
low current consumption startup circuit, and advanced
protection features.
in
When the switch is closed, the inductor current increases
A description of the switching over an ac line cycle is
. A derivation of equations found in AND8123 leads to
V
out
out
drain
R
R
L(peak)
C
out1
out2
Control
is the output power, L is the inductor value, h
COMP
) is floating and begins to decrease. If the next
FB
) to scale with the ac line voltage. The
t on +
R
FB
2 @ P
h @ Vac
on
Figure 29. Error Amplifier and On Time Regulation Circuits
out
+
) of the switch is constant
+
+
@ L
V
2
Control
V
V
V
OVP
−
+
REF
+
−
−
+
UVP
EA
gm
OVP
UVP
drain
rings towards
(Enable EA)
OVP Fault
http://onsemi.com
(eq. 1)
POK
UVP Fault
t
on
12
t
on(MAX)
t
PWM
Error Amplifier Regulation
an internal error amplifier (EA). The negative terminal of
the EA is pinned out to FB, the positive terminal is
connected to a 2.5 V ± 1.6% reference (V
output is pinned out to Control (Figure 29).
that the FB pin voltage is only determined by the resistor
divider network connected to the output voltage, not the
operation of the amplifier. This enables the FB pin to be
used for sensing overvoltage or undervoltage conditions
independently of the error amplifier.
Figure 28. Inductor Waveform During CrM Operation
The NCP1608 regulates the boost output voltage using
A feature of using a transconductance error amplifier is
MOSFET
V
I
I
in(peak)
in(peak)
L(peak)
Slope +
OFF
Ct
ON
(offset)
PWM BLOCK
I
charge
Ct
V
Control
V
EAH
REF
V
in
(t)
), and the EA
I
L
(t)
I
in
(t)