NCL30002 Datasheet, PDF (16/17 Pages) ON Semiconductor – Power Factor Corrected Buck LED Driver

Shutdown
NCL30002
Bias
R₁
MFP
R₂
R_FB
+ ^OVP
−
V_OVP
− ^UVP
+
V_UVP
EA
−
+
gm
V_REF
OVP Fault
POWER GOOD
UVP Fault
(Enable EA)
COMP
C_COMP
V_Control
Figure 29. Multi−Function Pin Operation
The positive input of the transconductance amplifier is
connected to a 2.51 V (nominal) reference. A filtered line
feed-forward signal (see Figure 2) is connected to the
negative input of the error amplifier and used to control the
on-time of controller.
V_CC Management
The NCL30002 incorporates a supervisory circuitry to
manage the startup and shutdown of the circuit. By
managing the startup and keeping the initial startup current
at less than 35 mA, a startup resistor connected between the
rectified ac line and V_CC charges the V_CC capacitor to
V_CC(on). Turn on of the device occurs when the startup
voltage has exceeded 12 V (nominal) when the internal
reference and switching logic are enabled. A UVLO
comparator with a hysteresis of 2.5 V nominal gives ample
time for the device to start switching and allow the bias from
the auxiliary winding to supply V_CC.
Design Tool
The NCL30002 implements a unique control method to
achieve high power and superior current regulation even
though the average current is not directly sensed. There are
a number of design tradeoffs that can be made between peak
switch current, inductor size, and desired power factor that
can impact the current regulation accuracy, efficiency, and
physical size. These tradeoffs can be made by adjusting the
amount of line feed forward applied, selecting the amount of
time where the controller is operating in mode 2 and 3, as
well as factoring in the LED forward voltage range. To
simplify the component selection process and allow the
designer to interactively make these tradeoffs,
ON Semiconductor has developed an EXCELt based
Design Guide which allows step-by-step analysis. This tool
is available at onsemi.com along with a supporting
application note that illustrates a complete design and
provides typical application performance.
http://onsemi.com
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