Datasheet

NCL30000
http://onsemi.com
12
ON
OFF
MOSFET
I
S
(t)
I
pr(peak)
I
pr(t)
I
in(peak)
I
in
(t)
Figure 24. Theoretical Switching Waveform
The LED current is compared to a reference and an error
signal is passed to the NCL30000 controller to maintain the
desired average level. This error signal adjusts the on-time
of the power switch to pass the required energy through the
flyback transformer to achieve proper regulation of the LED
load. Just like in a traditional PFC boost converter, the loop
bandwidth must be low enough to filter out the twice line
frequency ripple otherwise the power factor correction
element of the circuit will be compromised. In the event of
an open LED fault, a constant voltage loop regulates the
output voltage across the output capacitor to assure safe
operation.
The NCL30000 (refer to the block diagram − Figure 1) is
composed of 4 key functional blocks along with protection
circuitry to ensure reliable operation of the controller.
 On-time Control
 Zero Current Detection Control
 MOSFET Gate Driver
 Startup and V
CC
Management
On Time Control
The on-time control circuitry (Figure 25) consists of a
precision current source which charges up an external
capacitor (C
t
) in a linear ramp. The voltage on C
t
(after
removing an internal offset) is compared to an external
control voltage and the output of the comparator is used to
turn off the output driver thus terminating the switching
cycle. A signal from the driver is fed back to the on-time
control block to discharge the C
t
capacitor thus preparing the
circuit for the start of the next switching cycle.
The state of V
control
is determined by the external
regulation loop and varies with the rms input voltage and the
output load. To achieve high power factor, the regulation
loop is designed so that in steady state, the V
control
value is
held constant over a line half cycle. This results in fixed on
time operation. The range of on-time is determined by the
charging slope of the C
t
capacitor and is clamped at 4.93 V
nominal. The C
t
capacitor is sized to ensure that the required
on-time is reached at maximum output power and the
minimum input line voltage condition. Because the ramp has
a wide dynamic range, the control loop can accommodate
wide variation of line voltage and load power range.