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Design Information
Figure 12. CRM Flyback Current Waveforms
The input current shaping happens instantly in CRM due to the feed-forward mechanism; however, the
converter must also regulate the flyback output voltage with a traditional feedback loop. This is
accomplished with a narrow bandwidth error amplifier coupled with energy storage capacitance at the
output to limit the twice line frequency ripple. The output of the error amplifier is multiplied with the scaled
rectified AC voltage to achieve both input current shaping and output voltage regulation. Refer to the
datasheet for a more detailed explanation of the power factor controller.
The LM3450A also has a phase decoder that interprets the phase dimming angle and maps it to a 500Hz
PWM open-drain output at the DIM pin. This signal is directly connected to an opto-isolator to send across
the isolation boundary to the second stage LED driver. In addition, the LM3450A provides a dynamic hold
circuit to ensure that the holding current requirement is satisfied in forward phase dimmers. Refer to the
datasheet for a more detailed explanation of the phase dimmer decoder.
9.2
2
ND
Stage - Buck LED Driver
The second stage of the evaluation board is a buck LED driver controlled with the LM3409HV. The input
to this stage is the flyback output voltage and the output is a regulated constant current of 700mA to a
stack of <45V of LEDs. The LM3409HV is a hysteretic PFET controller using peak current detection and a
constant off-timer to provide regulated LED current with a constant switching frequency ripple. Coupled
with the flyback energy storage capacitance, the LM3409HV is able to remove all 120HZ ripple content
from the LED output. The 500Hz PWM signal from the first stage is used as the dimming input to the
LM3409HV. The output of the opto-isolator is connected directly to the EN pin of the LM3409HV to provide
a PWM dimmed LED current according to the detected phase angle at the primary.
The LM3409HV design is not included in this document. Refer to AN-1953 for a detialed design
procedure. The specifications for the second stage are:
•
Nominal Input Voltage = 50V
•
Regulated LED Current = 700mA
•
Nominal LED Stack Voltage = 45V
•
Switching Frequency at Nominal Input = 100kHz
•
Inductor/LED Current Ripple = 115mA
9.3
CRM Flyback Converter
Operating Points
9
SNVA485B – June 2011 – Revised May 2013
AN-2150 LM3450A Evaluation Board
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