LM3444-230VAC NSC [National Semiconductor], LM3444-230VAC Datasheet - Page 16

• Manufacturer Part Number: LM3444-230VAC
• Description: 8W Isolated Flyback LED Driver Industrial and Commercial Lighting
• Manufacturer: NSC [National Semiconductor]

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Circuit Analysis and Explanations
INJECTING LINE VOLTAGE INTO FILTER (ACHIEVING
PFC > 0.98)
If a small portion (750mV to 1.00V) of line voltage is injected
at FILTER of the LM3444, the circuit is essentially turned into
a constant power flyback as shown in Figure 5.
The LM3444 works as a constant off-time controller normally,
but by injecting the 1.0V
FIGURE 5. Line Voltage Injection Circuit
Pk
rectified AC voltage into the FIL-
FIGURE 7. Typical Operation of FILTER Pin
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TER pin, the on-time can be made to be constant. With a DCM
Flyback, Δi needs to increase as the input voltage line in-
creases. Therefore a constant on-time (since inductor L is
constant) can be obtained.
By using the line voltage injection technique, the FILTER pin
has the voltage wave shape shown in Figure 6 on it. Voltage
at V
limit is tripped. C11 is small enough not to distort the AC signal
but adds a little filtering.
Although the on-time is probably never truly constant, it can
be observed in Figure 7 how (by adding the rectified voltage)
the on-time is adjusted.
For this evaluation board, the following resistor values are
used:
R3 = R8 = 309 kΩ
R20 = 1.91 kΩ
Therefore the voltages observed on the FILTER pin will be as
follows for listed input voltages:
For VIN = 180V
For VIN = 230V
For VIN = 265V
Using this technique, a power factor greater than 0.98 can be
achieved without additional passive active power factor con-
trol (PFC) circuitry.
FILTER
peak should be kept below 1.25V. At 1.25V current
FIGURE 6. FILTER Waveform
RMS
RMS
RMS
, V
, V
, V
FILTER, Pk
FILTER, Pk
FILTER, Pk
= 0.78V
= 1.00V
= 1.15V
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