User manual

FILTER

R15 C11

FILTER

COFF

FILTER

VCC

GND

GATE

ISNS

LM3444

V+

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Circuit Analysis and Explanations

16 Circuit Analysis and Explanations

16.1 Injecting Line Voltage Into FILTER (Achieving PFC > 0.99)

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 23.

Figure 23. Line Voltage Injection Circuit

The LM3444 works as a constant off-time controller normally, but by injecting the 1.0V rectified AC voltage

into the FILTER pin, the on-time can be made to be constant. With a DCM Flyback, Δi needs to increase

as the input voltage line increases. 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 24 on it. Voltage at V

FILTER

peak should be kept below 1.25V. At 1.25V current 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 25 how (by adding the

rectified voltage) the on-time is adjusted.

Figure 24. FILTER Waveform

SNVA454E–September 2010–Revised May 2013 AN-2082 LM3444 -120VAC, 8W Isolated Flyback LED Driver

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