Datasheet

FLTR2

R15

C11

V+

LM3448

11 FLTR2

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Design Guide

13.1 DCM Flyback Converter

This LED driver is designed to accurately emulate an incandescent light bulb and therefore behave as an

emulated resistor. The resistor value is determined based on the LED string configuration and the desired

output power. The circuit then operates in open-loop, with a fixed duty cycle based on a constant on-time

and constant off-time that is set by selecting appropriate circuit components. Like an incandescent lamp,

the driver is compatible with both forward and reverse phase dimmers. A key aspect of this design is that

the converter operates in discontinuous conduction mode (DCM). DCM is implemented by ensuring that

the flyback transformer current reaches zero before the end of the switching period.

By injecting a voltage proportional to the line voltage at the FLTR2 pin (see Figure 27), the LM3448 circuit

is essentially turned into a constant power flyback converter operating in discontinuous conduction mode

(DCM).

Figure 27. Direct Line-Injection Circuit

Figure 28. FLTR2 Waveform with No Dimmer

The LM3448 normally works as a constant off-time regulator, but by injecting a 1.0V

rectified AC voltage

into the FLTR2 pin, the on-time can be made to be constant. With a DCM flyback converter the primary

side current, i

(t), needs to increase as the rectified input voltage, V

(t), increases as shown in the

following equations,

(1)

or,

(2)

Therefore a constant on-time (since inductor L is constant) can be obtained.

SNOA554C–October 2011–Revised May 2013 AN-2090 LM3448 -120VAC, 6W Isolated Flyback LED Driver

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