Datasheet

FLTR2

R20

C18

V+

LM3448

11 FLTR2

Design Guide

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By injecting a voltage proportional to the line voltage at the FLTR2 pin (see Figure 24), the LM3448 circuit

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

(DCM).

Figure 24. Direct Line-Injection Circuit

Figure 25. 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.

By using the line voltage injection technique, the FLTR2 pin has the voltage wave shape shown in

Figure 25 on it with no TRIAC dimmer in-line. Peak voltage at the FLTR2 pin should be kept below 1.25V

otherwise current limit will be tripped. Capacitor C18 in conjunction with resistor R20 acts a filter for noise.

Using this technique a power factor greater than 0.90 can be achieved.Figure 26 shows how a constant

on-time is maintained.

AN-2091 LM3448 - 230VAC, 6W Isolated Flyback LED Driver SNOA555C–April 2011–Revised May 2013

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