User manual

R15

C15

V+

LM3448

11 FLTR2

C14

INJECT

Design Guide

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13.1 Buck Converter

The following section explains how to design a non-isolated buck converter using the LM3448. Refer to

the LM3448 datasheet for specific details regarding the function of the LM3448 device. All reference

designators refer to the Evaluation Board Schematic in Figure 25 unless otherwise noted. The circuit

operates in open-loop based on a 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.

AC-Coupled Line Injection

By injecting a voltage V

INJECT

which is proportional to the line voltage into the FLTR2 pin (see Figure 26),

input current shaping is obtained which improves power factor performance. By AC-coupling the V

INJECT

signal through capacitor C14, improved line-regulation of the LED current is also achieved (see

Figure 27).

Figure 26. FLTR2 Waveform with No Dimmer

Figure 27. AC-Coupled Line-Injection Circuit

Figure 28 shows how line shaping of the input current is implemented. Peak voltage at the FLTR2 pin

should be kept below 1.25V otherwise current limit will be tripped. A good starting point is to set up the

resistor divider consisting of resistors R2, R7 and R15 to provide a V

INJECT

peak input voltage of 1.0V at

the input of capacitor C14 at the nominal input voltage. Recommended values for the AC-coupling

capacitor C14 is 0.47µF and for the FLTR2 capacitor C15 is 0.1µF.

With a 1.0V V

INJECT

voltage, the voltage at the FLTR2 pin at the maximum and minimum input voltages can

be calculated using the following equations,

(1)

These V

FLTR2

voltages will be used later to determine ripple and peak inductor currents.

AN-2127 LM3448 A19 Edison Retrofit Evaluation Board SNOA559B–October 2011–Revised May 2013

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