LM3445MM datasheet(8/26 Pages) NSC

8 / 26 page

Application Information

FUNCTIONAL DESCRIPTION

The LM3445 contains all the necessary circuitry to build a line-

powered (mains powered) constant current LED driver whose

output current can be controlled with a conventional triac dim-

mer.

OVERVIEW OF PHASE CONTROL DIMMING

A basic "phase controlled" triac dimmer circuit is shown in

Figure 2.

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FIGURE 2. Basic Triac Dimmer

An RC network consisting of R1, R2, and C1 delay the turn

on of the triac until the voltage on C1 reaches the trigger volt-

age of the diac. Increasing the resistance of the potentiometer

(wiper moving downward) increases the turn-on delay which

decreases the on-time or "conduction angle" of the triac (

θ).

This reduces the average power delivered to the load. Voltage

waveforms for a simple triac dimmer are shown in Figure 3.

Figure 3a shows the full sinusoid of the input voltage. Even

when set to full brightness, few dimmers will provide 100%

on-time, i.e., the full sinusoid.

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FIGURE 3. Line Voltage and Dimming Waveforms

Figure 3b shows a theoretical waveform from a dimmer. The

on-time is often referred to as the "conduction angle" and may

be stated in degrees or radians. The off-time represents the

delay caused by the RC circuit feeding the triac. The off-time

be referred to as the "firing angle" and is simply 180° -

θ.

Figure 3c shows a waveform from a so-called reverse phase

dimmer, sometimes referred to as an electronic dimmer.

These typically are more expensive, microcontroller based

dimmers that use switching elements other than triacs. Note

that the conduction starts from the zero-crossing, and termi-

nates some time later. This method of control reduces the

noise spike at the transition.

Since the LM3445 has been designed to assess the relative

on-time and control the LED current accordingly, most phase-

control dimmers, both forward and reverse phase, may be

used with success.

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