Datasheet

DELAY
DELAY
(a)
(b)
(c)
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LM3448
www.ti.com SNOSB51C –SEPTEMBER 2011–REVISED MAY 2013
An RC network consisting of R1, R2, and C1 delay the turn on of the TRIAC until the voltage on C1 reaches the
trigger voltage 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.
Figure 16. Line Voltage and Dimming Waveforms
Voltage waveforms for a simple TRIAC dimmer are shown in Figure 16. Figure 16(a) 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).
Figure 16(b) 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 can be referred to as the "firing angle" and is simply (180° - θ).
Figure 16(c) shows a waveform from a 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 terminates some time later. This method of
control reduces the noise spike at the transition. Since the LM3448 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.
A bridge rectifier converts the line (mains) voltage of Figure 17(b) into a series of half-sines as shown in
Figure 17(a).
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