LM3450 Datasheet, PDF (22/40 Pages) National Semiconductor (TI) – LED Driver with Active Power Factor Correction and Phase Dimming Decoder

LM3450
SNVS681D – NOVEMBER 2010 – REVISED MAY 2013
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DYNAMIC HOLD
A forward phase “triac” dimmer requires a minimum amount of current to be flowing through it during the entire
conduction angle. This is referred to as hold current. If the minimum hold current requirement is not met, the triac
will shut off (misfire). During normal operation, the converter will demand some amount of input current.
However, at any point during the cycle, the input current can be low enough to cause a misfire.
During an LM3450/50A sampling period, the triac should not misfire or the decoded angle will be inaccurate as
shown in Figure 32. Since the triac is asymmetrical phase-to-phase, misfires can occur at different points in the
waveform. After the triac misfires, the voltage returns to zero exponentially. This can create a large difference
between decoded angles which can be observed as a “fluttering” of the light.
Misfire
356 mV
V_AC
Original
Angle
D_IN
1
2
Figure 32. Forward Phase Waveform
To ensure the triac does not misfire during a sampling period and the angle is correctly decoded, a dynamic hold
function is enabled. The input current is sensed with a resistor (R_SEN) from GND to I_SEN (the return of the full
bridge rectifier). If the voltage across this resistor is less than 200mV, the device adds holding current via the
HOLD circuitry to maintain 200mV across R_SEN.
The hold current is added by linearly adjusting the gate voltage of Q_HLD as shown in Figure 31. As the gate
voltage of Q_HLD is increased, the HOLD pin voltage decreases, forcing a voltage across the resistance (R_HLD)
from the source of Q_PS to HOLD. This extra current is drawn from the input through the triac, but is not
processed by the converter. Figure 33 shows a typical dynamic hold waveform of the LM3450 where interval 1 is
a non-sampled conduction angle, 2 is the firing angle, and 3 is a sampled conduction angle. It should be noted
that using the LM3450A will ensure every conduction angle looks like interval 3 in Figure 33.
I_IN-MIN-REG
I_IN
Misfire
Added
Hold
Current
1
2
3
~V_CC
V_HOLD
1
2
Figure 33. Dynamic Hold Waveform
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