1

IIN-MIN-REG

Added

Hold

Current

~VCC

2

3

1

VHOLD

IIN

2

Misfire

1

356 mV

Misfire

2

Original

Angle

VAC

DIN

LM3450

SNVS681D – NOVEMBER 2010 – REVISED MAY 2013

www.ti.com

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.

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 (RSEN) from GND to ISEN (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 RSEN.

The hold current is added by linearly adjusting the gate voltage of QHLD as shown in Figure 31. As the gate

voltage of QHLD is increased, the HOLD pin voltage decreases, forcing a voltage across the resistance (RHLD)

from the source of QPS 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.

Figure 33. Dynamic Hold Waveform

22

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