Rev. A 06/10

4

LNK454/456-458/460

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This document contains information on a new product. Specifications and information herein

are subject to change without notice.

PRELIMINARY

TRIAC (Phase-Controlled) Dimming

The LinkSwitch-PL integrates several features to improve

dimming range and reduce external circuit complexity when

using a phase-controlled TRIAC dimmer. The output LED

current is controlled by the FEEDBACK pin voltage which

changes proportionally to the TRIAC dimmer conduction angle.

The conduction angle decreases, the voltage at the FEEDBACK

pin decreases causing the average LED current to decrease.

The FEEDBACK pin voltage adjustment is initiated at approxi-

mately 20% of the main half-cycle duration. When this (jOS)

threshold is exceeded, V

reduced until a second phase angle threshold is reached.

When this happens, with the TRIAC conduction angle being

very limited, the IC runs open loop (jOL region) and the

integrated MOSFET processes as much power as the heavily

chopped input voltage will allow creating a light output that is

deeply dimmed.

The 520 mV clamping feedback threshold is also linearly reduced

during dimming to control LED current ripple.

To help maintain the holding current of the TRIAC in the dimmer,

LinkSwitch-PL extends the MOSFET on-time towards the zero

crossings of the AC input voltage providing an active bleed or

holding current function. This on-time extension is carefully

limited to avoid deterioration of input current harmonic content

and limit total harmonic distortion (THD).

Figure 6.

Feedback Voltage vs. Phase Angle Dimming Characteristics.

ϕOL

ϕ

ϕ

180°

0°

Phase

Angle

Phase

Angle

TRIAC

Conduction

TRIAC

Conduction

Leading Edge

TRIAC Dimmers

Trailing Edge

TRIAC Dimmers

180°

0°

0°

0°

PI-5894a-052610

ϕOS

ϕOL

ϕOS

Phase

Angle

Phase

Angle

Phase

Angle

Phase

Angle