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VFB
ϕ
ϕ
ϕ
OL
ϕ
OS
OL
OS
VFB(
VFB(
ϕ
)
ϕ
)
Phase
Angle
Phase
Angle
0°
180°
0°
180°
VLINE
VTRIAC
ϕ
ϕ
Phase
Angle
Phase
Angle
0°
0°
Phase
Angle
Conduction
Angle
Conduction
Angle
Phase
Angle
Leading Edge
TRIAC Dimmers
Trailing Edge
Dimmers
PI-5894a-091010
Figure 6. Feedback Voltage vs. Phase Angle Dimming Characteristics.
TRIAC (Phase-Controlled) Dimming
j
The LinkSwitch-PL integrates several features to improve
( OS) threshold is exceeded, VFB and the output LED current
are reduced until a second phase angle threshold is reached.
At this point, with the TRIAC conduction angle being very
limited, the IC runs open loop at constant frequency and duty
cycle ( OL region) and the integrated power MOSFET
processes as much power as the heavily chopped input voltage
will allow creating a light output that is deeply dimmed.
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.
When the conduction angle decreases, the voltage at the
FEEDBACK pin decreases causing the average LED current to
decrease.
j
The 520 mV clamping feedback threshold is also linearly reduced
during dimming to control LED current ripple.
The FEEDBACK pin reference voltage adjustment is initiated at
approxi-mately 25% of the AC line half-cycle duration. When this
4
Rev. A 11/01/10
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