9

Allegro MicroSystems, Inc.

115 Northeast Cutoff

Worcester, Massachusetts 01615-0036 U.S.A.

1.508.853.5000; www.allegromicro.com

48102.002

LED Drivers

LC5200 Series

About TRIAC Dimming Control (Phase Control)

Commonly used TRIAC dimmers are designed for mainly resis-

tive loads and they require TRIAC holding current for proper

phase controls. LC5200 series does not respond to this type of

dimmers because it does not have function to create the holding

current during phase off period.

Power Factor Improvement

Making the LED current proportional to the AC input voltage

improves the power factor, and it can be realized using LC5200

series REF pin function. Figure 8 shows the application circuit.

There is no AC rectification capacitor, and R2 and R3 divide the

AC voltage to create a proportional low voltage as the AC voltage

for the REF pin. This way, LED current follows the AC voltage

shape and improves the power factor. In case the REF voltage

fluctuates widely, place a clamp diode in parallel with R2 to pro-

tect the REF pin. In that case, the REF voltage becomes distorted

(lower waveform in figure 8) and could cause the power factor to

decrease.

Figure 9 shows actual waveforms of the operation. Panel A shows

operation with fixed REF pin voltage, and panel B shows opera-

tion with AC proportional REF pin voltage. For both operations,

there is no AC rectification capacitor used. The yellow wave-

form is the AC input current, and the black waveform is a 2 kHz

low pass filtered waveform. In panel B, the current forms a sine

waveform, which means the power factor is improved.

Figure 8. Power factor improvement circuit

AC proportional voltage, PF = 82.9%

LC5200

GND

PF improved

PF loss with clamping

REF

R

Optional

clamp diode

2

VBB

BB

Ref

v

R

R

R

v

3

2

2





v

Ref

2 kHz LPF

2 kHz LPF