VAC

TSNS

LM3447

FLT1

FLT2

BIAS

HOLD

AUX

VCC

FF

INV

COMP

GATE

ISNS

GND

+

–

+

–

R

AC

C

PFC

C

FLT

R

FLT

R

NTC

C

FF

C

COMP

R

BS

R

HLD2

C

VCC

R

SN

R

AUX1

R

AUX2

Q

PASS

Q

SW

C

BULK

R

O

PRI

SEC

N

P

N

S

AUX

C

Y1

n:1

LM3447

SNOSC65 A – APRIL 2012 – REVISED MAY 2012

www.ti.com

Figure 18. Typical Primary Side Power Regulated Flyback LED Driver

APPLICATION INFORMATION

DESCRIPTION

LM3447 is an AC-DC power factor correction (PFC) controller for phase-cut dimmer compatible LED lighting

applications. The device incorporates an innovative primary side input power regulation technique for controlling

the LED light output over a wide input AC voltage and ambient temperature range. Operating LEDs with constant

power allows the controller to compensates for the LED forward voltage variations caused by temperature

modulation and LED aging. This also provides improved lamp lumen output maintenance and higher luminous

efficacy.

Smooth, flicker free LED dimming is performed by varying the power regulation set-point based on the dimmer

phase angle. The device includes internal angle detection and decoding circuitry to accurately interpret the phase

angle from a forward phase (leading edge) and reverse phase (trailing edge) based dimmers Power factor

correction (PFC) with low input current total harmonic distortion (THD) is maintained by forcing discontinuous

conduction mode (DCM) using a trimmed internal oscillator and valley detect circuitry.

These features, along with LED open circuit and short circuit protection, LED thermal foldback and cycle-by-cycle

FET overcurrent protection, make the LM3447 an ideal device for implementing a compact single stage isolated

Flyback AC-DC LED driver for 5–30W power output range. In addition, it is also possible to configure LM3447

with minor modifications to control SEPIC and Cúk based dimmable AC-DC PFC LED drivers. In this datasheet,

a discussion of the LM3447 functionality is presented using a typical Flyback LED driver circuit, as shown in

Figure 18.

10

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