FF

FF

REF

AC

M IN S

R

G

V

=

R

4

L P

f

p

VAC

LM3447

VCC

FF

INV

COMP

GATE

+

–

INPUT

FEEDFORWARD

I

FF = IVAC / 10

INTERNAL

REGULATORS

REFERENCE

GENERATOR

CTRL

RAMP

PWM

RAC

RFF

CFF

CCOMP

CVCC

RSN

QSW

PRI

NP

LOGIC

&

CONTROL

S

Q

Q

R

VAL

VAL

Rectified AC Line

VREC(t)

LM

GND

,

2

OUT (AVG)

IN (AVG)

IN (RM S)

LED (AVG)

FLY

F LY

OUT

OUT

OUT

e

P

P

V

I

=

= η

= η

V

V

V

R

LM3447

SNOSC65 A – APRIL 2012 – REVISED MAY 2012

www.ti.com

(7)

The LED current will have a ripple component varying at twice line frequency due to the PFC operation. The

magnitude of the ripple component is based on the energy storage capacitor connected in parallel with the LED

string at the output of Flyback PFC. In typical application, a low voltage aluminum electrolytic bulk capacitor is

used as an energy storage device and is connected across the LED string to limit the ripple current within an

acceptable range.

INPUT POWER REGULATION AND INPUT VOLTAGE FEEDFORWARD CONTROL

Using the LM3447, it is possible to regulate the LED current by implementing a control scheme using the duty

cycle, D, as the control variable. The duty cycle is generated using an internal GM error-amplifier and a fixed

frequency, triggered ramp generator, as shown in Figure 23. This technique should not be confused with other

current mode control schemes where switch current, ISW, is used for control.

With the LM3447, LED current can be directly controlled using a series sense resistor and a conventional closed-

loop feedback control scheme. Typically, for systems that need galvanic isolation between primary and

secondary sides of the transformer, feedback control is complicated and expensive as it requires an additional

signal processing amplifier and an opto-isolator. For improved luminous efficacy and simplicity, the LM3447

incorporates an innovative primary side input power regulation scheme based on input voltage feedforward

control techniques. By commanding input power, the DCM Flyback PFC output is matched with the LED load

characteristics to achieve indirect control of LED string current. The feedforward loop, consisting of input voltage

sensing circuitry, the GM error amplifier and PWM comparator, is able to reject any input voltage disturbance by

adjusting the duty cycle, thus achieving tight line regulation.

Figure 23. Feedforward Control Circuit

The reference power level, PIN, for the LM3447, is set choosing resistors, RFF and RAC, based on the

magnetizing inductance, LM, the internal reference voltage, VREF, the switching frequency, fS and the feedforward

gain, GFF, such that

(8)

The feedforward gain, GFF = IVAC/IFF = 10 and internal reference voltage VREF = 1 V.

14

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