VVF-CAP =

2

VAC-RMS

3

+

+

D3

C7

C9

C10

D4

D8

D9

VBUCK

V+

+

-

+

-

VBUCK

VBUCK

LM3444

www.ti.com

SNVS682D – NOVEMBER 2010 – REVISED DECEMBER 2015

Feature Description (continued)

Figure 11. Two Stage Valley-Fill Circuit when AC Line is Low

A three stage valley-fill circuit performs exactly the same as two-stage valley-fill circuit, except now three

capacitors are charged in series when the line voltage decreases, as shown in Equation 2:

(2)

Diode D3 is reverse-biased and three capacitors are in parallel to each other.

The valley-fill circuit can be optimized for power factor, voltage hold-up, and overall application size and cost.

The LM3444 operates with a single-stage or a three-stage valley-fill circuit as well. Resistor R8 functions as a

current limiting resistor during start-up, and during the transition from series to parallel connection. Resistors R6

and R7 are 1-M

Ω bleeder resistors, and may or may not be necessary for each application.

7.3.4 Buck Converter

The LM3444 is a buck controller that uses a proprietary constant off-time method to maintain constant current

through a string of LEDs. While transistor Q2 is on, current ramps up through the inductor and LED string. A

resistor R3 senses this current and this voltage is compared to the reference voltage at FILTER. When this

sensed voltage is equal to the reference voltage, transistor Q2 is turned off and diode D10 conducts the current

through the inductor and LEDs. Capacitor C12 eliminates most of the ripple current seen in the inductor. Resistor

R4, capacitor C11, and transistor Q3 provide a linear current ramp that sets the constant off-time for a given

output voltage.

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