2

3

+

+

D3

C7

C9

C10

D4

D8

D9

V+

+

-

+

-

2

2

LM3445

SNVS570L – JANUARY 2009 – REVISED MAY 2013

www.ti.com

The peak voltage of a two stage valley-fill capacitor is:

(1)

As the AC line decreases from its peak value every cycle, there will be a point where the voltage magnitude of

the AC line is equal to the voltage that each capacitor is charged. At this point diode D3 becomes reversed

voltage.

Figure 20. 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 now charged in series, and when the line voltage decreases to:

(2)

Diode D3 is reversed 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

LM3445 will operate 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.

BUCK CONVERTER

The LM3445 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 FLTR2. 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.

14

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