LM3445MM datasheet(12/26 Pages) NSC

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The valley-fill circuit allows the buck regulator to draw power

throughout a larger portion of the AC line. This allows the ca-

pacitance needed at V

BUCK to be lower than if there were no

valley-fill circuit, and adds passive power factor correction

(PFC) to the application. Besides better power factor correc-

tion, a valley-fill circuit allows the buck converter to operate

while separate circuitry translates the dimming information.

This allows for dimming that isn’t subject to 120Hz flicker that

can be perceived by the human eye.

VALLEY-FILL OPERATION

When the “input line is high”, power is derived directly through

D3. The term “input line is high” can be explained as follows.

The valley-fill circuit charges capacitors C7 and C9 in series

(see figure 9) when the input line is high.

30060319

FIGURE 9. Two stage Valley-Fill Circuit when AC Line is

High

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

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 biased, and the ca-

pacitors are placed in parallel to each other (figure 10), and

BUCK equals the capacitor voltage.

30060321

FIGURE 10. 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:

Diode D3 is reversed biased and three capacitors are in par-

allel to each other.

The valley-fill circuit can be optimized for power factor, volt-

age hold up and overall application size and cost. The

LM3445 will operate with a single stage or a three stage val-

ley-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 con-

stant 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. Re-

sistor 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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