2

90

2

x SIN(135

= 45V

2

#stages

x SIN(T)

t

135°

90°

45°

LM3445

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SNVS570L – JANUARY 2009 – REVISED MAY 2013

Figure 30. AC Line with Firing Angles

will always be:

(27)

Once the triac is firing at an angle greater than 90° the peak voltage will lower and equal to:

(28)

The purpose of the valley fill circuit is to allow the buck converter to pull power directly off of the AC line when

the line voltage is greater than its peak voltage divided by two (two stage valley fill circuit). During this time, the

capacitors within the valley fill circuit (C7 and C8) are charged up to the peak of the AC line voltage. Once the

line drops below its peak divided by two, the two capacitors are placed in parallel and deliver power to the buck

converter. One can now see that if the peak of the AC line voltage is lowered due to variations in the line voltage,

(29)

Example:

Valley-Fill = two stage

(30)

Depending on what type and value of capacitors are used, some derating should be used for voltage droop when

the capacitors are delivering power to the buck converter. When the triac is firing at 135° the current through the

LED string will be small. Therefore the droop should be small at this point and a 5% voltage droop should be a

sufficient derating. With this derating, the lowest voltage the buck converter will see is about 42.5V in this

example.

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