VBUCK(MIN) =

2

90

2

x SIN(135

o)

= 45V

VBUCK(MIN) =

2

VAC-RMS(MIN)

#stages

x SIN(T)

VAC-RMS-PK 2 x SIN(T)

VAC-RMS-PK 2

VAC

t

135°

90°

45°

VPEAK

LM3445

SNVS570M – JANUARY 2009 – REVISED NOVEMBER 2015

www.ti.com

Application Information (continued)

8.1.6.2 Determining Maximum Number of Series Connected LEDs Allowed

The LM3445 is an off-line buck topology LED driver. A buck converter topology requires that the input voltage

(VBUCK) of the output circuit must be greater than the voltage of the LED stack (VLED) for proper regulation. One

must determine what the minimum voltage observed by the buck converter will be before the maximum number

of LEDs allowed can be determined. Two variables will have to be determined in order to accomplish this.

1. AC line operating voltage. This is usually 90 VAC to 135 VAC for North America. Although the LM3445 can

operate at much lower and higher input voltages a range is needed to illustrate the design process.

2. How many stages are implemented in the valley-fill circuit (1, 2 or 3).

In this example the most common valley-fill circuit will be used (two stages).

Figure 27. AC Line with Firing Angles

Figure 28 shows three TRIAC dimmed waveforms. One can easily see that the peak voltage (VPEAK) from 0° to

90° will always be:

(27)

Once the TRIAC is firing at an angle greater than 90° the peak voltage will lower and equal to Equation 28.

(28)

The voltage at VBUCK with a valley fill stage of two will look similar to the waveforms of Figure 29.

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,

or if the TRIAC is firing at an angle above 90°, the DC offset (VDC) will lower. VDC is the lowest value that voltage

VBUCK will encounter.

(29)

Example:

Line voltage = 90 VAC to 135 VAC

Valley-Fill = two stage

(30)

26

Submit Documentation Feedback

Copyright © 2009–2015, Texas Instruments Incorporated

Product Folder Links: LM3445