LM3445M datasheet(21/39 Pages) TI

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VBUCK(MAX) = VAC-RMS(MAX) x 2

VLED(MIN)

tON(MIN) =

VBUCK(MAX) fSW

VBUCK (V)

1.50

1.25

1.00

0.75

0.50

0.25

100

150

200

3 LEDs

5 LEDs

7 LEDs

9 LEDs

Series

connected LEDs

LM3445

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SNVS570L – JANUARY 2009 – REVISED MAY 2013

The input voltage to the buck converter (VBUCK) changes with both line variations and over the course of each

half-cycle of the input line voltage. The voltage across the LED string will, however, remain constant, and

therefore the off-time remains constant.

The on-time, and therefore the switching frequency, will vary as the VBUCK voltage changes with line voltage. A

good design practice is to choose a desired nominal switching frequency knowing that the switching frequency

will decrease as the line voltage drops and increase as the line voltage increases (see Figure 26).

Figure 26. Graphical Illustration of Switching Frequency vs VBUCK

The off-time of the LM3445 can be programmed for switching frequencies ranging from 30 kHz to over 1 MHz. A

trade-off between efficiency and solution size must be considered when designing the LM3445 application.

The maximum switching frequency attainable is limited only by the minimum on-time requirement (200 ns).

Worst case scenario for minimum on time is when VBUCK is at its maximum voltage (AC high line) and the LED

string voltage (VLED) is at its minimum value.

(12)

The maximum voltage seen by the Buck Converter is:

(13)

INDUCTOR SELECTION

The controlled off-time architecture of the LM3445 regulates the average current through the inductor (L2), and

therefore the LED string current. The input voltage to the buck converter (VBUCK) changes with line variations and

over the course of each half-cycle of the input line voltage. The voltage across the LED string is relatively

constant, and therefore the current through R4 is constant. This current sets the off-time of the converter and

therefore the output volt-second product (VLED x off-time) remains constant. A constant volt-second product

makes it possible to keep the ripple through the inductor constant as the voltage at VBUCK varies.

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