1

K

1

u

1.50

1.25

1.00

0.75

0.50

0.25

0

50

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

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

therefore the off-time remains constant.

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).

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).

(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

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

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