VLED(MIN)

tON(MIN) =

VBUCK(MAX) fSW

1

K

1

u

VBUCK (V)

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

1

K

VLED

VBUCK

u

¸

¹

·

¨

©

§

fSW =

1

tOFF

VLED

= K u D

VBUCK

LM3444

www.ti.com

SNVS682D – NOVEMBER 2010 – REVISED DECEMBER 2015

Application Information (continued)

With efficiency of the buck converter in mind, Equation 10 shows:

(10)

Substituting and rearranging the equations, Equation 11 shows:

(11)

Off-time and switching frequency can now be calculated using the previous equations.

8.1.3 Setting the Switching Frequency

Selecting the switching frequency for nominal operating conditions is based on tradeoffs between efficiency

(better at low frequency) and solution size and cost (smaller at high frequency).

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, however, remains constant, and therefore

the off-time remains constant.

The on-time, and therefore the switching frequency, varies 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

decreases as the line voltage drops, and increases as the line voltage increases (Figure 14).

Figure 14. Graphical Illustration of Switching Frequency vs VBUCK

The off-time of the LM3444 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 LM3444 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, as shown in Equation 12.

(12)

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