LM3444
SNVS682C –NOVEMBER 2010–REVISED MAY 2013
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1.50
Series
connected LEDs
1.25
1.00
0.75
0.50
0.25
3 LEDs
5 LEDs
7 LEDs
9 LEDs
50
0
100
150
200
V
BUCK
(V)
Figure 17. 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.
VLED(MIN)
1
h
1
ì
tON(MIN)
=
VBUCK(MAX)
fSW
(12)
(13)
The maximum voltage seen by the Buck Converter is:
VBUCK(MAX) = VAC-RMS(MAX)
x
2
INDUCTOR SELECTION
The controlled off-time architecture of the LM3444 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.
14
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