LM3444MM Datasheet, PDF (14/30 Pages) Texas Instruments – AC-DC Offline LED Driver

LM3444
SNVS682C – NOVEMBER 2010 – REVISED MAY 2013
www.ti.com
1.50
Series
connected LEDs
1.25
3 LEDs
1.00
5 LEDs
0.75
7 LEDs
0.50
9 LEDs
0.25
0
50
100
150
200
V_BUCK (V)
Figure 17. Graphical Illustration of Switching Frequency vs V_BUCK
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 V_BUCK is at its maximum voltage (AC high line) and the LED
string voltage (V_LED) is at its minimum value.
t_ON(MIN) =
1
K
u
V_LED(MIN)
V_BUCK(MAX)
1
f_SW
(12)
The maximum voltage seen by the Buck Converter is:
V_BUCK(MAX) = V_AC-RMS(MAX) x 2
(13)
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 (V_BUCK) 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 (V_LED 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 V_BUCK varies.
14
Submit Documentation Feedback
Product Folder Links: LM3444
Copyright © 2010–2013, Texas Instruments Incorporated