Texas Instruments LM3448 Evaluation Board LM3448-EDSNEV/NOPB LM3448-EDSNEV/NOPB Hoja De Datos
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LM3448-EDSNEV/NOPB
C12
D10
V
LED
V
BUCK
V
L2
L2
R3
12
13
5
ISNS
GND
GND
LM3448
16
1
SW
SW
15
2
SW
SW
BUCK(MAX)
AC RMS(MAX)
V
V
2
u
LED(MIN)
ON(MIN)
LED(MAX)
SW
V
1
1
t
V
f
§
·
¨
¸
u
¨
¸
K
©
¹
SNOSB51C – SEPTEMBER 2011 – REVISED MAY 2013
The on-time (t
ON
) and therefore the switching frequency, will vary as the V
BUCK
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.
will decrease as the line voltage drops and increase as the line voltage increases.
The off-time of the LM3448 can be programmed for switching frequencies ranging from 30 kHz to over 1MHz. A
trade-off between efficiency and solution size must be considered when designing the LM3448 application.
trade-off between efficiency and solution size must be considered when designing the LM3448 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.
(14)
The maximum voltage seen by the Buck Converter is:
(15)
INDUCTOR SELECTION
The controlled off-time architecture of the LM3448 regulates the average current through the inductor (L2), and
therefore the LED string current (see
therefore the LED string current (see
). 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
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.
Figure 34. Simplified LM3448 Buck Converter
Copyright © 2011–2013, Texas Instruments Incorporated
25
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