LM3444_14 Datasheet, PDF (7/27 Pages) Texas Instruments – AC-DC Offline LED Driver

LM3444
www.ti.com
SNVS682B – NOVEMBER 2010 – REVISED NOVEMBER 2011
APPLICATION INFORMATION
FUNCTIONAL DESCRIPTION
The LM3444 contains all the necessary circuitry to build a line-powered (mains powered) constant current LED
driver.
Theory of Operation
Refer to Figure 11 below which shows the LM3444 along with basic external circuitry.
V+
BR1
R
2
VAC
D
1
D
3
C+
7
D
D
8
4
Q
1
D2
D
9
C
1
C +0
9
C
5
LM344
4
1 NC _U1
NC
1
0
2 NC
VC
C
9
V_BUCK
C
1
2
V_LED
-
R
4
V_LED-
D1
0
Q3
L
2
I_COLL
3
N
C
GAT ₈
E
4
CO
FF
ISN ₇
S
C
5
FILT
ER
G
N6
D
4
Q2
R
3
C
1
1
Figure 11. LM3444 Schematic
VALLEY-FILL CIRCUIT
V_BUCK supplies the power which drives the LED string. Diode D3 allows V_BUCK to remain high while V+ cycles on
and off. V_BUCK has a relatively small hold capacitor C10 which reduces the voltage ripple when the valley fill
capacitors are being charged. However, the network of diodes and capacitors shown between D3 and C10 make
up a "valley-fill" circuit. The valley-fill circuit can be configured with two or three stages. The most common
configuration is two stages. Figure 12 illustrates a two and three stage valley-fill circuit.
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