LM3444
SNVS682C –NOVEMBER 2010–REVISED MAY 2013
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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.
D
3
V
BUCK
V+
D
9
C
7
+
C
1
0
BR1
D
8
C
1
2
V
LED
-
C
9
+
R
2
R
4
D
4
Q
1
V
LED-
D2
VAC
D
1
D1
0
C
Q3
5
L
2
LM344
4
1
0
1 NC
2 NC
NC
U1
I
COLL
VC
C
9
8
7
6
N
C
GAT
E
3
Q2
CO
FF
ISN
S
4
R
3
G
N
D
FILT
ER
5
C
4
C
1
1
Figure 11. LM3444 Schematic
VALLEY-FILL CIRCUIT
VBUCK supplies the power which drives the LED string. Diode D3 allows VBUCK to remain high while V+ cycles on
and off. VBUCK 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.
8
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