Datasheet

R20 C18
FLTR2
R3
R8
DIM
R16
C16
R2
R7
D5
Q1
R11
V+
C14
C15
V
CC
R13
D9
R22
R21
VLED+
VLED±
L1
C6
R6
V+
LINE
NEUTRAL
INPUT EMI FILTER AND RECTIFIER CIRCUIT
D1
D3
D4
D6
C12
DIM
V
CC
FLTR2
R18
C17
R17
TRIAC HOLDING
CIRCUIT
R23
C20
Q4
C2
C5
L2
R4
R12
C8
R5
D2
T1
C1
+
LM3448
9
10
11
12
13 4
5
6
7
8
BLDR
ASNS
ISNS
DIM
COFF
GND
VCC
FLTR1
GND
FLTR2
14
3NC
NC
15 2SWSW
16 1
SW
SW
COFF
COFF
R9
C7
COFF CURRENT
SOURCE
V
CC
V
AUX
R14
V
AUX
C11
Q3
OVP CIRCUIT
FLTR2
D11
V
AUX
Q2
VCC AUXILIARY BIAS CIRCUIT
R15
D10
D8
V
CC
VR1
F1
C9 C10
R10
C4
R1
C3
PGND SGND
D7
C13
www.ti.com Design Guide
12 Design Guide
Figure 23. Evaluation Board Schematic
The following section explains how to design an isolated flyback converter using the LM3448. Refer to the
LM3448 datasheet for specific details regarding the function of the LM3448 device. All reference
designators refer to the Evaluation Board Schematic in Figure 23 unless otherwise noted.
12.1 DCM Flyback Converter
This LED driver is designed to accurately emulate an incandescent light bulb and therefore behave as an
emulated resistor. The resistor value is determined based on the LED string configuration and the desired
output power. The circuit then operates in open-loop, with a fixed duty cycle based on a constant on-time
and constant off-time that is set by selecting appropriate circuit components. Like an incandescent lamp,
the driver is compatible with both forward and reverse phase dimmers. A key aspect of this design is that
the converter operates in discontinuous conduction mode (DCM). DCM is implemented by ensuring that
the flyback transformer current reaches zero before the end of the switching period.
11
SNOA555C–April 2011–Revised May 2013 AN-2091 LM3448 - 230VAC, 6W Isolated Flyback LED Driver
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