Datasheet
R
FLTR1
R
SNS
PWM
I-LIM
1.27V
ISNS
ASNS
GND
FLTR1
FLTR2
DIM
DIM DECODER
4.9V
Tri-State
50k
370k
C
FLTR2
C
FLTR1
1k
RAMP GEN.
5.9 kHz
3V
1V
750 mV
125 ns
LEADING EDGE BLANKING
1V
Nearly a constant on-
time as the line varies
The PWM reference increases
as the line voltage increases.
As line voltage increases, the voltage across the
inductor increases, and the peak current increases.
LED Current
RAMP
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Design Guide
Figure 26. Typical Operation of FLTR2 Pin
Turns Ratio
The first step with an isolated design is to determine the transformer turns ratio. This can be an iterative
process that will depend on the specified operating conditions, maximum stresses allowed for the LM3448
SW FET and re-circulating diode as well as transformer core parameters. For many LM3448 flyback
designs, an integer turns ratio of 4 or 5 is a good starting point. The next step will be to verify that the
chosen turns ratio results in operating conditions that do not violate any other component ratings.
Duty Cycle Calculation
The AC mains voltage at the line frequency f
L
is assumed to be perfectly sinusoidal and the diode bridge
ideal. This yields a perfect rectified sinusoid at the input to the flyback. The peak nominal input voltage V
IN-
PK(NOM)
is defined in terms of the input voltage V
IN(NOM)
,
(3)
Duty cycle is calculated at the nominal peak input voltage V
IN-PK(NOM)
. Note that this is the duty cycle for
flyback operation at the boundary of continuous conduction mode (CCM) operation. In order to ensure that
the converter is operating in DCM, the primary inductance of the transformer will be adjusted lower (refer
to "Transformer" section).
(4)
Peak Input Current Calculation
Due to the direct line-injection, the flyback converter operates as a constant power converter. Therefore
average input power over one line cycle will approximately equal the output power,
(5)
However since the input power has 120Hz ripple, the “peak” input power P
IN-PK
will be equal to twice the
output power,
(6)
Figure 27 illustrates the input current going into the primary side winding of the flyback transformer over
one-half of a rectified input voltage line cycle.
13
SNOA555C–April 2011–Revised May 2013 AN-2091 LM3448 - 230VAC, 6W Isolated Flyback LED Driver
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