LM3410_08 Datasheet, PDF (17/32 Pages) National Semiconductor (TI) – PowerWise® 525kHz/1.6MHz, Constant Current Boost and SEPIC LED Driver with Internal Compensation

analysis of the LM3410 Boost Converter is applicable to the
LM3410 SEPIC Converter.
SEPIC Design Guide:
SEPIC Conversion ratio without loss elements:
The average inductor current of L2 is the average output load.
Therefore:
Small ripple approximation:
In a well-designed SEPIC converter, the output voltage, and
input voltage ripple, the inductor ripple I_L1 and I_L2 is small in
comparison to the DC magnitude. Therefore it is a safe ap-
proximation to assume a DC value for these components. The
main objective of the Steady State Analysis is to determine
the steady state duty-cycle, voltage and current stresses on
all components, and proper values for all components.
In a steady-state converter, the net volt-seconds across an
inductor after one cycle will equal zero. Also, the charge into
a capacitor will equal the charge out of a capacitor in one cy-
cle.
Therefore:
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FIGURE 13. Inductor Volt-Sec Balance Waveform
Applying Charge balance on C1:
Since there are no DC voltages across either inductor, and
capacitor C3 is connected to Vin through L1 at one end, or to
ground through L2 on the other end, we can say that
Therefore:
V_C3 = V_IN
Substituting I_L1 into I_L2
I_L2 = I_LED
This verifies the original conversion ratio equation.
It is important to remember that the internal switch current is
equal to I_L1 and I_L2 during the D interval. Design the converter
so that the minimum guaranteed peak switch current limit
(2.1A) is not exceeded.
FIGURE 14. HB/OLED SEPIC CONVERTER Schematic
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