Datasheet

2
AREA
1
AREA
S
T
S
DT
( )
tL
V
(s)
t
I
R
V
O
L2
=
x
=
R
¸
¹
·
V
O
¨
©
§
¨
©
§
D
D
'
¸
¹
·
I
L1
=
and
L2
I
x
L1
I
D
¸
¹
·
¨
©
§
'
D
D =
O
V
O
V +
IN
V
V
o
V
IN
=
D'
D
LM2735
www.ti.com SNVS485F –JUNE 2007–REVISED APRIL 2013
SEPIC Converter
The LM2735 can easily be converted into a SEPIC converter. A SEPIC converter has the ability to regulate an
output voltage that is either larger or smaller in magnitude than the input voltage. Other converters have this
ability as well (CUK and Buck-Boost), but usually create an output voltage that is opposite in polarity to the input
voltage. This topology is a perfect fit for Lithium Ion battery applications where the input voltage for a single cell
Li-Ion battery will vary between 3V & 4.5V and the output voltage is somewhere in between. Most of the analysis
of the LM2735 Boost Converter is applicable to the LM2735 SEPIC Converter.
SEPIC Design Guide:
SEPIC Conversion ratio without loss elements:
(49)
Therefore:
(50)
Small ripple approximation:
In a well-designed SEPIC converter, the output voltage, and input voltage ripple, the inductor ripple and is small
in comparison to the DC magnitude. Therefore it is a safe approximation 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 cycle.
Therefore:
(51)
Substituting I
L1
into I
L2
(52)
The average inductor current of L2 is the average output load.
Figure 32. Inductor Volt-Sec Balance Waveform
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