Datasheet

I
RMS
=
I
SWPEAK
2
- I
SWPEAK
('I
L1
+ 'I
L2
) +
('I
L1
+ 'I
L2
)
2
3
(1-D) - I
OUT
2
I
CIN(RMS)
=
'I
L1
/
12
V
IN
- V
Q
(L1)f
S
D
3
2
=
C
S
t
L1
I
OUT
2
(V
IN
- V
Q
)
2
'I
L1
=
(V
IN
- V
Q
) D
(L1)f
S
C
S
'V
S
2
=
(L1)'I
L1
2
1
2
1
2
LM3481
www.ti.com SNVS346E –NOVEMBER 2007–REVISED APRIL 2012
The SEPIC capacitor must be rated for a large ACrms current relative to the output power. This property makes
the SEPIC much better suited to lower power applications where the rms current through the capacitor is small
(relative to capacitor technology). The voltage rating of the SEPIC capacitor must be greater than the maximum
input voltage. Tantalum capacitors are the best choice for SMT, having high rms current ratings relative to size.
Ceramic capacitors could be used, but the low C values will tend to cause larger changes in voltage across the
capacitor due to the large currents, and high C value ceramics are expensive. Electrolytics work well for through
hole applications where the size required to meet the rms current rating can be accommodated. There is an
energy balance between C
S
and L1, which can be used to determine the value of the capacitor. The basic
energy balance equation is:
Where
is the ripple voltage across the SEPIC capacitor, and
is the ripple current through the inductor L1. The energy balance equation can be solved to provide a minimum
value for C
S
:
INPUT CAPACITOR SELECTION
Similar to a boost converter, the SEPIC has an inductor at the input. Hence, the input current waveform is
continuous and triangular. The inductor ensures that the input capacitor sees fairly low ripple currents. However,
as the input capacitor gets smaller, the input ripple goes up. The rms current in the input capacitor is given by:
The input capacitor should be capable of handling the rms current. Although the input capacitor is not as critical
in a SEPIC application, low values can cause impedance interactions. Therefore a good quality capacitor should
be chosen in the range of 100 µF to 200 µF. If a value lower than 100 µF is used, then problems with impedance
interactions or switching noise can affect the LM3481. To improve performance, especially with V
IN
below 8V, it
is recommended to use a 20Ω resistor at the input to provide a RC filter. This resistor is placed in series with the
V
IN
pin with only a bypass capacitor attached to the V
IN
pin directly (see Figure 32). A 0.1 µF or 1 µF ceramic
capacitor is necessary in this configuration. The bulk input capacitor and inductor will connect on the other side
of the resistor with the input power supply.
OUTPUT CAPACITOR SELECTION
The output capacitor of the SEPIC sees very large ripple currents similar to the output capacitor of a boost
converter. The rms current through the output capacitor is given by:
The ESR and ESL of the output capacitor directly control the output ripple. Use capacitors with low ESR and ESL
at the output for high efficiency and low ripple voltage. Surface mount tantalums, surface mount polymer
electrolytic and polymer tantalum, Sanyo- OSCON, or multi-layer ceramic capacitors are recommended at the
output for low ripple.
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