LM3481 Datasheet, PDF (20/22 Pages) National Semiconductor (TI) – High Efficiency Low-Side N-Channel Controller for Switching Regulators

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 interac-
tions. 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, es-
pecially with V_IN below 8V, it is recommended to use a 20Ω
resistor at the input to provide a RC filter. This resistor is
Other Application Circuits
placed in series with the V_IN pin with only a bypass capacitor
attached to the V_IN pin directly (see Figure 15). 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 cur-
rents 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 ca-
pacitors are recommended at the output for low ripple.
FIGURE 18. Typical High Efficiency Step-Up (Boost) Converter
20136543
www.national.com
20