IRMS =

ISWPK

2 - I

SWPK (

'IL1 + 'IL2)+ ('IL1 + 'IL2)

2

3

(1-D) - IOUT

2

27

LM3478

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SNVS085X – JULY 2000 – REVISED DECEMBER 2017

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Typical Applications (continued)

8.2.2.2.6

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

using Equation 54.

(54)

The input capacitor should be capable of handling the RMS current. Although the input capacitor is not as critical

in a boost application, low values can cause impedance interactions. Therefore a good quality capacitor should

be chosen in the range of 10µF to 20µF. If a value lower than 10 µF is used, then problems with impedance

interactions or switching noise can affect the LM3478. To improve performance, especially with VIN below 8 volts,

TI recommends that the user uses a 20Ω resistor at the input to provide a RC filter. The resistor is placed in

series with the VIN pin with only a bypass capacitor attached to the VIN pin directly (see Figure 31). 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.

8.2.2.2.7

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 using Equation 55.

(55)

The ESR and ESL of the output capacitor directly control the output ripple. Use low 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.

8.2.2.3 Application Curves

Figure 35. Efficiency vs Load Current (3.3-V In and 12-V

Out)

Figure 36. Efficiency vs Load Current (5-V In and 12-V Out)