LM3478MA_15 Datasheet, PDF (23/26 Pages) Texas Instruments – High-Efficiency Low-Side N-Channel Controller for Switching Regulator

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LM3478MA
SNVS705B – FEBRUARY 2011 – REVISED FEBRUARY 2013
(51)
is the ripple voltage across the SEPIC capacitor, and
(52)
is the ripple current through the inductor L1. The energy balance equation can be solved to provide a minimum
value for C_S:
(53)
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:
(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 10µF is used than problems with impedance
interactions or switching noise can affect the LM3478. To improve performance, especially with V_IN below 8 volts,
it is recommended to use a 20Ω resistor at the input to provide a RC filter. The resistor is placed in series with
the V_IN pin with only a bypass capacitor attached to the V_IN pin directly (see Figure 33). A 0.1µF or 1µF 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:
IRMS =
I_SWPK2 - I_SWPK ('I_L1 + 'I_L2)+ ('I_L1 + 'I_L2)² (1-D) - I_OUT2
3
(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.
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