8.2.2.9 Filter Inductor and Output Capacitor Selection

In a buck regulator, selection of the filter inductor and capacitor will affect many key system parameters, including stability, transient response and efficiency The LM2854 can accommodate relatively wide ranges of output capacitor and filter inductor values in a typical application and still achieve excellent load current transient performance and low output voltage ripple.

The inductance is chosen such that the peak-to-peak inductor current ripple, Δi_L, is approximately 25 to 40% of I_OUT as shown in Equation 16.

Equation 16.

Note that the peak inductor current is the DC output current plus half the ripple current and reaches its highest level at lowest duty cycle (or highest V_IN). It is recommended that the inductor should have a saturation current rating in excess of the current limit level.

When operating the LM2854 at input voltages above 5.2 V, the inductor should be sized to keep the minimum inductor current above –0.5 A. For most applications this should only occur at light loads or when the inductor is drastically undersized. To ensure the current never goes below –0.5 A for any application, the peak-to-peak ripple current (Δi_L) in the inductor should be less than 1 A. Keeping the minimum inductor current above –0.5 A limits the energy storage in the inductor and helps prevent the switch node voltage from exceeding the absolute maximum specification when the low side FET turns off.

Table 3 lists examples of off-the-shelf powdered iron and ferrite based inductors that are suitable for use with the LM2854. The output capacitor can be of ceramic or electrolytic chemistry. The chosen output capacitor requires sufficient DC voltage rating and RMS ripple current handling capability.

The output capacitor RMS current and peak-to-peak output ripple are given respectively as in Equation 17.

Equation 17.

In general, 22 µF to 100 µF of ceramic output capacitance is sufficient for both LM2854 frequency options given the optimal high frequency characteristics and low ESR of ceramic dielectric. It is advisable to consult the manufacturer’s derating curves for capacitance voltage coefficient as the in-circuit capacitance may drop significantly with applied voltage.

Tantalum or organic polymer electrolytic capacitance may be suitable with the LM2854 500 kHz option, particularly in applications where substantial bulk capacitance per unit volume is required. However, the high loop bandwidth achievable with the LM2854 obviates the necessity for large bulk capacitance during transient loading conditions.

Table 4 lists some examples of commercially available capacitors that can be used with the LM2854.