The LM21305 is designed to be used with a wide variety of LC output filters. It is generally desired to use as little output capacitance as possible to keep cost and size down. The output capacitor or capacitors, C_OUT, should be chosen with care since it directly affects the steady state output voltage ripple, loop stability and the voltage overshoot or undershoot during a load transient. The output voltage ripple is composed of two parts. One is related to the inductor current ripple going through the equivalent series resistance (ESR) of the output capacitors:

ΔV_OUT-ESR = Δi_LP-P × ESR

The other is caused by the inductor current ripple charging and discharging the output capacitors:

Equation 5.

Since the two components in the ripple are not in phase, the actual peak-to-peak ripple is smaller than the sum of the two peaks:

Equation 6.

Output capacitance is usually limited by system transient performance specifications, particularly if the system requires tight voltage regulation in the presence of large current steps and fast slew rate. To maintain a small overshoot or undershoot during a load transient, small ESR and large capacitance are desired. But these also come with the penalty of higher cost and size. Clearly, the control loop should also be fast to reduce the voltage droop.

One or more ceramic capacitors are recommended because they have very low ESR and remain capacitive up to high frequencies. The dielectric should be X5R, X7R, or comparable material to maintain proper tolerances. Other types of capacitors also can be used if large capacitance is needed, such as tantalum, POSCAP and OSCON. Such capacitors have lower ESR zero frequency, 1 / (2πESR × C), than ceramic capacitors. The lower ESR zero frequency can affect the control loop if it is close to the crossover frequency. If high switching frequency and high crossover frequency are desired, an all ceramic capacitor design is sometimes more appropriate.