The device is designed to be used with a wide variety of LC filters. Minimize the output capacitance to keep cost and size down. The output capacitor or capacitors, C_OUT, must be chosen with care because it directly affects the steady state output voltage ripple, loop stability, and output voltage overshoot and undershoot during load current transient. The output voltage ripple is essentially composed of two parts. One part is caused by the inductor ripple current flowing through the Equivalent Series Resistance (ESR) of the output capacitors:

Equation 12.

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

Equation 13.

The two components of the voltage ripple are not in-phase, therefore, the actual peak-to-peak ripple is less than the sum of the two peaks.

Output capacitance is usually limited by transient performance specifications if the system requires tight voltage regulation with presence of large current steps and fast slew rates. When a large load step occurs, output capacitors provide the required charge before the inductor current can slew to an appropriate level. The control loop of the converter usually requires eight or more clock cycles to regulate the inductor current equal to the new load level during this time. The output capacitance must be large enough to supply the current difference for eight clock cycles to maintain the output voltage within the specified range. Equation 14 shows the minimum output capacitance needed for a specified V_OUT overshoot and undershoot.

Equation 14.

where

• K_IND is the ripple ratio of the inductor current (Δi_L / I_OUT).
• I_OL is the low level output current during load transient.
• I_OH is the high level output current during load transient.
• V_{OUT_SHOOT} is the target output voltage overshoot or undershoot.

For this design example, the target output ripple is 30 mV. Assuming ΔV_{OUT_ESR} = ΔV_{OUT_C} = 30 mV, choose K_IND = 0.3. Equation 12 yields ESR no larger than 75 mΩ and Equation 13 yields C_OUT no smaller than 12 µF. For the target overshoot and undershoot limitation of this design, ΔV_{OUT_SHOOT} = 5% × V_OUT = 250 mV. The C_OUT can be calculated to be no less than 32 µF by Equation 14. In summary, the most stringent criterion for the output capacitor is 44 µF. For this design, two 22-µF, 25-V, X7R ceramic capacitors with 5-mΩ ESR are used.