The device is designed to be used with a wide variety of LC filters. It is generally desired to minimize the output capacitance to keep cost and size down. The output capacitor or capacitors, COUT, must be chosen with care since 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:
The other part is caused by the inductor current ripple charging and discharging the output capacitors:
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 12 shows the minimum output capacitance needed for a specified VOUT overshoot and undershoot.
For this design example, the target output ripple is 30 mV. Assuming ΔVOUT_ESR = ΔVOUT_C = 30 mV, choose KIND = 0.4. Equation 10 yields ESR no larger than 75 mΩ and Equation 11 yields COUT no smaller than 2.38 µF. For the target overshoot and undershoot limitation of this design, ΔVOUT_SHOOT = 8% × VOUT = 400 mV. The COUT can be calculated to be no less than 14.3 µF by Equation 12. In summary, the most stringent criteria for the output capacitor is 14.3 µF. Considering derating, one 22-µF, 10-V, X7R ceramic capacitor with 10-mΩ ESR is used.