9.2.2.5 Output Capacitor Selection

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

Equation 10.

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

Equation 11.

The two components in the voltage ripple are not in phase, so the actual peak-to-peak ripple is smaller 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 rate. When a large load step happens, output capacitors provide the required charge before the inductor current can slew up to the appropriate level. The converter’s control loop usually needs 8 or more clock cycles to regulate the inductor current equal to the new load level. The output capacitance must be large enough to supply the current difference for 8 clock cycles to maintain the output voltage within the specified range. Equation 12 shows the minimum output capacitance needed for specified V_OUT overshoot and undershoot.

Equation 12.

where

• K_IND = Ripple ratio of the inductor current (Δi_L / I_OUT)
• I_OL = Low level output current during load transient
• I_OH = High level output current during load transient
• V_{OUT_SHOOT} = Target output voltage overshoot or undershoot

For this design example, the target output ripple is 30 mV. Presuppose ΔV_{OUT_ESR} = ΔV_{OUT_C} = 30 mV, and chose K_IND = 0.4. Equation 10 yields ESR no larger than 125 mΩ and Equation 11 yields C_OUT no smaller than 0.91 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 smaller than 4.3 µF by Equation 12. In summary, the most stringent criteria for the output capacitor is 4.3 µF. Consider of derating, one 10-µF, 10-V, X7R ceramic capacitor with 10-mΩ ESR is used.