The output capacitors smooth the inductor current ripple and provide a source of charge for load transient conditions. The output capacitor selection is primarily dictated by the following specifications:

• Steady-state output peak-peak ripple (ΔV_PK-PK)
• Output voltage deviation during transient condition (ΔV_Transient)

For the 5-V output design example, ΔV_PK-PK = 50 mV (1% of VOUT) and ΔT_Transient = 100 mV (2% of VOUT) were chosen. The magnitude of output ripple primarily depends on ESR of the capacitors while load transient voltage deviation depends both on the output capacitance and ESR.

When a full load is suddenly removed from the output, the output capacitor must be large enough to prevent the inductor energy to raise the output voltage above the specified maximum voltage. In other words, the output capacitor must be large enough to absorb the maximum stored energy of the inductor. The stored energy equations of both the inductor and the output capacitor can be calculated:

Equation 16.

Evaluating, the above equation with a ΔVout of 100 mV results in an output capacitance of 475 µF. As stated earlier, the maximum peak to peak ripple primarily depends on the ESR of the output capacitor and the inductor ripple current. To satisfy the ΔV_PK-PK of 50 mV with 40% inductor current ripple, the ESR must be less than 15 mΩ. In this design example, a 470-µF aluminum capacitor with an ESR of 10 mΩ is paralleled with two 47-µF ceramic capacitors to further reduce the ESR.