The peak current mode control scheme of the LMR60420 device allows operation over a wide range of inductor and output capacitor combinations. The output capacitance is responsible for maintaining the desired output voltage during operation. The output capacitance impacts several key performance factors including:

• The amount of output voltage ripple during steady state operation
• The overshoot and undershoot of the output voltage when a load transient occurs
• Loop stability

During steady state operation, the inductor supplies a triangular current to the load. The AC portion of this triangular current is filtered out by the output capacitance while the DC portion passes through to the load. The AC current through the output capacitance and the equivalent series resistance (ESR) of this capacitance both contribute to the output voltage ripple. Equation 10 can be used to estimate the amount of peak to peak output voltage ripple required for a given output capacitance:

Where:

• ΔI_L = the peak to peak inductor current

The output capacitance is responsible for maintaining the output voltage during transient conditions in the load current. To determine the output capacitance and ESR required for a desired output voltage transient Equation 11, Equation 12, and Equation 13 can be used:

Where:

• ΔV_OUT = output voltage transient
• ΔI_OUT = output current transient
• K = inductor current ripple factor

The output capacitance is also important for overall loop stability and careful study must be done with the selected capacitance to confirm adequate phase margin and transient performance.

In this example, a single 22μF multilayer ceramic capacitor is used.