8.2.2.4 Output Capacitor Selection

The three primary considerations for selecting the value of the output capacitor are: 

• Minimum capacitance to meet the load transient requirement
• Minimum capacitance to meet the output voltage ripple requirement
• Maximum ESR to meet the output voltage ripple requirement

The output capacitor must be selected based on the most stringent of these three criteria.

The first criterion is the desired response to a large change in the load current. The output capacitor needs to supply the load with current when the regulator cannot. This situation would occur if there are desired hold-up times for the regulator where the output capacitor must hold the output voltage above a certain level for a specified amount of time after the input power is removed. The regulator is also temporarily not able to supply sufficient output current if there is a large, fast increase in the current needs of the load such as transitioning from no load to a full load. The regulator usually needs two or more clock cycles for the control loop to see the change in load current and output voltage and adjust the duty cycle to react to the change. The output capacitor must be sized to supply the extra current to the load until the control loop responds to the load change. The output capacitance must be large enough to supply the difference in current for 2 clock cycles while only allowing a tolerable amount of droop in the output voltage. Equation 22 shows the minimum output capacitance necessary to accomplish this.

Equation 22.

where

• ΔIout is the change in output current.
• ƒsw is the regulator's switching frequency.
• ΔVout is the allowable change in the output voltage.

For this example, the transient load response is specified as a 4% change in Vout for a load step of 0.75 A. Using these numbers (ΔI_OUT = 0.75 A and ΔVout = 0.04 × 3.3 = 0.132 V) gives a minimum capacitance of 23.7 μF. This value does not take the ESR of the output capacitor into account in the output voltage change. For ceramic capacitors, the ESR is usually small enough to ignore in this calculation.

Equation 23 calculates the minimum output capacitance needed to meet the output voltage ripple specification. Where ƒsw is the switching frequency, Voripple is the maximum allowable output voltage ripple, and Iripple is the inductor ripple current. In this case, the maximum output voltage ripple is 33 mV. Under this requirement, Equation 23 yields 6.4 µF.

Equation 23.

Equation 24 calculates the maximum ESR an output capacitor can have to meet the output voltage ripple specification. Equation 24 indicates the ESR should be less than 40 mΩ. In this case, the ESR of the ceramic capacitor is much smaller than 40 mΩ.

Equation 24.

The capacitance of ceramic capacitors is highly dependent on the DC output voltage. Equation 25 is used to select output capacitors based on their voltage rating. For 6.3-V ceramic capacitors, the minimum capacitance that meets the load step specification is 49.7 µF. For this example, one 47-μF, 6.3-V, X5R ceramic capacitor with 4 mΩ of ESR is used.

Equation 25.

Capacitors generally have limits to the amount of ripple current they can handle without failing or producing excess heat. The designer must specify an output capacitor that can support the inductor ripple current. Some capacitor data sheets specify the root mean square (RMS) value of the maximum ripple current. Equation 26 can be used to calculate the RMS ripple current the output capacitor needs to support. For this application, Equation 26 yields 235 mA.

Equation 26.