8.5 Output Capacitor Selection

The output capacitor helps in providing a stable output voltage during sudden load transients and reduces the output voltage ripple. As with the input capacitor, X5R and X7R ceramic capacitors are well suited for this application.

The output capacitor voltage rating must be a minimum of V_OUT plus margin.

To determine the steady-state voltage ripple, the ESR component and the capacitive ripple need to be taken into account (Equation 8-5).

Equation 8-5. Output Capacitor Ripple

${∆ V}_{r i p p l e, o u t} = E S R \times {∆ I}_{L} + \frac{{∆ I}_{L}}{8 \times f_{S W} \times C_{O U T}}$

Where:


ESR	=	Output capacitor series resistance
f_SW	=	Switching frequency
C_OUT	=	Output capacitance
ΔI_L	=	Inductor current ripple

The worst case load transient for output capacitor calculations is an instantaneous load release from full load to no load. In this situation, the energy stored in the inductor, which is at its peak value, must be absorbed by the output capacitor.

The output voltage overshoot in such situations can be calculated using Equation 8-6.

Equation 8-6. Output Voltage Overshoot

${∆ V}_{O U T} = \sqrt{{V_{O U T}}^{2} + \frac{L}{C_{O U T}} \times {I_{L P E A K}}^{2}} - V_{O U T}$

Where:


V_OUT	=	Output voltage
L	=	Inductance value
I_LPEAK	=	Inductor peak current
C_OUT	=	Output capacitance

As in the case of the input capacitors, enough design margin needs to be taken into account to mitigate temperature and DC bias variations.