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.

When calcluating the steady-state voltage ripple, both the ESR component and the capacitive ripple must be considered, as shown in 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 resulting output voltage overshoot 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 with input capacitors, it is important to include sufficient design margin for output capacitors to account for variations due to temperature and DC bias.