8.4 Input Capacitor Selection

The step-down converter input capacitor must filter the high input current ripple as a result of pulsing or chopping the input voltage. The MCP16367/8/9 input voltage pin is used to supply voltage for the power train and as a source for internal bias. A low equivalent series resistance (ESR), preferably a ceramic capacitor of at least 10 µF capacitance, is recommended. Depending on the loading profile and conditions, the application will benefit from additional bulk capacitance.

The minimum capacitance needed for a given input peak-to-peak voltage ripple can be calculated using Equation 8-4.

Equation 8-4. Input Capacitor Ripple

C_{I N (M I N)} = \frac{I_{O U T} \times D \times (1 - D)}{{Δ V}_{r i p p l e, i n} \times f_{S W}}

$C_{I N (M I N)} = \frac{I_{O U T} \times D \times (1 - D)}{{∆ V}_{r i p p l e, i n} \times f_{S W}}$

Where:


I_OUT	=	Nominal output current
D	=	Duty-cycle
ΔV_ripple,in	=	Required input voltage ripple
f_SW	=	Switching frequency

The value of ceramic capacitors varies significantly with temperature and DC bias; therefore, design margins must be taken into account to ensure enough capacitance at the input of the regulator. To mitigate temperature variations, X5R and X7R capacitors are recommended. The DC bias must also be considered, as the capacitance decreases with the voltage applied.