8.2.2.3 Capacitor Selection

The LM3241 is designed for use with ceramic capacitors for its input and output filters. Use a 10-µF ceramic capacitor for the input and a 4.7-µF ceramic capacitor for the output. The capacitors should maintain at least 50% capacitance at DC bias and temperature conditions. Ceramic capacitors type such as X5R, X7R, and B are recommended for both filters. These types provide an optimal balance between small size, cost, reliability, and performance for cell phones and similar applications. Table 4 lists some recommended part numbers and suppliers. DC bias characteristics of the capacitors must be considered when selecting the voltage rating and case size of the capacitor. For C_IN, use of an 0805 (2012) size may also be considered if room is available on the system board.

The input filter capacitor supplies AC current drawn by the PFET switch of the LM3241 in the first part of each cycle and reduces the voltage ripple imposed on the input power source. The output filter capacitor absorbs the AC inductor current, helps maintain a steady output voltage during transient load changes, and reduces output voltage ripple. These capacitors must be selected with sufficient capacitance and sufficiently low equivalent series resistance (ESR) to perform these functions. The ESR of the filter capacitors is generally a major factor in voltage ripple.