8.2.2.1.3 Output Capacitor Selection

Use ceramic capacitors, X7R or X5R types; do not use Y5V or F. DC bias voltage characteristics of ceramic capacitors must be considered. DC bias characteristics vary from manufacturer to manufacturer, and DC bias curves should be requested from them as part of the capacitor selection process. The output filter capacitor smooths out current flow from the inductor to the load, 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 ESR and ESL to perform these functions. The minimum effective output capacitance to ensure good performance is 10 μF per output voltage rail at the output voltage DC bias, including tolerances and over ambient temperature range.

The output voltage ripple is caused by the charging and discharging of the output capacitor and also due to its R_ESR. The R_ESR is frequency dependent (as well as temperature dependent); make sure the value used for selection process is at the switching frequency of the part. See Table 10.

A higher output capacitance improves the load step behavior and reduces the output voltage ripple as well as decreases the PFM switching frequency. For most applications one 22-μF 0603 capacitor for C_OUT per voltage rail is suitable. A point-of-load (POL) capacitance C_POL can be added as shown in Figure 19. Although the loop compensation of the converter can be programmed to adapt to virtually several hundreds of microfarads C_OUT, it is preferable for C_OUT to be < 50 µF . Choosing higher than that is not necessarily of any benefit. Note: the output capacitor may be the limiting factor in the output voltage ramp, especially for very large (> 100 µF) output capacitors. For large output capacitors, the output voltage might be slower than the programmed ramp rate at voltage transitions, because of the higher energy stored on the output capacitance. Also at start-up, the time required to charge the output capacitor to target value might be longer. At shutdown, if the output capacitor is discharged by the internal discharge resistor, more time is required to settle V_OUT down as a consequence of the increased time constant.