9.2.1.3 Output Capacitor Selection

The output capacitors C_OUT0, C_OUT1, C_OUT2, and C_OUT3 are shown in Typical Applications. A ceramic local output capacitor of 22 μF is required per phase. Use ceramic capacitors, X7R or X7T types; do not use Y5V or F. DC bias voltage characteristics of ceramic capacitors must be considered. The output filter capacitor smooths out current flow from the inductor to the load, helps keep a steady output voltage during transient load changes and decreases output voltage ripple. These capacitors must be selected with sufficient capacitance and sufficiently low ESR and ESL to do these functions. The minimum effective output capacitance to make sure performance is good is 10 μF for each phase including the DC voltage roll-off, tolerances, aging and temperature effects.

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 12.

POL capacitors (C_POL0, C_POL1, C_POL2, C_POL3) can be used to improve load transient performance and to decrease the ripple voltage. A higher output capacitance improves the load step behavior and decreases the output voltage ripple as well as decreases the PFM switching frequency. However, output capacitance higher than 100 µF per phase is not necessarily of any benefit. Note that the output capacitor may be the limiting factor in the output voltage ramp and the maximum total output capacitance listed in electrical characteristics for the specified slew rate must not be exceeded. At shutdown the output voltage is discharged to 0.6 V level using forced-PWM operation. This can increase the input voltage if the load current is small and the output capacitor is large. Below 0.6 V level the output capacitor is discharged by the internal discharge resistor and with large capacitor more time is required to settle V_OUT down as a consequence of the increased time constant.