7.2.2.2.3 Selecting the Output Capacitors

Texas Instruments recommends using ceramic capacitors with low ESR values to provide the lowest output voltage ripple. The output capacitor requires an X7R or an X5R dielectric. Y5V and Z5U dielectric capacitors, aside from their wide variation in capacitance over temperature, become resistive at high frequencies. At light load currents, the controller operates in PFM mode, and the output voltage ripple is dependent on the output-capacitor value and the PFM peak inductor current. Higher output-capacitor values minimize the voltage ripple in PFM mode. To achieve specified regulation performance and low output voltage ripple, the DC-bias characteristic of ceramic capacitors must be considered. The effective capacitance of ceramic capacitors drops with increasing DC bias voltage. TI recommends the use of small ceramic capacitors placed between the inductor and load with many vias to the power-ground (PGND) plane for the output capacitors of the controller. This solution typically provides the smallest and lowest cost solution available for DCAP controllers. The selection of the output capacitor is typically driven by the output transient response. Equation 4 provides a rough estimate of the minimum required capacitance to make sure the transient response is correct.

Equation 4.

where

• I_TRAN(max) is the maximum load current step.
• L is the selected inductance.
• V_OUT is the minimum programmed output voltage.
• V_IN is the maximum input voltage.
• V_OVER is the maximum allowable overshoot from programmed voltage.

Because the transient response is affected significantly by the board layout, some experimentation is expected to confirm that values derived in this section are applicable to any particular use case. Equation 4 is not provided as an absolute requirement, but as a starting point. Alternatively, lists recommended capacitor values.