8.2.1.2.6 Input Capacitor Selection

High quality input capacitors are necessary to limit the input voltage ripple while supplying switching-frequency AC current to the buck power stage. It is generally recommended to use X5R or X7R dielectric ceramic capacitors, thus providing low impedance and high RMS current rating over a wide temperature range. To minimize the parasitic inductance in the switching loop, position the input capacitors as close as possible to the PVIN and PGND pins. A good approximation for the required ripple current rating is given by Equation 9.

Equation 9.

The highest input capacitor RMS current occurs at 50% duty cycle, at which point the RMS ripple current rating should be greater than half the output current. Place low ESR ceramic capacitors in parallel with higher value bulk capacitance to provide optimized input filtering for the regulator and damping to mitigate the effects of input parasitic inductance resonating with high-Q ceramics. One bulk capacitor of sufficiently high current rating and one or two 22-μF 10-V X7R ceramic decoupling capacitors are usually sufficient. Select the input bulk capacitor based on its ripple current rating and operating temperature.

When operating at low input voltages (3.3 V or lower), additional capacitance may be necessary to avoid triggering an undervoltage condition during an output current transient. This depends on the impedance between the input voltage supply and the LM21215A, as well as the magnitude and slew rate of the load transient.

The AVIN pin requires a 1-µF ceramic capacitor to AGND and a 1-Ω resistor to PVIN. This RC network filters inherent noise on PVIN from the sensitive analog circuitry connected to AVIN.