The input capacitor supplies the AC switching current drawn from the switching action of the internal power FETs. The input current of a buck converter is discontinuous, so the ripple current supplied by the input capacitor is large. The input capacitor must be rated to handle both the RMS current and the dissipated power.
The device is designed to be used with ceramic capacitors on the input of the buck regulator. The recommended dielectric type of these capacitors is X5R, X7R, or of comparable material to maintain proper tolerances over voltage and temperature.
The device requires a minimum of 22 µF of ceramic capacitance at the input. TI recommends 2 × 10 µF, 10 µF for PVIN1 and 10 µF for PVIN2. Place these capacitors close to the PVIN1 and PGND1 / PVIN2 and PGND2 pads.
In addition, it is especially important to have small ceramic capacitors of 10 nF to 100 nF very close to the PVIN1 and PVIN2 inputs in order to minimize ringing and EMI generation due to the high speed switching of the device coupled with trace inductance.
Many times it is desirable to use an additional electrolytic capacitor on the input, in parallel with the ceramics. This is especially true if longs leads/traces are used to connect the input supply to the regulator. The moderate ESR of this capacitor can help damp any ringing on the input supply caused by long power leads. The use of this additional capacitor will also help with voltage dips caused by input supplies with unusually high impedance.