The ceramic input capacitors provide a low impedance source to the regulator in addition to supplying the ripple current and isolating switching noise from other circuits. A minimum of 2.2 µF of ceramic capacitance is required on the input of the LM516x-Q1 regulator, connected directly between VIN and GND. This must be rated for at least the maximum input voltage that the application requires; preferably twice the maximum input voltage. This capacitance can be increased to help reduce input voltage ripple and maintain the input voltage during load transients. More input capacitance is required for larger output currents. Keep in mind that the value of 2.2 µF is the actual value after all derating is applied. For this example, 4x 1-µF, 250-V, X7R (or better) ceramic capacitors are chosen due to voltage derating. If larger case size, higher voltage capacitors, or both can be used, then the total number may be reduced. Designs with reduced input voltage range can use capacitors with lower voltage ratings.

Many times, it is desirable to use an electrolytic capacitor on the input in parallel with the ceramics. This is especially true if long leads or traces (greater than about 5 cm) 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 the long power leads. The use of this additional capacitor also helps with voltage dips caused by input supplies with unusually high impedance.

Most of the input switching current passes through the ceramic input capacitor or capacitors. Use Equation 35 to calculate the approximate RMS current. This value must be checked against the manufacturers' maximum ratings.