SNVSBK9C November 2019 – September 2020 LM63635-Q1
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 4.7 µF of ceramic capacitance is required on the input of the LM63635-Q1, connected directly between VIN and PGND. 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. In addition, a small case size, 220-nF ceramic capacitor must be used at the input as close a possible to the regulator, typically within 1 mm of the VIN and PGND pins. This provides a high frequency bypass for the control circuits internal to the device. For this example, a 10-µF, 50-V, X7R (or better) ceramic capacitor is chosen. Two 4.7-µF capacitors can also be used. The 220 nF must also be rated at 50 V with an X7R dielectric and preferably a small case size, such as an 0603.
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 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 11 to calculate the approximate RMS current. This value must be checked against the manufacturers' maximum ratings.