9.2.2.1.3 Input Capacitor Selection

The buck converter has a natural pulsating input current; therefore, a low-ESR input capacitor is required for best input voltage filtering and minimizing the interference with other circuits caused by high-input voltage spikes. For most applications, a 4.7-μF to 10-μF ceramic capacitor is recommended (Table 4). Because ceramic capacitors lose up to 80% of their initial capacitance at 5 V, TI recommends using a 10-μF input capacitor for input voltages greater than 4.5 V. The input capacitor can be increased without any limit for better input voltage filtering.

Take care when using only small ceramic input capacitors. When a ceramic capacitor is used at the input, and the power is being supplied through long wires, such as from a wall adapter, a load step at the output, or V_IN step on the input, can induce ringing at the VIN pin. The ringing can couple to the output and be mistaken as loop instability, or could even damage the part by exceeding the maximum ratings.