C1: The purpose of this capacitor is to supply most of the switch current during the on-time and to limit the voltage ripple at VIN, on the assumption that the voltage source feeding VIN has an output impedance greater than zero. At maximum load current, when the buck switch turns on, the current into pin 8 suddenly increases to the lower peak of the output current waveform, ramps up to the peak value, and then drops to zero at turn-off. The average input current during this on-time is the load current (150 mA). For a worst-case calculation, C1 must supply this average load current during the maximum on-time. To keep the input voltage ripple to less than 2 V (for this exercise), C1 calculates to Equation 11:

Equation 11.

Quality ceramic capacitors in this value have a low ESR that adds only a few millivolts to the ripple. The capacitance is dominant in this case. To allow for the capacitor tolerance, temperature effects, and voltage effects, a 1.0-µF, 100-V, X7R capacitor is used.

C5: This capacitor helps avoid supply voltage transients and ringing resulting from long lead inductance at VIN. A low-ESR, 0.1-µF ceramic chip capacitor is recommended, located close to the LM5009.