PVIN is the supply voltage for the switcher power stage. It is the supply that delivers the output power. The input capacitors supply the large AC switching current drawn by the switching action of the internal MOSFETs. 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 this current. To prevent large voltage transients, a low-ESR input capacitor sized for the maximum RMS current should be used. The maximum RMS current is given by:

The power dissipated in the input capacitor is given by:

P_{D_CIN} = I_RMS²R_{ESR_CIN}

where

• R_{ESR_CIN} is the ESR of the input capacitor.

This equation has a maximum at PVIN = 2 V_OUT, where I_RMS ≅ I_OUT / 2. This simple worst-case condition is commonly used for design because even significant deviations do not offer much relief. Several capacitors can also be paralleled to meet size or height requirements in the design. For low input voltage applications, sufficient bulk input capacitance is needed to minimize transient effects during load current changes. For optimal high frequency decoupling, a 1-µF ceramic bypass capacitor is also recommended adjacent the IC between the PVIN and PGND pins. Please refer to the PCB layout recommendation section in the LM21305 data sheet for more details. Note that the ESR of an electrolytic capacitor is used in this eval board to damp any oscillations that may occur when the supply lines have parasitic series inductance.