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The next component consideration are the input capacitors. The purpose of the input capacitor is to reduce both the surge current drawn from the input supply as well as the switching noise from the device. The input capacitor must sustain the ripple current produced during the on-time of the high-side FET. It must have a low ESR, (Equivalent Series Resistance), to minimize power dissipation due to the RMS input current. The RMS current rating of the input capacitor is a critical parameter and must be higher than the actual RMS input current. As a rule of thumb, select an input capacitor with an RMS current rating greater than half of the maximum load current. Due to the large dI/dt through the input capacitor, electrolytic or ceramic capacitors with low ESR should be used. If using a tantalum capacitor, it must be surge protected or else capacitor failure could occur. To be effective on the PCB, the input capacitor needs to be placed as closely as possible across the VIN and GND pins of the device. The goal is to place the input capacitor as close to the IC with as little trace resistance as possible. It is recommended to use at least X7R type ceramic capacitors.
