How is the input capacitance determined for an LM2641-based regulator?
The response time of the regulator will be the same regardless of the input capacitance value. It does not care if the input voltage dips, as long as it does not drop below the minimum input voltage required for operation (or regulation if the output is higher than 5V or so). It will supply the instantaneous current if there is a fast load increase, but only until the source catches up. So the input capacitance depends a lot on the source. If using a battery with high ESR, another regulator with slow response time, or some other source with high source resistance, you will need a larger capacitor if a large voltage dip cannot be tolerated. If the source is very low ESR or has fast response, the input capacitor value can be reduced. For an approximation, start by making sure the RMS current rating of the capacitor is at least 75% of the max load current (50% if the ambient will not exceed 40 degC). This is only to help in sizing, this will often give a minimum value if cross referenced
