Say the seventh of the capacitor: the equivalent series resistance of the capacitor ESR

The general view is that a relatively large external capacitor with a small equivalent series resistance (ESR) can well absorb the peak (ripple) current during fast switching. However, sometimes such a choice is liable to cause instability of the regulator (especially the linear regulator LDO), so the capacitance of the small-capacity and large-capacity capacitors must be properly selected. Always remember that a voltage regulator is an amplifier that can appear in a variety of situations that an amplifier can appear.

Since the response speed of the DC/DC converter is relatively slow, the output decoupling capacitor dominates the initial stage of the load step, so an extra large capacity capacitor is needed to slow down the fast conversion relative to the DC/DC converter. Use high frequency capacitors to slow down the fast transition relative to large capacitors. In general, the equivalent series resistance of bulk capacitors should be chosen to be appropriate so that the peaks and spurs of the output voltage are within the device's Dasheet specifications.

In high-frequency conversion, small-capacity capacitors can meet the requirements on the order of 0.01μF to 0.1μF. Surface-mount ceramic capacitors or multilayer ceramic capacitors (MLCC) have a smaller ESR. In addition, under these values, their volume and BOM cost are reasonable. If the local low frequency decoupling is insufficient, the input voltage will be lowered when switching from low frequency to high frequency. The voltage drop process can last for a few milliseconds, depending on the regulator's gain adjustment and the time it takes to supply a large load current.

It is certainly more cost effective to use a large ESR capacitor in parallel than a single capacitor that is just as low as the ESR. However, this requires you to find a trade-off between PCB area, device count and cost.