Work out decoupling capacitance requirement instantly with clear inputs, formula shown and shareable results.
Decoupling capacitance must supply the charge the load demands before the regulator responds: C = I x t / allowed droop. ESR eats into that droop budget directly, so a high-ESR bulk capacitor can make the requirement impossible however large its capacitance.
Decoupling
C = delta_I x t_response / delta_V_allowed; ESR contributes I x ESR of droop immediately
Because the ESR drop appears instantly and cannot be reduced by adding capacitance in the same part. Paralleling capacitors reduces effective ESR, which is why many small ones beat one large one.
The voltage regulator's control loop bandwidth. A 1 MHz loop needs decoupling to cover roughly a microsecond of transient on its own.