As a first-order approximation, the VRM can simply be modeled as a series connected resistor and inductor, as shown in Figure 2.
At low frequencies in the range of tens of KHz, the VRM, being primarily resistive, provides very low impedance and thus it is capable of providing the instantaneous current requirements at these lower frequencies. However, beyond a few tens of KHz, the VRM impedance being primarily inductive makes it incapable of providing the transient current requirements. You can get the ESR and ESL values of the VRM from the VRM manufacturer and you can choose a low ESR/ESL regulator for best transient performance.
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