How much inrush current (startup current)?

When power is first applied both the processor and the bypass caps absorb higher than normal current, The extra current can cause voltage overshoot on the processor and can exceed the current rating of the regulator.

The bypass caps around the processor (both bulk and high frequency) are required to keep the voltage constant under changing loads. The ideal capacitor will have infinite current if a voltage is applied instantaneously. If you suspect that most of the inrush current is caused by the IC (processor or memory or whatever) then you probably do not have enough large bypass caps. This is why most IC datasheets do not specify inrush current. Inrush current is situation common to all switched power systems.

Using a power regulator with internal current limit or Soft Start capability can reduce the peak current rating for your power supply. This can result in smaller switches and reduced cost.

Inrush current can be a much bigger issue for some processors that can wake from a hibernate state. For normal power up, the voltage source to the regulator may be rising slowly and therefore limit the regulator’s output current. When waking from hibernate, the voltage source to the regulator may be at full voltage at the start. Currents could be much higher than normal. A soft start regulator is needed. If the processor has a power good input, use that to signal the processor when it is ok to begin operation.

_BobK

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