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Can we use bulk electrolytic capacitors on the power rails of an IC? I know that we will use bulk capacitors in switching regulators at input. Can anyone explain why or why not and how to find ripple current of an IC? Is there any formula?

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    \$\begingroup\$ Data sheets on ICs normally give some indication of current. \$\endgroup\$
    – Andy aka
    Aug 3, 2021 at 7:02

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It all depends on the necessary power supply impedance. The IC maker either suggests a maximum power supply impedance over a certain frequency range in their datasheet or they give you some guideline as to what the required ripple current and voltage tolerances are, which allow you to calculate the necessary supply impedance.

After you know the \$Z_{max}(f)\$, you can decide which type of capacitor is most appropriate for your bulk caps.

An aluminum electrolytic capacitor (the standard type with liquid electrolytes) will have an intermediate ESR/capacitance ratio. Solid tantalum capacitors have a rather high ESR/capacitance ratio. Ceramics and aluminum polymer capacitors have a rather low ESR/capacitance ratio.

Now when do you need high or low ESR/capacitance ratio? You need low ESR if you need to achieve very low values of supply impedance, e.g. \$m\Omega\$ and below. High ESR/capacitance ratio capacitors are good if you have a sluggish voltage regulator and need to cover rather low frequencies with the bulk capacitors (e.g. below 1 kHz) but don't need a very low impedance. In this case it is cheaper to provide the large capacitance with e.g. aluminum electrolytic capacitors instead of ceramics. High ESR capacitors are also a simple way to work as a snubber with its ESR in the ohms range. That means that it can be wise to provide a single tantalum or aluminum capacitor even if you provide the decoupling capacitance in the form of ceramics.

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