Best practice for handling decoupling guidelines from both source and load

Question

I'll often use tools for designing switching regulators that will give me a recommended amount of bulk decoupling to handle some amount of step load to get a desired transient response and ripple. Additionally, most high power ICs I am trying to power will have their own guidelines for bulk decoupling. I imagine these requirements from the IC vendor are both conservative and are targeting the sort of step loads I am already designing my switching regulators around.

I am curious what the best practice would be for combining/handling these requirements. I can't imagine I would need both sets of decoupling capacitors recommended for source and load if the path is short and on the same board. I also want to minimize required board surface area. Should I just take which requirement desires more bulk capacitance and call that good? What sort of due diligence would give high confidence the power distribution network will function as intended? Is simulation/PDN analysis sufficient even if it maybe subverts what the IC requirements might state?

For this kind of thing, if you have exacting requirements on ripple or line and load transient response, the best approach is to do a rough calculation, simulate, then test hardware. If you have a very good PDN model with all the important parasitics modelled you can get really close in simulation, but you pretty much still have to verify in hardware. — John D
– John D, Commented Mar 19 at 20:48

Stefan Wyss · Accepted Answer · 2025-03-20 07:55:44Z

Bulk capacitance refers to the capacitance that is distributed throughout a circuit or a system, rather than being concentrated at a specific component or location. So by definition of bulk capacitance, you just take which requirement desires more bulk capacitance and all is OK.

But it's a different story if we talk about decoupling capacitance. Decoupling capacitance refers to capacitors that are used to isolate different parts of a circuit, particularly to prevent voltage fluctuations caused by transient loads. These capacitors are usually placed close to power pins of integrated circuits (ICs) to provide a local charge reservoir. When the IC requires sudden bursts of current, the decoupling capacitors can quickly supply that current, thus stabilizing the voltage seen by the IC and preventing it from dropping or oscillating.

Concerning decoupling capacitance, you would need both sets of decoupling capacitors recommended for source and load.

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