Distributing the output capacitance of a buck regulator

Question

Buck regulators often specify the value and placement of the output capacitor. Typically 10µF placed very close to the buck regulator.

However, what would the effect be of replacing the 10µF output capacitor with 10x 1µF capacitors. Where only one of those are placed immediately at the buck regulator, and the other 9 are decoupling capacitors at the load. Within an inch or so from the buck regulator.

The 1µF capacitor close by the buck regulator should handle the high frequency components of the buck switching just as well or better than a 10µF would, and the total capacitance is 10µF so the regulator loop should function properly.

For point-of-load buck regulators, this distributed output capacitor approach would have quite some practical advantages nowdays when 1µF is readily available in 0402 footprint.

Is there a downside which I am not seeing?

Clarification 1: The current at the output of a buck regulator is DC plus a small triangle wave at the switching frequency (typically 1-2MHz). Triangle waves do not have any significant HF content. Examples can be found here: http://electronicsbeliever.com/sizing-the-inductor-of-buck-converter-and-setting-its-operation/

Clarification 2: Physically smaller capacitors have lower ESL (which is better). See for example: SMD capacitor package size and high frequency performance

Answer 1

There could be a downside, but it depends. The manufacturer's recommendation is usually based on characterization with a given capacitor or range of capacitors. The safe approach is to stay within those guidelines. Be sure you know if the capacitance specified is effective capacitance, or a specific recommended capacitor.

By deviating from the guidelines you may introduce extra ESR, moving the ESR zero and changing the response of the control loop. 0402 capacitors have worse derating with voltage and can have worse ESR) so the extra parasitics and loss of effective capacitance could affect performance.

Over temperature an 0402 biased near its voltage rating can lose more than 80% of its capacitance, depending on dielectric and value:

You may also not be satisfied with the output ripple given the extra ESL and ESR you're adding. You could run some simulations with various stray inductance and ESR values to get a feel for it.

It would likely cause less of an issue with a current mode topology vs. voltage mode, and it might be fine, but you should at least prototype it, and measure the loop and transient response before committing anything to production.

Further info:

An 0402 1uF cap does show higher impedance than an 0805 case size 50V MLC cap- From Murata's Simsurfing tool here's an 0805 in blue and an 0402 in green:

And here's the derating curve for an 0402 50V 1uF X5R:

And ESR vs. frequency, 0805 in blue again:

So derating might not be a factor for 1.8V rails, but with temperature and tolerance even at 3.3V you're losing significant capacitance.

Answer 2

The main issue with distributing that capacitance is that the ripple currents, which are often the largest unintentional radiators in a design, are now also distributed throughout the design. Depending on your other circuitry or your environment, this may cause crosstalk with other devices or sections of your own device. It's true that you're increasing ESR as well, but depending on your switching frequency ESL may be a more critical issue. Over an area of about an inch the ESR probably remains in spec.