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What's the best solution for noise reduction in switching power supplies?

I'm going to use a switching dc to dc buck converter to supply an STM32 Cortex-M4 microcontroller. But I'm a little concerned about reliability of the supply.

What considerations must be made when an mcu connected directly to a switching regulator that works at very high frequency?

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    \$\begingroup\$ A properly designed SMPS should not require noise reduction to power a microcontroller. If it does, fix the problem, not put a band-aid on it. \$\endgroup\$ – Matt Young Apr 15 '17 at 22:18
  • \$\begingroup\$ usually a big cap and a ferrite bead work wonders. Compare scope shots before and after and tell us if you see a difference. \$\endgroup\$ – Sredni Vashtar Apr 16 '17 at 1:52
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The switchreg will have parasitic ringing at 50X or 100X the switching frequency. Consider 100MHz ringing with 100mV amplitude. With close to ZERO ohms source impedance (the ringing is coming from a Power Circuit, after all).

You could just let that 100MHz ringing, at 100mV, simply enter your MCU. But that will trash any onboard ADC. And the clock-multiply PLL will be wonderfully jittery.

You need 3 regions of GND, to be successful

1) the SwitchReg

2) the filter: bead in series, low-esr cap to GND#2

3) the MCU

These 3 regions of GND should only connect through narrow pieces of foil.

schematic

simulate this circuit – Schematic created using CircuitLab

Here is the noise attenuation of the PI filter shown in the previous schematic, for 4 values of Rout of the SwitcherReg (5 / 0.5/ 0.05 / 0.005 ohms)

enter image description here

Notice the 12dB peaking for 0.005 ohms, the 3dB peaking for 0.05 ohms, then the well-dampened behavior for 0.5 and 5.0 ohms. Thus some discrete resistance between SwitchReg and the PI filter seems useful.

And here is the wide-spectrum filtering, assuming the 3 regions of GND are effective, for 0.005 Ohms Switcher Rout; above 3MHz, you get at least 47dB attenuation.

enter image description here

Quiet GND and VDD have to be designed; once quietness is achieved, guard it, protect it, do not expose GND and VDD to trash sources.

How quiet is quiet? 47dB below 100mV ringing of SwitchReg is 200:1 reduction, or 500 microVolts. Given 2.5 volt ADC fullscale, you have dynamic range of 5,000:1, only 12 bits. To reach 16 bits, VDD trash needs 24 dB more filtering. To reach 24 bits (and 24 bits ADCs will be trashed by all aggressors entering the VDD pin), you need 72 dB more filtering, or 2 or 3 more PI filters.

And once 24-bit clean GND and VDD are achieved, you better protect them.

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The most obvious advice is to follow manufacturer's specifications and follow their suggested board layout. If possible, follow their BOM precisely for all high-current switching components. When embedding their layout into your PCB, take all measures to separate all return currents from your main ground and power ground planes, connecting the DC-DC area to your load only at a single point. Use additional C-L-C filters in close proximity you the switcher to limit the inherent ripples from entering and propagating along power rails of your PCB.

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