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I am designing a board for my Bachelors thesis. The board takes 12V - 24V as an input. The dimensions are 100mmx100mm. It will be a 4 layer board and has an onboard antenna, multiple sensors and a Raspberry Pi compute module 4 as well. I am thinking of reserving 1 layer as a power rail and 1 layer for GND. My question is:

  1. Should I regulate the 24V voltage into 3.3V & 5V near the VCC, use wide track width and then just use low pass filters near the Vin of ICs?

or

  1. Should I feed 24V into the power rail and regulate the voltage locally at multiple points on the board?
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2 Answers 2

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A single regulator for 5 volts and a single regulator for 3.3 volts is all you'll need by the sounds of it. Use your power layer to route 5 volt and 3.3 volt supplies but, try and place those regulators reasonably close to where they are needed. Don't forget to calculate heat dissipation of these regulators if linear types are used. I might consider using a buck regulator to generate the 5 volt and then tee-off 5 volts with a linear regulator for 3.3 volts.

The on-board antenna may require removal of any copper on all layers directly below it.

use wide track width and then just use low pass filters near the Vin of ICs

Local decoupling capacitors to ground is probably all that is needed. Rarely, something like a ferrite bead or small value series resistor can improve things and a bit more rarely, a series inductor can help. It really depends on what the chips are and what the data sheets say about supply filtering.

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Just a single 5V and 3V3 regulator, as Andy suggested. 5V is often better as a switcher unless current is very low, because a linear 5V regulator will have 19V across it, which tends to make it get hot. Decoupling caps are key - look at the data sheets for your chips. More is better, you can always leave a few unfitted if you go over the top. Insufficient decoupling leading to power rails being too noisy causes horrible problems and is equally horrible to diagnose and debug, usually requires another board spin with modern SMD devices. So be generous.

(EDIT) Another thing to keep in mind is that you generally want to power all your digital logic from one common supply at each voltage. Why?

Let's say you have a bunch of devices at nominal 5V, +/-10% (old fashioned example). However, let's say that the maximum voltage on a digital input is (Vcc + 0.6V)

If you power them from two separate regulators which are also 5V +/- 10%, you could be unlucky and have half your board running on 4.5V and the other half at 5.5V. Individual devices will not have a problem, but if a 5.5V device drives a 4.5V device, you just violated a constraint. So in general, try to keep things running off a common rail to avoid having to think about this.

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  • \$\begingroup\$ I have LM2596 buck converter for 5V and I was thinking of a AP2112K LDO from 5V to 3.3V. Or should I also use a buck conv from 24V to 3.3V \$\endgroup\$
    – Emre Mutlu
    Jan 5, 2021 at 15:34
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    \$\begingroup\$ @EmreMUTLU Note that the AP2112K has somewhat limited PSRR at the 150 khz switching frequency of that buck converter. This won't matter for most for digital components, but if any of your sensors require an analog voltage rail (e.g. do you have an ADC in there?), consider using a better regulator. For one off designs, I usually pick a better regulator than I think I need. \$\endgroup\$ Jan 5, 2021 at 15:39
  • \$\begingroup\$ @user1850479 yes I do have an ADC. Okay, do you have a recommendation? \$\endgroup\$
    – Emre Mutlu
    Jan 5, 2021 at 16:23
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    \$\begingroup\$ Be generous with decouplers = +1 \$\endgroup\$
    – Andy aka
    Jan 5, 2021 at 17:11

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