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I have designed a few PCBs, all of which have worked fine. I'm starting to get into some more complex designs, switching power supplies, etc.. I have a few questions related to proper power handling as it relates to current flow and current loops.

For the questions, I am referencing a 2 layer board. Bottom and top layer are GND pours, with very few signal traces on the bottom pour. Using this simple schematic as a reference:

simple schematic example

  1. The main supply voltage gets regulated to 5v. Where should the 5v trace connect to the circuit at? I assume at one of the two decoupling caps.
  2. Letting my assuming for number 1 be true, should I take all additional power traces from that point(this schematic doesn't show any other components, but if there was)? Similar to a star ground, but with 5v.
  3. So I know return current wants to flow under the original trace for high frequencies, does that apply for the LED?
  4. When the return current flows from the LED back to the IC GND, which GND pin will it flow to?
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2 Answers 2

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You need to put the bypass capacitors as close as possible to the power supply pins on the processor. If you are using an uninterrupted bottom layer ground plane (I assume surface mount given the -au suffix for the atmega328) assume the ground path is on the bottom and put a decent via very close to the ground side of each bypass cap. Ideally there should be a physical flow ground plane -> via -> bypass cap pin -> processor pin, all as short and wide as possible. Power should ideally be run in a star with wide traces but most complex boards end up a little more tree-like.

BTW, you'll need a current limiting series resistor for the LED and routing for the LED traces is generally pretty non-critical.

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  • \$\begingroup\$ Yeah, edited to add the resistor, overlooked that.. So where would the star point for the power be located at? Tie the main 5v at the decoupling cap? What if they are on opposite sides of the IC? \$\endgroup\$ Oct 10, 2017 at 22:28
  • \$\begingroup\$ C4 goes as close to the LM1117 as possible, the star between C4 and C1/C2. BTW, the ATMEGA328 is pretty tolerant of bad layout with the exception of power supply noise coupled into the ADC (both inside the CPU and through the sensor). \$\endgroup\$ Oct 10, 2017 at 23:02
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Supply Voltage drop and ground shift occurs from IR and (ESL)dI/dt

Can you compute these properly? There are many calculators for prop delay, track inductance, impedance, data sheets for micro and nanosecond rise times.

Do LED’s have xx ns rise times?

Supply decoupling is necessary.

Ringing occurs from mismached impedance when prop delay exceeds rise time.

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