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NebraLtd / nebra-lora-hardware

Hardware Files for a variety of Nebra LoRa Hardware
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Poor PI voltage regulator layout #6

Open Funkton opened 3 years ago

Funkton commented 3 years ago

The Raspberry pi and computer reference design have examples of layout for the PAM2306 dual output regulators. The Nebra layout in the compute module carrier board has a very poor layout implementing the regulator. Due to this fact the regulated supplies providing power to the pi and the eMMC card have very noise, unstable supplies. I will not go into the reasons switch mode regulators require small loop paths, but will show examples of best practice regarding this regulator from readily available information.

Here is the example reference design provided by Diodes inc. https://www.diodes.com/assets/Evaluation-Boards/PAM2306-User-Guide.pdf

Here is the pi compute IO board which further reduces the loop currents by placing switching components closer together. Pi Comute IO Board PAM2306 Layout

Here is the Nebra layout. You can clearly see that the components which are critical to keep next to the pins and short traces are spread out and arbitrarily placed. This design has a major negative impact on the supply quality and can lead to unstable performance and/or damaged components. Nebra PAM2306 Layout

This shows the peak to peak noise on the 3.3V line on the Raspberry Pi 3B.

Stock PI 3B 3 3V Noise

This shows the 10x increase in peak to peak noise on the Nebra module's 3.3V lines, peaking over 3.6V. While this is under the absolte maximum of 4.1V, it is at the typical upper limit of SD cards and eMMC modules.

Nebra Compute Module 3 3V Noise
shawaj commented 3 years ago

Design was created by @ryanteck so would need his input on why it was designed like that, or whether it was an oversight