Powering through USB vs powering through VDD vs 5V Pin - STM32 F4 Discovery board

[Vtrai]

Hi,

I am just going through manuals to understand power circuitry on STM32F4 Discovery board for using ADC for voltage sensing.

Kindly help in understand difference between internal voltages (VDDA,VSSA,VDD, VREF+ etc) and other impact on power circuitry if I power the board using USB vs power the board using other Voltage pins (VDD, 5V etc).

[Ozone]

If you check the schematics of the F4 discovery, you might notice the following:

1. Both VDDA and Vref are basically the same potential level as VDD, only with an additional LC filter and caps.

2. The different power supply paths (including from USB) are decoupled from each other with a series schottky diode (BAT60).

This means, current draw transients in any part will create a VDD/VDDA transient, via the forward voltage of said diode.

In addition to a "constant" drop of about 0.2V, ie. you get a VDD/VDDA of 3.1V instead of 3.3V.

With the design, ST sacrificed analog performance/consistency for "foolproofness" and ease of use, which is comprehensible.

If you want higher accuracy, you will need a separate, and careful designed VDDA supply, with a proper layout.

For my (non-commercial) projects involving analog peripherals on the F4 discovery, I fared quite well with the current state of affairs.