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I'm working on an STM32F407 PCB for a flight simulator that has the following conditions:

  1. Normally powered via 12V and an AMS1117-3.3 to power the MCU.
  2. Needs to provide 5V for certain peripherals that run off of 5V. This adds an AMS1117-5 connected to 12V.
  3. Needs to be able to run off of VUSB (5V) for programming, is it needs another AMS1117-3.3 because i dont want to connect the 12V and VUSB together in case i accidentally leave the 12V on when connecting USB.

So a total of 3 LDOs seems a bit overkill, is there a simpler solution? Afaik a diode won't be enough to connect the 12V and VBUS together right? I'm trying to keep it to "basic components" on jlcpcb.com/parts to keep cost down.

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  • \$\begingroup\$ Does it matter if the 5V rail is also powered from USB? Schottky diode from the USB to the 5V rail? You have enough dropout voltage to still get 3.3V after the diode drop and regulator \$\endgroup\$
    – Criticizing Israel not allowed
    Commented Sep 24, 2023 at 13:08

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Take away U3, add a 3-pin header and jumper to select between USB and 12 V. Middle pin connected to U2 Vin. Or if pcb is enclosed, wire a 3 way toggle switch the same as the header.

"i'd like to keep it automatic if possible"

Most simple approach is adding VUSB to U2 Vin through a diode. If there are devices on the 12 V rail that possibly consume current when 12 V is disconnected, another diode, now between 12 V and Vin will eliminate that. Now U3 is redundant.

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  • \$\begingroup\$ Thats an option, but i'd like to keep it automatic if possible. There will be a lot of switching between programming (5V) and testing (12V). \$\endgroup\$
    – Maciej Swic
    Commented Sep 24, 2023 at 12:47
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    \$\begingroup\$ Thanks, so what i need is a diode rated for the right current and a reverse breakdown voltage of at least 12V, do i understand that correctly? In this case an SS54 would work for both the 12V and 5V inputs? datasheet.lcsc.com/lcsc/… \$\endgroup\$
    – Maciej Swic
    Commented Sep 24, 2023 at 16:12
  • \$\begingroup\$ Max reverse voltage would actually be 12-5 = 7, so from 20 (MDD SS52) and up works fine. \$\endgroup\$
    – MiNiMe
    Commented Sep 24, 2023 at 16:30
  • \$\begingroup\$ But only the SS54 is a "basic" component at my fab, each non-basic is an extra $3/order which is why i asked about the SS54. Anyway, thanks a lot for the help! \$\endgroup\$
    – Maciej Swic
    Commented Sep 24, 2023 at 16:36
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    \$\begingroup\$ "from 20 (MDD SS52) and up" But not over SS54.. Vf increases \$\endgroup\$
    – MiNiMe
    Commented Sep 24, 2023 at 16:43

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