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I'm working on designing a PCBA which uses the STM32F446 as the micro. The issue I'm dealing with is that something seems to be shorted between 3.3V and GND, likely due to some fault with my schematic.

Things I've tried:

  • 3.3V supply is known good with micro not populated. No excess current is drawn. Micro has been populated by board house so that is also known good. I tested a couple boards with the same result as well, so some manufacturing error can be ruled out.
  • The micro is oriented correctly (I've been particularly careful about this because STM has a weird marking system).
  • Resistance drops from ~5k to ~400 ohms when the micro is present.
  • I grabbed the schematic and PCB footprint from STM, so that is also known good.

At this point I've ruled it down to an error in my schematic, but after poring over the datasheet I've been unable to find where that might be. Any and all help would be greatly appreciated. Please let me know if there's any more information I can provide.

enter image description here

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Why did you shortcut the V_CAP1 and VCAP_2 to VDD? These voltages are internally connected to the end of the internal voltage regulator and the voltages are typically at about 1.14 up to 1.32 V ! You have to place a capacitor from pin 48 to VSS (GND) and an extra cap from pin 73 to VSS (GND) - typically 2.2 µF (or up to 4.7 µF) with low ESR (< 1 Ω). All information found in datasheet STM32F446xC/E at chapter 3.17.2 ff

It is also very useful to go from reference designs and development boards.

Overview: enter image description here

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  • \$\begingroup\$ Thanks for your help. Once you read a datasheet a couple times I guess you stop actually reading. The micros I normally work with don't have an internal regulator so I guess that's where my mistake happened. Have a good day and I appreciate the help! \$\endgroup\$
    – A Ryan
    Apr 10 '20 at 1:34
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You have connected VCAP pins to 3.3V supply.

The VCAP pins are outputs from the internal core voltage regulator. You are essentially feeding 3.3V into the low voltage core, and the chips are now fried.

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  • \$\begingroup\$ Yep. I feed my Vcap 1.2V \$\endgroup\$
    – DKNguyen
    Apr 9 '20 at 22:04
  • \$\begingroup\$ Forehead moment. Thanks a lot for the help. \$\endgroup\$
    – A Ryan
    Apr 10 '20 at 1:34

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