Is this decoupling configuration for the STM32F407ZET6 correct

[Noah_Stoessel]

Hello im creating one of my first stm32 boards and need some help ive watched many tutorials about decoupling capacitors and have referenced what i could from the datasheet could anyone confirm if this decoupling setup would work or if modifications are needed:grinning_face:. I am using a usb-c connector for programming and power with an ams1117 3.3 volt regulator with the appropriate solid tantalum capacitors. For a little more info these are all smd 0603 capacitors and the component marked fb1 is a ferrite bead with a resitance of 120ohms at 100mhz.

 

[TDK]

Should work just fine. The ferrite bead isn't necessary if you aren't very concerned with analog performance.

> I am using a usb-c connector for programming

Through an STLINK-V3MINIE? Note that connecting USB directly to the chip itself won't allow for debugging.

 

For general reference, in case you haven't seen it:

https://www.st.com/resource/en/application_note/an4488-getting-started-with-stm32f4xxxx-mcu-hardware-development-stmicroelectronics.pdf

[Noah_Stoessel]

Wouldnt wiring the d+ and minus pins to the chip then using a 5,1k pulldown resistor to ground for the cc pins work? then leaving the sbus1 and 2 pins unconnected should work? or not because i was looking of using usb c for programming without and extra board if there any ics that would work if my solution doesnt work and you knew of them could you give a link or a reference. thank you for your help anyways.

[TDK]

> Wouldnt wiring the d+ and minus pins to the chip then using a 5,1k pulldown resistor to ground for the cc pins work?

Debugging requires a standalone debugger. They're cheap. I would recommend buying one, implementing the header on your board, and using that for debugging and programming. Here is one:

https://www.st.com/en/development-tools/stlink-v3minie.html

ST dev boards generally have an ST-LINK debugger built onto the dev board itself. It's a separate STM32 chip that sits between the target MCU and the USB.

[MSing.1713]

Unless my eyes are playing tricks, it looks like you have 12 VDD pins and 11 0.1u bypass caps.  Probably best to make sure there is a cap on each VDD pin.

[MSing.1713]

...and FWIW, tantalum is considered a 'conflict' material.  I don't think I've see tantalum caps in a design for years.  I cannot speak to the necessity of using tantalum, though.  I can say that the LDOs I've used have generally stated that they can provide stable regulation with ceramics.  Often, though, I'm only looking for a few 100mAs. Once we get to the range of 1A, generally we place a buck regulator (e.g. ACOT) for better efficiency and these, too, operate fine with ceramic caps.

[LCE]

Tantalum - wasn't that something last seen in the 90s? :D

Depending on what else is powered from these 3.3V, a switching regulator might be more appropriate than the LDO. Check power dissipation.

And I would also recommend putting the SW DEBUG pins on a header.

And take care of the boot pins, at least that you can access them in case you need to.

[MSing.1713]

I think folks still use tantalum because datasheets, like this one, recommend it.  I was curious about the part since I hadn't seen it placed.  It's quite inexpensive.  It also expects a large output capacitor to stabilize the output.  Modern LDOs operate fine with 1u.  This device recommends 22u.

At 1A, you're looking at (5-3.3)*1W dissipation, so roughly 2W.  Shrug.  It'll get warm.  The DS makes it look like it can handle this configuration.  If it is mounted over a large copper, it should survive.

[LCE]

I guess he will not need 1A, because for 1.7W you need a lot of thick copper or some other cooling.

Anyway, even for space reasons I would check what the max. current requirement is, then decide if a switching regulator makes sense - or even if a SOT23-5 LDO might be enough.
Interestingly, there are almost pin-compatible LDO / buck regulators in SOT23-5/6 out there, so for some designs I basically put the buck circuit on the board, but if an LDO was good enough, I just bridged the inductor:

[RhSilicon]

It might be interesting to know this:

Transmission Lines - Signal Transmission and Reflection

https://www.youtube.com/watch?v=ozeYaikI11g

 

Why is reflected power bad?

 

The reflected power from a bad load will put additional stresses on the transmission lines. As an example, if the normal transmitter power is 10,000 watts and the load has a VSWR of 1.05:1, the maximum RMS voltage in a 50-ohm transmission line will be 725 volts with a peak voltage of 1,025 volts.

 

https://www.radioworld.com/news-and-business/the-abcs-of-swr-vswr-reflected-power-and-return-loss

[LCE]

@RhSilicon WTF? 10kW? What do you want to tell us? That's mostly irrelevant for this topic, ignore above post @Noah_Stoessel 

[RhSilicon]

Keep ignoring it's better.

[LCE]

It's some interesting information, but rather off topic concerning a low power MCU power supply.

Or I don't get it, so please explain @RhSilicon 

Or isn't that about high power radio frequency transmission, and not about DC power supply?

[LCE]

Okay, now I get it... you refer to my switching regulator layout hints.
But still, it is a little too far-fetched IMO concerning the layout of a switching regulator.

That's why I tried to sum things up with:

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When doing the PCB layout, read the datasheet, understand where big (switching) currents are flowing and keep these paths short.

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