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I am developing a low power board using the STM32L053C8T6TR and have written a schematic to power the board with a 3V coin cell battery and have a few questions.

The datasheet mentions N x 100nf + 1 x 10uF caps on VDD, VSS and while looking at the discovery board schematic, I replicated and came up with the following LDO

In addition to that, the schematic shows the following. Based on the DS STM32 Getting Started I have the following VDDA-VDD

My question, I am trying to power the board primarily through a coin cell at 3V with the ability to run power through USB if needed while programming (If not using the STLink SWD breakout) Based on the schematics and documentation, I am not really able to tell if I am using too many caps around the LDO, and around the bead between VDD-VDDA. The documentation states N * 100nF + 1 10nF. What is the optimal capacitance and number of caps needed on the 5V input side, and on the output side?

Same goes for the VDD-VDDA capacitors and bead. Am I over subscribing capacitance for a 3V coin cell?

Edit: After looking through some more schematics and datasheets, i'm not even sure there needs to be caps between VDD-VDDA with the bead. Most schematics I see only have the bead. Since this is going to be a super low power dev board working on a coin cell, is all that needed?

Edit: Edited the question to focus on power input and capacitance around the LDO and VDD-VDDA

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    \$\begingroup\$ Do you have a specific question? As written, this is a very broad question. \$\endgroup\$
    – Elliot Alderson
    Aug 15, 2019 at 20:23
  • \$\begingroup\$ I'm really looking to verify my pin out and schematic. I want to make sure i'm not overusing the caps on the LDO, and VDD-VDDA. The pinouts for this MCU are not difficult to understand, but I want to make sure I'm not missing something it the few schematics I put down. The pins should be OK, but just looking for some advice on if this is correct or not. \$\endgroup\$
    – TyrantUT
    Aug 15, 2019 at 20:29
  • \$\begingroup\$ Ok, LDO caps, look at the datasheet, one 10nF on the BYPASS pin and a 1uF on the input and output(1uF is minimum for stability). As far as the STM32 goes, trust me, while the cap may not be right beside the FB on the schematic, there will be 1 or 2 caps between the power pin and the bead. If your not going to be using analog at ALL(ADC, comp, op amp), you can loose the 1uF/bead and reduce the other to about 40nF. \$\endgroup\$
    – GB - AE7OO
    Oct 4, 2019 at 17:30

2 Answers 2

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Ok, Your missing the STM32 schematic itself, to verify everything, we need that.

Next, are you planning on being primarily battery powered? If so, lose the TLC5940! If your just wanting to have a few LED available, just drive them from the STM32, if your concerned about using too many pins, have a look at Charlieplexing.

TLC5940: First off, this thing is a beast! Unless you need ALL of the bells and whistles it offers, I would start off with something easier, like the TLC59108 or the TLC59116.
Most likely you meant it as a decoupling cap, but that 0.1uF in the VCC line needs to be changed. Next, how are you planning on programming it? You need SCLK, SIN, XLAT connected to program it.

The power, SWD, and USB(What type of interface are you using for the USB renum?) all look OK. Oh, BTW, you can reduce the 100uF decoupling caps down to about 40uF, since your battery powered. I can't remember the exact App Note, but ST does talk about it.

I've never used that BT module before, sorry. But there seems to be a slight confusion to my eyes. It looks like your connecting SPI to the BT module, but that EEPROM is also SPI, and there are no active Chip Selects. As it is it looks like you'll talk to both at the same time....

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  • \$\begingroup\$ I am planning on having it primarily battery powered, yes - and USB for programming only, but powered while plugged into USB. \$\endgroup\$
    – TyrantUT
    Aug 16, 2019 at 18:14
  • \$\begingroup\$ Then I would drive the LEDs directly. At most I would use a transistor/FET to switch the LEDs current; that TLC5940 is both overkill AND a battery killer. And have you looked into that SPI problem I mentioned? \$\endgroup\$
    – GB - AE7OO
    Aug 16, 2019 at 18:20
  • \$\begingroup\$ As for USB, the discovery board schematic has a transistor coming off the pull up on USB_DP with lines to 3v and USB_RENUMn (PIN 38) PA15, but I didn't connect that since from my understanding, that is for host mode detection? I want to be able to program from USB and SWD (External STLink chip). I followed the Discovery board schematic and left off the discovery jumpers so it uses STLink only. \$\endgroup\$
    – TyrantUT
    Aug 16, 2019 at 18:29
  • \$\begingroup\$ As for the LEDs, perhaps driving directly, or using the 59116 may be a better solution. I like the ability to run LEDs from a driver as that's what i'm used to. I would assume omitting the driver would help save power as well, but I like the ability to dim if needed. \$\endgroup\$
    – TyrantUT
    Aug 16, 2019 at 18:30
  • \$\begingroup\$ The BLE module, I followed the X-NUCLEO-IDB05A1 schematic, and I had the same concern you do. It seems weird.I think I'm not following it correctly and will need to revisit. The BLE should talk to the EEPROM, and the BLE to the MCU, which would make sense. \$\endgroup\$
    – TyrantUT
    Aug 16, 2019 at 18:32
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N x 100nf + 1 x 10uF caps on VDD

  1. Not really mandatory.
  2. Please provide provisions for the caps but do not mount all the 10uF capacitors.
  3. Wait for the feedback from lab testing and EMI EMC testing.
  4. You do not need a ferrite on the VDDA line if your product does not need such a noise isolation. (for example: if it is a simple NTC reading for temperature)
  5. Follow the datasheet recommendation for the LDO capacitors
  6. Since it is battery powered, there is no noise on the VIN as such
  7. Every capacitor extra you add is a leakage path to ground. Please provide option in the PCB but experiment later for the best possible least possible option. I am not awre about the whole application(RF, Motor or others)
  8. Consider adding safety diodes for reverse voltage protection (a PMOS in series with VBAT provides both reverse voltage protection as well as lest drop across it unlike a diode
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