Alternative BluePill sized F4

[racemaniac]

In this thread we're designing an F405 board that is pin compatible with a bluepill (and with some nice extra features).

But for myself, i was wondering: if i get a more powerful chip, what would i want? one of the main things would be SDIO (but those pins aren't broken out on the bluepill clone). For fabricating, <5cm length would be ideal (but the bluepill is slightly longer due to its 20 pin rows), and the huge boot header & the 32Khz crystal are nice to have, but not necessary.

This is my first attempt at something slightly smaller than a bluepill (4.85cm long), the pins are further apart (on a breadboard, you'll have 1 row of pins free on 1 side, 2 on the other), so i could fit a micro sd card slot in between the pins. No 32Khz crystal, but the pins are available, and the capacitors for it can be fitted (so just take one of the cylidrical ones, and put it on the pins). And except for the pins used for SDIO (PA8, PC8-12, PD2), all pins are broken out (PA8 was chosen for the card detection pin, as PA9-12 are USB, so most of the higher 8 PA ports is already in use for an important peripheral).
Besides that the features would be the same as the other one (dual power supply so the analog rail is separated, and if we figure out a good way: USB OTG support) :

BPF4SDFront.jpg (72.98 KiB) Viewed 887 times

BPF4SDBack.jpg (71.05 KiB) Viewed 887 times

What do you guys think about a board like that?

[Pito]

1. use an 800mA voltage regulator for Vdd
2. do use more vias (heat, grounding)
3. wait on stm32H743/753 in 100pin flatpack

[racemaniac]

1. use an 800mA voltage regulator for Vdd
2. do use more vias (heat, grounding)
3. wait on stm32H743/753 in 100pin flatpack

you don't think 600mA is enough?
and you're expecting heat issues?? (i've only got experience with the 72/84 Mhz microcontrollers, and heat doesn't seem to be one of their issues ^^). or do you mean heat at the voltage regulators (i'll have to check their datasheet on how hot they can get)

[Pito]

Heat around the voltage regulators. You have got a large gnd plane beneath them, but you do not have any vias for a thermal connection to that plane (the heatsink). Calculate the temperature of the Vdd chip (look for the topic I posted few weeks back). That sot23 package will smoke with 5V input and >=200mA output.

[racemaniac]

indeed good idea

[bobc]

3. wait on stm32H743/753 in 100pin flatpack

Oh yes, that would be awesome

Haven't looked, maybe there is some pin compatibility? Even if not, experience with F4 should help with H7 design.

[Pito]

indeed good idea

Do use a larger package with Vdd regulator (ie SOT-223 or Dpack - a package with a heatsink fin) - with large copper around/beneath the heatsink fin. Use a lot of vias to thermally connect the top/bottom copper.
Read the chapter 10.1. carefully:
http://www.ti.com/lit/ds/symlink/lm1117.pdf

[racemaniac]

btw, i've done some reading about the absolute maximum power conditions i can expect:
the stmf4 is rated to draw max 240mA, and sd card writing full speed probably max 100mA
The f4 running full speed with all peripherals will require 100mA (and then has 140mA extra to drive things with on its pins). and then +10mA for the 2 leds.

350mA from that ap2112@5V would mean it heats up to about 100°C at ambient temperature, which would be well within its max rating (160° i think according to the datasheet), and is an absolute max ratings for everything on the board (but that's then of course not calculating the various modules people could connect to the board...).
I'd consider changing it to something else if i found something with better thermal properties and an equally low voltage drop. Moving to an LM1117 (or an AMS1117) would mean it not being able to properly run of a lithium battery without first boosting the voltage >_<...

but it's an interesting point. it's a difficult design tradeoff to make... would 2 of those ap2112's in parallel work (i would say no as one of both might end up doing all the work and burning out if it has small differences to the other one). anyone got experience with that?

i've just made a testsetup here with a maple mini with a ap2112 doing 270mA with 4.6V in, and it's handling it graciously so far ^^

[Squonk42]

(4.6 - 3.3) * 0.27 = 0.35W
(5.0 - 3.3) * 0.35 = 0.6W. Assuming a SOT25 with 184°C/W for tja, this is 0.6 * 184 = 110°C above ta, so you fry your chip if ta > 40°C. Not so unlikely in temperate countries behind a car windshield... Anyway, going to these extreme temperatures will decrease substantially your chip lifetime, beside being dangerously hot.

Putting LDOs is parallel is calling for disaster.

Why don't you use the bigger AP2114 in a larger package like DPack or D2Pack? It is rated at up to 1A with lower dropout voltage, same Iq, same PSRR and better thermal resistance, down to 90 or even 73°C/W for these packages?

Is it your final layout? If so, there is something really strange above the reset button (top side): why are there vias on both the 2-signal group and the 3-signal group? It is completely useless.

I also suggest moving all the passive components on the top side (except for the SD connector), such that in the case this latter is not mounted, the bottom side is flush. I also suggest turning the SD Card connector 180°, so that the card exit faces the short PCB edge.

For silkscreen text, every character with height below 32 mils (0.8 mm) won't be printed correctly with standard PCB manufacturing quality.

[racemaniac]

(4.6 - 3.3) * 0.27 = 0.35W
(5.0 - 3.3) * 0.35 = 0.6W. Assuming a SOT25 with 184°C/W for tja, this is 0.6 * 184 = 110°C above ta, so you fry your chip if ta > 40°C. Not so unlikely in temperate countries behind a car windshield... Anyway, going to these extreme temperatures will decrease substantially your chip lifetime, beside being dangerously hot.

Putting LDOs is parallel is calling for disaster.

Why don't you use the bigger AP2114 in a larger package like DPack or D2Pack? It is rated at up to 1A with lower dropout voltage, same Iq, same PSRR and better thermal resistance, down to 90 or even 73°C/W for these packages?

Is it your final layout? If so, there is something really strange above the reset button (top side): why are there vias on both the 2-signal group and the 3-signal group? It is completely useless.

I also suggest moving all the passive components on the top side (except for the SD connector), such that in the case this latter is not mounted, the bottom side is flush. I also suggest turning the SD Card connector 180°, so that the card exit faces the short PCB edge.

For silkscreen text, every character with height below 32 mils (0.8 mm) won't be printed correctly with standard PCB manufacturing quality.

the ap2114 indeed seems great, but is a lot more expensive, but indeed maybe worth it
gonna have a look at it .
there is only 1 seller of it on aliexpress, about 0.7$ a piece (10 times more expensive than the 2112, but he's only selling per 5), but it looks like a great regulator indeed.

about the sd card slot, i put it like that on purpose but indeed, other orientations are also possible.

i think most silkscreen is 0.8-1mm, i usually go for dirtypcb where they specify the silkscreen lines have to be at least 0.15mm thick, and KiCad can go to 0.7mm with that line thickness.