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set321go:
Hi Folks

I'm trying to make a stroke computer for rowing, there are some products out there but I thought it would be an easyish first real project. I've prototyped the idea on a mkr1000 and a lucky shield and it works fairly well, I did have some problems with the SPI and breadboard connections to an SD board but I think that was mainly due to poor connections and wires moving around (i had similar issues with i2c until i taped them all up).

I've been using kicad to put together a design and i'm at the point where I think i'm ready to place components on a pcb but it would be great if I could have some input from those more knowledgable than I (i'm a software guy no background in EE) on the schematic. I really struggled with were inductor sizing and I feel i've only really scratched the surface in that area. Some feedback on my power & charging circuitry especially would be much appreciated.

For a design like mine that is primarily going to be low current + low voltage how important is the inductor & capacitor sizing?

(I don't seem to be able to generate a jpg from kicad so i've attached a pdf)
nAyPDJ:
Power flows from up to down, and signal flows from left to right. You mostly follow this convention, but you could improve things in a few spots.

I generally also prefer to use global labels in KiCad, since they allow me to indicate in which direction the signal is flowing (see attachment). This is just personal preference though.

What is the "-BATT" and why have it if it is equivalent to GND?

At first glance, 33u seems huge for the LM2731 boost, especially with 1.6MHz switching. Did you follow the "Detailed Design Procedure" in the LM2731 datasheet? Is it the right value? You display claims to have a typical power consumption of 18mA.

> It might be more efficient to drive directly from VBAT

Nope. The SYS pin on the bq25895 says it's connected directly to the battery unless Vbat is under 3.5V, in which case Vbat is boosted up to 3.5V.


--- Quote from: set321go on April 23, 2019, 01:22:21 am ---For a design like mine that is primarily going to be low current + low voltage how important is the inductor & capacitor sizing?

--- End quote ---

Too big and it's going to increase the board size and cost. Too small and it won't work.
set321go:
Thanks for your feedback, much appreciated, I will go and work on making things a bit more 'standard'


--- Quote ---What is the "-BATT" and why have it if it is equivalent to GND?
--- End quote ---
I got rid of it already, I labelled it to remind me what was going on.


--- Quote ---At first glance, 33u seems huge for the LM2731 boost, especially with 1.6MHz switching. Did you follow the "Detailed Design Procedure" in the LM2731 datasheet? Is it the right value? You display claims to have a typical power consumption of 18mA.
--- End quote ---
Yes but i didn't find enough info there for my limited knowledge I used another document from TI  "How to Select a Proper Inductor for Low Power Boost
Converter" http://www.ti.com/lit/an/slva797/slva797.pdf

I found this much more useful for my limited knowledge but looking back at my math


--- Code: ---IL = (13 x 0.05) / (3.8 x 0.8) = 0.214
L = 3.8 x (13 + 0.5 -3.8) / (0.3 x 0.214 x 1.6e6 x 13.5)  = 32uH
--- End code ---

I think I used 0.05 (50mA) for my output current. I selected 33uH because it was the closest standard size. I think maybe I did something wrong there because now that comes out as 26.5uH. Using 20mA as a starting point i ended up with 66uH.
ebastler:
I am a bit confused about your supply voltages: 3.3.V, 3.8V, and 5V. Do you need them all?
set321go:

--- Quote ---I am a bit confused about your supply voltages: 3.3.V, 3.8V, and 5V. Do you need them all?
--- End quote ---

3.3v is the logic voltage (ideally i could just use this everywhere)
5v is the external input voltage (usb)
3.8v is the output voltage (i think) from the bq25895
13v is the output from LM2731 for the display

I took the output of bq25895 (3.8v) for the input of LM2731 (used for the display) as it was already there instead of using the 3.3v
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