[Arm-netbook] Laptop PCB schematics

Luke Kenneth Casson Leighton lkcl at lkcl.net
Tue Feb 21 02:21:43 GMT 2017

crowd-funded eco-conscious hardware: https://www.crowdsupply.com/eoma68

On Mon, Feb 20, 2017 at 1:07 PM, Internet <internet at devpi.de> wrote:

> I took another look and i am pretty satisfied with the results ;).

yeah, thank you for encouraging me to do it.

> However, a few things still need attention (i wrote it down on
> rhombus-tech.net
> <http://t224.mjt.lu/lnk/AEAAHq9bWRkAAAAAAAAAAGbE2wkAASHlFNUAAAAAAAZHgABYquow53AL_eKiTEKLFD2cPfS26wAGAjk/1/8CS5JnYHwfGJh6YxzsKZ1g/aHR0cDovL3Job21idXMtdGVjaC5uZXQ>
> )
ok let's go through them:

PCB3 - Power Board

   - connect thermistor for BQ24193 USB OTG Charger between BAT_NTC and GND
   (not TS which is wired to the fuel guage)

yep, cut/paste error there

   - change DVCC to SYS-V (otherwise LEDs won't work)

yep done

   - pin 2 of connector J2 (to PCB1) should go to VCC-5V0 (we need power
   for the EOMA68 card)

ah no.   rrright, the way it works is:

* EOMA68 Card Power is actually "OTG-like".  as in, you should expect it to
be *reversible* on demand, because there could actually be an OTG charger
plugged into the card, providing power *in* to the Housing.  this is why
the SY6280 is required.

* the laptop's main components need a *separate* 5.0V power rail, for the
USB Hub, Audio, LCD, USB2 connectors etc. etc.

* power could actually come *in* through the EOMA68-5V0 line and has to go
*in* to VBUS... which... argh, is missing so that needs sorting.

* power can also go *out* through the EOMA68-5V0 line (from VBUS) when the
bq24193 is put in "boost" mode (this is for OTG power compliance) ... which
is missing and entirely defeats the purpose of *using* the bq24193 in the
first place :)

*sigh* so still have to sort that out, creating a two-way power path
between VBUS and EOMA68-5V0 at *specifically* 5.0v, coping with 1.5A
sustained current, and under user-control... preferably without using too
many components.  SY6280's are my favourite "power cutting IC".  there's
one spare pin on PCB3 J4 which could be used to control its enabling.

my first guess at a circuit would be a schottky diode pointing from
EOMA68-5V0 to VBUS to act as "power in to VBUS" and the SY6280 to act as
"power out".  however, a *five amp* power regulator is going to be needed
on VIN to ensure that the input voltage never exceeds 5V (which it could at
the moment).

thoughts appreciated.

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