[Arm-netbook] [flying squirrel] need quick review of power input for RTC battery on ATSAM4S

[joem]

On Tue, 2013-11-05 at 12:04 +0000, luke.leighton wrote:
> this is mainly a question for joe (and anyone else with schematics
> review experience!)
> 
> the goal is to provide dual-power for the ATSAM4S.  atmel recommend
> using 2 diodes however the voltage for the IO needs to be above 3.0v
> in order to run USB, the AXP209 can only output 3.3v max and a diode
> would suck 0.7v off of that.
> 
> so i've been advised to use a MOSFET instead, and need to check it's
> going to work.  so here's the schematic that needs checking:
> 
> * http://hands.com/~lkcl/eoma/kde_tablet/rev3_review/atsam_vddio_power.png
> 
> * note the diode which provides backup power (from the AXP209 RTC supply)
> 
> * also i'd like to use the AXP209's "PWROK" signal to control when the
> ATSAM4S powers up.  the datasheet says it's "HIGH" when power is "OK"
> and it starts off "LOW" when the AXP209 starts up.
> 
> as the AXP209 PWROK signal is "HIGH" when "OK", that's why i *believe*
> that the transistor (Q6) is needed, to invert that so that when PWROK
> is "HIGH", the MOSFET delivers power.
> 
> greatly appreciated a quick review of that logic.


It will work as you describe it. You can replace diodes like 1N4148 with
SS16 or similar schottky diode - their voltage drop is 0.2 to 0.3V.
They are rated 1A / 60V (farnell stocks them) - check to make sure the
current and voltage is handled as needed.