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

[joem]

On Tue, 2013-11-05 at 13:48 +0000, luke.leighton wrote:
> perfect.  thanks.
> 
> On Tue, Nov 5, 2013 at 1:18 PM, joem <joem at martindale-electric.co.uk> wrote:
> > 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


hmmmp....

I just scanned it again - notice a problem - the WNM2306 is an n-FET.
You need a p-FET with similar ratings to work it correctly.
:
Also when fitting FETs, use the FET symbol with FETs 
(such as for the 2N7002) to avoid layout problems.