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

[luke.leighton]

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
>>
>> * 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.
>
>
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