SIMtrace v0.8

historical

Hi Kevin,

On Tue, May 10, 2011 at 09:57:29AM +0200, Kevin Redon wrote:

> then it would be 4xAAA/AA (3xAA will only work if fully charged)

fine with me.  I honestly don't care about the number of batteries...
and the PCB will not be smaller than 4x AAA/AA anyway.

> the other idea would be :
> 
> 0.7-4.5 --- VREG 5V --->|----+
>                              |
>                              +--- VREG 3.3V
>                              |
> USB 5V ---------------->|----+

that should work, but you should use something like a 1N4001 as the
diode, to cope with more current.  However, I would want to have that
tested on a breadboard before sending the gerber off for PCB
manufacturing, just to make sure the power supply side is working.  We
still have a problem if somebody decides USB _and_ the battery  power
supply :/

In fact, now that I'm thinking off it, we should use one of those jacks
which disconnnect one contact when plugging in the power supply.  This
way we can actually interrupt the USB 5V once a power supply plug has
been plugged in.  This should be done independent of the decision on the
voltage regulator.

> or another nice idea (like the arduino does for the 5v) :

looks not that trivil to me, as you don't only need the FDN304 but also
the LM358 op-amp and external circuitry.  Probably a bit overkill.

So I think we should route the 5V USB via the power plug, and simply use
a low-drop-out 3.3V regulator, like in the current schematics.  This way
the LDO input will be USB 5V _until_ somebody plugs in an external power
supply or battery pack, which will have to provide at least 3.4V for
stable operation.

Regards,
	Harald
-- 
- Harald Welte <laforge at gnumonks.org>           http://laforge.gnumonks.org/
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