Minty Boost && FreeRunner
joerg at openmoko.org
Sat Nov 22 18:22:11 CET 2008
Am Fr 21. November 2008 schrieb Tobias Diedrich:
> Cédric Berger wrote:
> > On Thu, Nov 20, 2008 at 17:24, DJDAS <djdas at djdas.net> wrote:
> > > Cédric Berger ha scritto:
> > >> I did not have a look at neo's circuitry.
> > >> But whatever the method it uses, it cannot force 1A if 1A is not
> > >> available (wall charger unplugged from the wall won't give 1A :-p ) ?
> > >>
> > > Uhm...not exactly true... Ohm Law says: V = R * I -> I = V/R, and if
> > > R->0 then I->oo
> > > In practice if you power a load with a little impedance (in real systems
> > > the load is not always only resistive) the current requested will grow
> > > and the source could be damaged (try to short circuit a normal battery,
> > > you'll see a flash and if you maintain the circuit closed you'll meld
> > > the battery).
> > > This is why you should not ask 1000mA from the USB port (for example)
> > > unless you're sure the hardware could give it.
> > > Bye :)
> > >
> > Yes but I also have some car adapters that "did not mind" being
> > shorted (12v to 5v adapter, given for 350mA). So I doubt a device
> > wanting 1A would be worst than a short circuit... but what would be
> > the output in such a case I do not know.
> According to the MAX756 datasheet (the step-up converter used in the
> minty boost AFAICS), the switching mosfet should be protected due to
> the operating principle:
> The coil is shorted to ground until the current reaches about 1A,
> then switched off automatically (and then the coil discharges in
> series to the battery, effectively boosting the voltages).
> So the only things relevant to a overload situation are the
> coil rating and the diode rating.
> If both are capable of handling >1A continously, then switching
> the Freerunner to 1A can't break anything.
> The 1N5818 can handle 1A and the coil used has a saturation current
> of 1.5A.
> (However, it is _not_ short-circuit-proof, since in that case the
> current would flow directly from the battery over coil and diode to
> ground and will likely destroy the diode first)
> Switching noise is nothing to worry about (Switching frequency depends
> on load and battery voltage).
> Of course thermal dissipation can still be an issue.
you should use a polyfuse, to make device resistant against short on output
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