Minty Boost && FreeRunner

Tobias Diedrich ranma+openmoko at
Thu Nov 20 21:47:53 CET 2008

Cédric Berger wrote:
> On Thu, Nov 20, 2008 at 17:24, DJDAS <djdas at> 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.


Tobias						PGP:

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