[maemo-users] [maemo-users] Re: Charge from a USB port.

From: Jon Smirl jonsmirl at gmail.com
Date: Fri Feb 2 00:26:48 EET 2007
On 2/1/07, Larry Battraw <lbattraw at gmail.com> wrote:
> > For example, this guy has a power injector he's used for USB Host mode that
> > simply uses a 7805 regulator (pumps up to 1A) and a 9V battery. USB is
> > supposed to have a current limiting power source controlling the power sent
> > to a device based on the devices requests. That 7805 doesn't do this and
> > simply follows ohms law above, yet it doesn't tend to break things because
> > it keeps the 5V steady enough.
>  I'm not sure how this is relevant since this power injector is meant
> to provide a tiny amount of current at 5V, just enough to tell the 770
> something is plugged in.  It doesn't actually power the 770.

The OMAP3430 processor has a companion power management chip, the
TWL4030. Is this used in the N800? If so, it's spec sheet says that it
supports battery charging from USB. Hard to tell how it works with no


Maybe the next Nokia tablet will use something like the Freescale MX31
family which has public documentation.

> > (Link:
> > http://thoughtfix.blogspot.com/2006/01/usb-power-injector-2.html) This is
> > also the same method used by a whole slew of home made ipod and phone
> > chargers.
>    All the phone chargers I've seen have a significant internal
> resistance-- they are _not_ designed the same way as devices that
> charge directly from USB.  All the USB phone chargers I've seen have a
> inline adapter of some sort (including Nokia's new USB charger) to
> boost the voltage and limit the current.  As mentioned, Thoughtfix's
> injector tells the 770 it has a USB device plugged in and does not
> power the 770.
> >
> > Simon's solution sounds like what I would do.
>   Simon's solution scares me :-)  I messed around enough to see how
> easy it is to start feeding far too much current in very quickly.  USB
> ports are typically current-limited, but it still concerns me.
> Larry
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Jon Smirl
jonsmirl at gmail.com

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