Re: USB power supply for charging

mpm wrote:

On Nov 11, 1:35�pm, "Paul Hovnanian P.E." <p...@xxxxxxxxxxxxx> wrote:
Mike V wrote:

I've been asked to add 2 USB power outputs to a product to allow
ipods, phones, cameras etc to be charged.

Sounds easy. USB specs 5V, 500mA max per output. So the obvious
solution is a 5V 1A regulator driving 2 USB connectors in parallel.

But is it that easy? Do USB peripherals need to talk to the power
supply?

Yes. A USB slave must negotiate with the power supply to draw any more
than a small amount of power. Some USB loads ignore the spec. Some will
operate in low power mode if the power supply does not adhere to the
protocol (a dumb 5V wall wart for example).

iPods will refuse to charge at all (at least my Nano won't).

Do the outputs need to be independant:? What else don't I
know? �I'm getting concerned about the unknown unknowns.

Yes. Each USB host port will handle its own power handshaking.

Any useful thoughts welcome.

http://en.wikipedia.org/wiki/USB#Power

--
Paul Hovnanian �p...@xxxxxxxxxxxxx
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Have gnu, will travel.

Damn.
That answers the question I had about whether I could just cobble up a
small power supply to charge the iPod rather than plunk down $35 for
one at the big box stores....

I was hoping to just cut an old USB cord and attach it to a regulated
supply.
If I understand the above correctly, the iPod Nano will not charge
because it did not enumerate on the USB? Is that right?

Enumeration is not required. You have to adhere to the USB power
protocol, which does not entail actual USB date communication.

Mike posted that biasing the USB data lines to 2.5V is sufficient to
satisfy the iPod. This may work, although it doesn't address other high
power USB devices which expect load handshaking.

--
Paul Hovnanian mailto:Paul@xxxxxxxxxxxxx
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Some people are like Slinkies; they serve no specific purpose,
but they bring a smile to your face when you push them down the stairs.
.

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