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? Do the outputs need to be independant:? What else don't I know? I'm getting concerned about the unknown unknowns.
Any useful thoughts welcome.
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).
Yes. Each USB host port will handle its own power handshaking.
-- Paul Hovnanian paul@hovnanian.com ----------------------------------------------------------------------- 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?
I thought USB devices could still pull some decent current even when not enumerated. And that assumes a compliant controller (which my regulator most certainly would not be). I'd have to drag the book out - I know it's not much, but might be enough juice to keep the batteries up for long trips in the car?
I've put it off because the longest trip I generally make is a 5-hour trip to Mom's, and the iPod keeps up pretty good. It definitely requires recharging (and new podcasts!) for the trip home, though.
Actually, that brings up another issue. Eventually I expect the iPod battery to weaken & die. I suppose you can get replacements installed. But what I really need is one of those programs where you can back the entire device up to a hard drive. If anyone knows a particularly useful program that's worth the money, please post it here or email me. Thanks.
-mpm
-mpm
Still googling. But I read that biasing the 2 data pins to 2.5V with
47k resistors makes the Ipod work.I'll test this as soon as I can borrow the kids Ipods.
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@Hovnanian.com ------------------------------------------------------------------ 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.
possibly.
some do.
no.
I found burried on the usb website:
They give a standard which includes USB dedicated chargers.
The requirement seems to be 0.5A-1.5A, 4.75-5.25V D+ and D- linked by less than 200 ohms No processor required..
So it is that easy - provided of course that things like Ipods follow the USB standards properly.
In particular,
Battery Charging Spec v1.0 Spec and Battery Charging
..which contains
batt_charging_1_0_033007.pdf
(See section 1.3.3
...and a dead short is fine.
Alas, some cellphones refuse to charge with a USB dedicated charger. They attempt to force you to buy their chargers. :(
-- Guy Macon
Having finished reading the spec it's not quite that easy.
A dedicated charger has to current limit somewhere between 0.5A and
1.5A. This is so the charger output voltage drops and the load can minimize internal dissipation. So with 2 outputs I need seperate current limiters on each output.I reckon that I'll use a buck reg to get about 5.6V with seperate LDO regulators for each output.
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