Getting power from a USB port...

Hi,

I work on the train during my commute, and I'd like to be able to power my uP development board from my laptop (its a small dev board), so I can do code development and testing.

I thought about using the USB port to provide 5V, and then using a MAX1811 or LTC4053 to charge a 4.2V Li battery, which in turn feeds a LT1308B or MAX1771 boost converter to give 5/9/12V etc. at a few 10s of mA.

Is this a reasonable idea? It shouldn't load the USB port too much - both the MAX & LT Li chargers are designed to run from a USB port, so shouldn't overload it, and the boost converter should give me fairly clean power at whatever voltage I need. I can charge the unit at work from the desk PCs, and therefore the loading on the laptop should be minimal for the hour on the train.

Are there better ways of doing this? Is there a cheap commercial alternative already available? This should end up very small and should cost about USD

15 to 20 when made on a PCB. Alternative chips would be a help too...

Thanks for your input,

Mike

Reply to
Mike Deblis
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I remember seeing some USB current limiter/ management chips, prolly on maxim's site, might be worth search. There are some USB specs saying you MUST NOT exceeds xxx milliamps. This is what the managemant chips do, IIRC.

Wonder when they will bring out a USB soldering iron?

martin

After the first death, there is no other. (Dylan Thomas)

Reply to
martin griffith

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I don't know that I'd count on a laptop USB port meeting specs written for tower boxes.

...Jim Thompson

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|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
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Reply to
Jim Thompson

Ideally, (as far as the USB guys are concerned), there's individual port overcurrent protection that is set to the maximum of 10mA (I think -- number could be wrong) that the device is allowed prior to enumeration, and then set to the "high end" power limit of the port (100mA or 500mA or whatever) once enumeratoin is complete and the OS has determined that that much power actually is availble. Several manufacturers have chips out there that provide "per port" protection and have inputs for configuring whether the current limit is set "low" or "high" (e.g., 10mA or 500mA -- the actual values are set by resistors).

In actuality, it's rather _un_common to have a system that doesn't simply use a "one size fits all" approach to current limiting: If there are, say, 4 USB ports, they current limit the "+5V USB" line to 500mA*4=2A. The end. No port by port protection, no low/high power modes, etc.! In effect, you peripheral is connecting to a shared +5V bus that also an active fuse -- and nothing else.

I'd be surprised if a laptop can't manage to put out 500mA per USB port (if not more based on the above!) -- there are many "high power" (500mA) devices out there, and the manufacturers don't want to get the phone calls from everyone whose favorite peripheral doesn't work. That being said, though, I onve used a KVM that was very sensitive to the USB voltage -- in theory it should have worked down to something like +4.25V, in practive it appeared to need more like 4.75V or above to function correctly!

---Joel

Reply to
Joel Kolstad

There are two current levels available from USB ports. 100mA from Bus Powered ports and 500mA from self-powered ports. I would assume a laptop host port is self-powered. BTW, Manufacturers would only have to adhere to these standards if they are planning on USB org certification.

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Reply to
R. Jenkins

If you're going to use a battery anyway, why even worry about charging it from the USB port? A couple of AA size NiMH cells will give you around 1500 mAh at about 2.4 V, which is way more than you need. You can charge them from AC at work and at home, and maybe even keep a reserve set of AA alkalines in the laptop bag in case they run out unexpectedly.

Having said that... I think you can get reasonable amounts of juice out of the USB port. I was given a small "USB" aquarium that I have hooked up to my computer at work. It has a blue LED and a small pump that makes two plastic fish swim around. The power for the LED and pump can come from either the USB port or some AA batteries, and this is selected by slide switches on top of the tank. I don't think it touches the USB data lines at all; I don't think it has a chip to do the full USB negotiation and I don't see an aquarium show up in the device list when I plug it in. I figure maybe 3.6 V at 20 mA for the LED and at least that much for the motor.

If your laptop dates from the transition period when they still had PS/2 mouse/keyboard ports, you can also get 5 V from that port. Not sure of the available current but it should be a few tens of milliamps at least.

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says that if you have an IEEE 1284 level II parallel port, you ought to be able to get at least 2.4 V at at least 14 mA from each data pin.

There is probably a reasonable amount of juice available in the PCMCIA card slots. You might even be able to get some of the circuitry onto the card, further reducing the amount of stuff in an external box.

How about plugging into the sound card output and always having a loud MP3 playing? Or a solar cell and some tape to stick it against the light fixture on the train... :)

Matt Roberds

Reply to
mroberds

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