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View Full Version : USB Charging -- current limits, specs, etc?



Zoot
02-18-2012, 07:08 PM
I've been really enjoying some of my newer devices that can charge from any USB port, and I've been thinking about possibly using USB ports on my next rounds of projects so I can use the same port for programming the micro and for charging on-board batteries.

Then, fortituously, the following Maxim release showed up in my inbox -- http://www.maxim-ic.com/company/newsroom/pr_products/show.mvp/npk/1656 --.

In any case, I was wondering if anybody knows about current limits for charging and such (does the connected device need to request a 500ma supply?). Also, what kind of USB host device has a "CDP" or "DCP" -- aka "charging downstream port" and "dedicated charger ports". Are those non-comm higher current USB ports?

kwinn
02-19-2012, 04:42 AM
It depends. The standard is that a device can only draw 100mA to start with, but can negotiate with the connected device to draw up to 500mA. a lot of devices do not follow that standard and allow 500mA without negotiation. There are also "charging ports" that allow more than 500mA to be drawn. Take a look at http://en.wikipedia.org/wiki/Universal_Serial_Bus#Charging_ports_and_accessory_ charging_adapters for more info.

Peter KG6LSE
02-19-2012, 04:52 AM
I've been really enjoying some of my newer devices that can charge from any USB port, and I've been thinking about possibly using USB ports on my next rounds of projects so I can use the same port for programming the micro and for charging on-board batteries.

Then, fortituously, the following Maxim release showed up in my inbox -- http://www.maxim-ic.com/company/newsroom/pr_products/show.mvp/npk/1656 --.

In any case, I was wondering if anybody knows about current limits for charging and such (does the connected device need to request a 500ma supply?). Also, what kind of USB host device has a "CDP" or "DCP" -- aka "charging downstream port" and "dedicated charger ports". Are those non-comm higher current USB ports?

most ports are 2.0 and are max at 500mA ..... the 3.0 can do up to 750 . ( IMO 1 A would have bean better )
In some products like the Ect CD drive on the mac book air . apple sneeked in a port that can do do more then 500mA at 2.0 Specs .

The Ipad From what I hear drawes more then 500mA. so Ill bet most macs have impllementied a boosted USB spec ..

AFAIK you need to "request " a X value of current from a USB port . Else If its working as made the USB chip will treat it as a Over load and like a software fuse . remove power From the port . . this is VERY common with those bus ran laptop HDDs that need a boost to spin up .
some defeat the USB spec and share power off a 2ed port .. again AFAIK this is NOT kosher with the way USB was made to be used and is againts the white papers rules ....... but it works so its sadly done ..


Some USB ports that are not data Like on some car radios . ( that DONT do mass storage reading ) Use USB as a easy way to make a 5 V convenience outlet for a cell phone as most chargers are not fixed at the Brick end and are just a USB to ( phone DC plug ) cable and are 5V so USB is a common way to do this .. some of those chargers j just Float the data pins open and are a 5V rail . 500mA is not limited .. and Il bet that maxim chip looks for tthe lack of data as a way to say ( ohh a dumb charger ) and then goes to 500mA draw mode.

then Some 1A USB chargers are smart that will emulate the Power data front end of a USB computer port and say 500mA OK normal phone or 1A Ipad Ect....... depending on the device being charged to request a normal 500mA or a boosted 1A or whatever amount .



Ill bet the radio ones classify as DCP .

were you thinking of useing the maxim chip or just wing it??



JINK Kwinn!!! JK ! .. got to Love standards that are So loosely adhered to ..

again AFAK the 100ma was based on the idea that you dont need much for a Micon to start up to the point it needs to request more power .





Peter ...


Illl make a slight tip here for any USB tinkerers ..

A) Buy a USB Industrail 5kV rated Isolator If you can afford it !


Esp if a large batt is In the project as it could shove Amps Up your port and Fry it.,,

OR at the very least make a extender that on the 5V line has a fuse in it .


NOTE some better made USB hubs have on them some rather good power and Sig filters Ect so that may be enough .

Zoot
02-19-2012, 03:43 PM
I was possibly going to experiment with the Maxim chip as it includes 15kv isolation, etc. And I'm not thinking of charging 33ah gel cells, more like projects that run off 4-6 rechargeable AAs and the like.

FYI -- from the wikipedia entry:



Charging ports exist in two flavors: charging downstream ports (CDP), supporting data transfers as well, and dedicated charging ports (DCP), without data support. A portable device can recognize the type of USB port from the way the D+ and D- pins are connected. For example, on a dedicated charging port, the D+ and D- pins are shorted. With charging downstream ports, current passing through the thin ground wire may interfere with high-speed data signals. Therefore, current draw may not exceed 900 mA during high-speed data transfer. A dedicated charge port may have a rated current between 0.5 and 1.5 A. There is no upper limit for the rated current of a charging downstream port, as long as the connector can handle the current (standard USB 2.0 A-connectors are rated at 1.5 A).

kwinn
02-20-2012, 06:52 PM
OK Peter you stumped me. What does "JINK" mean?

Peter KG6LSE
02-21-2012, 03:26 AM
A penalty that one person can invoke on another when the two of them say the same thing at the same time.



http://en.wikipedia.org/wiki/Jinx