Universal fast USB charger - Page 1
Forum section: electronics / beginners

author: Elandril
author_rank: Contributor
timestamp: June 17, 2012, 05:51:54 pm
content: Hi,I'm quite a newbie when it comes to designing a circuit - mostly I've just hacked some existing devices.I'd like to design a small universal USB quick-charger with multiple outputs (for small gadgets like tablets, iPads, iPhone, other mobile phones). I've seen that there is the USB Battery Charging Specification that defines a standard for Dedicated Charger Ports which provide 1.5A current. Such a port has to have a resistance below 200R between the D+ and D+ pins (seethis image).Unfortunately Apple has brewed their own soup (again) and their devices detect a fast charging port when there is a potential of 2.8V on D+ and 2.0V on D- (created by two voltage dividers as seenhere). I suspect that the detection inside the device works with a current sense resistor or a direct detection of the 0.8V potential difference.Is there any way to combine these two specifications into one universal charging port? I've tried some variants with optoisolators and mosfets in LTSpice, but was not successful.Any input would be greatly appreciated!ElandrilPS: Images courtesy ofThe Dirty Truth about USB device charging.Edited for typos.

author: SeanB
author_rank: Super Contributor
timestamp: June 17, 2012, 06:11:36 pm
content: Why not just dedicate an output to iDevices, and have a few that implement the other methods as well. Just make sure the 5V rail you give can handle 2.5A by however many ports, and that it is happy running at any point from no load to full load. Remote sense at the USB pins for the power, choose thick traces for the power and preferably a ground pour for ground, and connect the shield to it via a ferrite bead and a capacitor ( Thanks to Free Electron for that tip earler on the ESD thrtead, worth reading), along with decoupling each socket with low ESR capacitors.

author: T4P
author_rank: Super Contributor
timestamp: June 17, 2012, 06:29:01 pm
content: They brew their own sour again ? I thought adafruit (I guess?) fixed that in their mintyboost ?

author: amyk
author_rank: Super Contributor
timestamp: June 18, 2012, 08:20:19 am
content: How about trying this?Code:550R 200R 500R+5V ---/\/\/\/---- D+ ---- /\/\/\/ ---- D- ---- /\/\/\/ --- GNDYou get 2.8V on D+, 2V on D-, and the resistance between D+ and D- is 200R. Unless Apple is relying on specific resistances (e.g. detecting the voltage drop with specific current) it should work for that. I haven't read into the 200R detection too much but I believe it involves placing +5V on D+ and checking to see if D- comes up to some voltage.If you want < 200R between D+ and D- then the other resistors will have to decrease appropriately; it all depends on how much quiescent current you want it to draw. (4mA as it's drawn above.)Edit: Yes, this configuration should work with the DCP standard if I'm reading it correctly.

author: rr100
author_rank: Frequent Contributor
timestamp: June 18, 2012, 09:04:23 am
content: What about moving the circuit in the cable? You still need to carry a special cable for your i* device and probably 1-3$ cheapies are available on ebay (if you don't want to mess up with an original cable).Bonus point: works with any other (reasonably powered) charger, like a car charger, etc.