The Dirty Truth about USB device Charging
How many devices do you have that charge over USB? How many devices require you to do something special, or have a special adapter to do it?
The last few cell phones I’ve had have USB micro or mini ports to charge from. I figure I can take a USB A Male to USB micro/mini Male cable and just plug it into a USB port and get power… except that doesn’t work some times. Then there are the Apple devices. You’ve got your iPods, iPhones and iPads. They do their own thing as well. What’s a geek to do?
Well, the Chinese started mandating a specific configuration for cell phone charging via USB (shorting DATA- and DATA+ pins together) back in 2007. The EU got the big cell phone manufacturers onboard in 2009, with the US wireless industry coming along also. The bad news is that the EU standard doesn’t have to be implemented until 2012.
So that problem is resolved, right? That’s just cell phones. Others will probably jump on the band wagon, but there will be hold outs. So, how do you design a USB charger in the mean time? The Chinese standard is here, but that leaves our device non-apple compliant. We don’t want to miss out on the apple market do we? I hear you say, “but that means another product, and that’s expensive.” We’ve got a handy generic circuit to allow you to use the same design for both charging standards by just replacing a couple resistors. First, let’s look at the standards.
I don’t have the Chinese standard, but all articles agree that it is just shorting the D+ and D- lines together. Easy enough. Now, the Europeans did their own thing… sort of… not really. They supported the USB Battery Charging Specification, which instead of shorting D+ to D-, puts a 200 Ohm resistor between them. It’s practically the same thing. So we can meet those two standards with a 200 Ohm resistor, or if you really want to short it out, a zero Ohm resistor.
Now onto Apple’s USB charging setup. Luckily, there are intrepid explorers who did the hard work there. There’s two Apple USB charging circuits, low and high current. To change between the two we just need to switch out the resistors. Here’s the circuit for the high current resistors.
Now, we just need to combine the two. Merging the two circuits we get:
Now realize, that installing all five resistors will not meet either of the charging standards. However, if we install R2, R3, R8 & R9, we will have an Apple format charging port. If we install just R6, we will have an EU standard charging port. If we replace R6 with a zero Ohm resistor and leave the other resistors off, we will have a Chinese standard charging port.
Now, instead of two different boards, we can use the same board with two or three assembly variants to support different devices. First, here’s the layout for the resistors connected to a USB A Female board mount connector:
The circuit requires two layers if using surface mount components.Blue is the bottom layer and red is the top layer. If using thru-hole, traces could be ran under the resistors. The resistor layouts shown are for 0805 resistors. They can be smaller, since they dissipate very little power (less than 1/8 W). The same can be said for the traces. I had the room, so I chose 0805s with larger traces. Make sure to keep your GND and VCC traces large to support the current on them. Here’s the 3D view of the layout:
And there you have it, one circuit and layout for multiple USB charging standards. Enjoy!