How to get 7W out of a USB 3.0 port? - Page 1
Forum section: electronics / projects

author: polar8
author_rank: Contributor
timestamp: September 21, 2015, 10:14:38 pm
content: I'm designing a bus-powered USB accessory for fun: I have a spare laptop LCD panel around, and I'd like to make a secondary monitor for my laptop. Power will be supplied by my laptop's USB port, and data will be sent through the DisplayPort connection.The panel I'm using draws 7W max. My 2014 MacBook Pro has USB 3.0 ports which support the USB BC 1.2 standard. According to the BC 1.2 spec, they can deliver up to 7.5W.I've done some research and I can't figure out how to actually get that much power out of them. Some say the D+/D- lines must be shorted together, some say a specific resistance must be applied to each line, etc.Has anyone solved this problem?

author: bfritz
author_rank: Regular Contributor
timestamp: September 22, 2015, 08:49:28 am
content: The USB 1.2 BC spec gives the details.http://composter.com.ua/documents/BC1.2_FINAL.pdfSection 4.6 on page 41 shows that 1.5A at 5V nominal is available.Section 4.4.3 on page 40 gives the rules for the resistance for a dedicated charging port, and defines the resistance as Rdcp_datTable 5-3 on page 45 gives the maximum value of Rdcp_dat on the line titled as "Dedicated Charging Port resistance across D+/-", and gives a max value of 200 ohms.So, it is perfectly acceptable to short the D+ and D- pins per the spec. A BC1.2 compliant device will recognize this, and will consume up to the 1.5A current.Enjoy!

author: polar8
author_rank: Contributor
timestamp: September 23, 2015, 03:17:58 am
content: Quote from: bfritz on September 22, 2015, 08:49:28 amGreat info. I had read through the spec and found that, but doesn't that apply to port (source) design? From what I understand, a USB DCP (dedicated charging port) has the D+/- lines shorted, allowing the portable device to identify it as such.But I'm designing the portable device, not the port. Do the same rules still apply? I.e. should I short the D+/- lines on my design?

author: rs20
author_rank: Super Contributor
timestamp: September 23, 2015, 03:41:46 am
content: Quote from: polar8 on September 23, 2015, 03:17:58 amYou are correct that the linked spec applies to the charger, not the device being charged/powered. You are incorrect to suggest that you should be shorting the D+/D- lines on your device. Obviously, you should betestingwhether you're are plugged into a charger with shorted D+/D- lines, and only turning on your display iff you are plugged into a charger with shorted D+/D- lines. You're receiving the "I can support 1.5A" message, not sending it.This means that you won't be able to draw 1.5A from computer USB ports, because of course computer USB ports don't have their D-/D+ lines shorted. Theremightbe a way to communicate to the PC over the D+/D- lines to request more power; I don't know if the USB spec allows for this. From myout-of-date knowledge from years ago, that requesting mechanism only goes up to 500mA, you cannot "legally" draw more than 500mA from a computer USB port. And indeed, modern phones plugged into modern computers with a micro-USB cable only pull in 500mA.

author: polar8
author_rank: Contributor
timestamp: September 23, 2015, 04:00:28 am
content: Quote from: rs20 on September 23, 2015, 03:41:46 amThat's exactly the mechanism that I'm trying to figure out. Regarding your last statement- modern phones and tablets can in fact pull more tham 500mA. If the USB port supports USB BC1.2 (Battery Charging)- and most modern USB 3.0 ports do- the portable device can request up to 1.5A. I've verified this in the real world using my USB multimeter. For example, my Android phone charges at 1A, and my Android tablet charges at 1.5A.So my question remains- as a portable device, how do you request the full 1.5A from a BC1.2-compatible port? I will keep searching and report back with any updates!

author: NiHaoMike
author_rank: Super Contributor
timestamp: September 23, 2015, 05:15:51 am
content: You can check if it's plugged into USB 3 by checking if the USB 3 ground actually connects to ground. But that only gets you up to 900mA, not enough for your application. BC1.2 checking could be done with a cheap comparator and a few resistors attached to a microcontroller.

author: timgiles
author_rank: Regular Contributor
timestamp: July 26, 2016, 03:54:54 pm
content: Not sure if it helps an old thread .http://www.ti.com/tool/bq24392evmis such an IC.