USB Battery Charging Spec clarification

Discussion in 'General Electronics Chat' started by tindel, Apr 13, 2014.

  1. tindel

    Thread Starter Active Member

    Sep 16, 2012
    568
    193
    I'm hoping some of you have done this before. I'm building a simple boost supply to power my usb devices with two AA batteries. I'm hoping to sell a few in a kit to make up for some of my time and expenses.

    I was looking at the USB Battery charging spec (see link below), and I can't decide if I meet the requirements, but I also can't decide which requirements I need to meet.

    I can't decide if I'm a Dedicated Charging Port (DCP) or a Accessory Charger Adapter (ACA). On one hand, I think I'm a ACA, because my voltage can technically go below 4.75V (I explain this point below). On the other hand, I believe I meet the DCP spec and load line requirements very well with one exception, that as my batteries become depleted my 5V supply voltage can sag at light loads - which technically doesn't meet the spec of no operation below 4.75V.

    Regarding the DCP... I could turn off my power supply between 4.75 and 4.85V. (My boost is guaranteed to provide a minimum of 4.85V until batteries reach a certain voltage level). My rough calculations show that this is probably obtainable (barely) with my reference and some 1% resistors. I'm just a bit concerned that this doesn't leave much room for error though, and that I won't be able to guarantee good depth-of-discharge out of my batteries.

    The spec does say "ACA operation is only required if for Charger Port voltages in the range of VACA_OPR." where VACA_OPER = 4.1V to 6V... this leads me to think I'm more of a ACA and that a 4.5V cut-off would be acceptable. But I don't have a 5th wire or current source that the ACA requires (I believe). I was planning on shorting the data lines which would again lead me to think I'm more of a DCP.

    I was just wondering if anyone here has done this before and knows if I could fudge a DCP a bit and let my power supply go down to ~4.5V before cutting it off.

    http://www.usb.org/developers/docs/devclass_docs/BCv1.2_070312.zip
     
  2. tindel

    Thread Starter Active Member

    Sep 16, 2012
    568
    193
    Trying one more time...
     
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