USB Battery Charging Spec clarification

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

  1. tindel

    Thread Starter Well-Known Member

    Sep 16, 2012
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    199
    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 Well-Known Member

    Sep 16, 2012
    593
    199
    Trying one more time...
     
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