USB charger design

Discussion in 'The Projects Forum' started by Wind23, Oct 29, 2012.

  1. Wind23

    Thread Starter New Member

    Sep 26, 2012
    22
    1
    Hi, i am planning to make some usb chargers to charge electronic devices like smartphones.

    I have the following questions to ask:

    (1) Is it better to use switching regulator or linear regulator ICs? The ICs will convert 12Vdc to 5Vdc. I know that switching regulator ICs have better operating efficiency but linear regulator(7805) is low cost.

    (2) After selecting the right IC and assembling the circuit, we can just directly plug in the usb charging cable or we have to consider other factors such as the cable's resistance ?

    Thanks.
     
  2. wayneh

    Expert

    Sep 9, 2010
    12,093
    3,031
    1) Depends what you mean by "better". From a technical and efficiency standpoint, SMPS is better than linear. But as you note, linear is cheap and moreso when you factor in design and build time. Using the 7805 for high-current charging presents some design challenges, particularly heat dissipation.

    2) Cable properties won't be a concern unless you're planning unusually long runs over, say, 8 feet. (Arbitrary guess - I don't recall the USB limit.)

    One issue you WILL have is that different devices need different states on the data lines in order to initiate high-current charging. Pull-up and/or pull-down resistors may be needed to put the required voltages onto the data lines.
     
  3. Wind23

    Thread Starter New Member

    Sep 26, 2012
    22
    1
    Hi, i am planning to use 500mA and 1A 7805 linear regulators. Some devices like mp3 players will draw around max 500mA and the 500mA regulator can provide the current.

    For most smartphones, i think the current draw is higher than 500mA and 1A regulator can do the job.

    Another question i would like to ask is the usb cable configuration. In most designs, the two data cables are usually left open. Can i just ground them and i do not understand the different states on the data lines in order to initiate high-current charging. Could you explain please?

    Thanks.
     
  4. DickCappels

    Moderator

    Aug 21, 2008
    2,647
    632
    The USB Implementers Forum http://www.usb.org/home has drafts of a standard, USB Battery Charging on their site. Get this document; it tells you what cell phones are looking for, what the charger needs to do in order to charge the phones and not destroy any of them. Its free -get them while they are hot!
     
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