12v to 5v Car to USB switched power supply

Discussion in 'The Projects Forum' started by claddc4, Aug 8, 2009.

  1. claddc4

    Thread Starter New Member

    Jul 29, 2009
    I need some help choosing the proper IC for this project. I'm working on a USB charging port integrated into the dash that would draw from the battery, meaning I need to step down from 12VDC to 5VDC and be able to draw .5-1.5 A max (depending on how many ports I choose to have). I'm assuming that a LM7805 circuit would be out of the question because of the inefficiency, so therefore I'm turning to a switched power supply IC to drive the circuit. I found this IC on mouser which seems simple enough to implement, however I am a chemical engineer and pretty unfamiliar with DC-DC.


    1. Am I using the right IC? ( datasheet here, its the NJM2360)

    2. My supply voltage is going to vary a bit with the engine, do I need to regulate Vin?

    3. How smooth is my Vout going to have to be to charge most USB devices?

    I'm aware you can buy one of these chargers for around $15, but that would ruin the fun.
  2. jj_alukkas

    Well-Known Member

    Jan 8, 2009
    LM7805 cant be said that inefficient,if used on a maximum of 8v. But considering your supply voltage of 12v and the drop of 7v, it will dissipate a lot of heat.
  3. mat sazilin

    New Member

    Feb 6, 2009
    you can try to used the la7805 and the transistor 2n3055
  4. nicsky

    New Member

    Aug 1, 2009
    Get a simple switcher from national semiconductor
    They have a easy design program which will give you everything including a circuit and parts list. its called web bench
    If you are nice they will send you sample parts for free in a few days
  5. claddc4

    Thread Starter New Member

    Jul 29, 2009
    National seems to be the way to go. I am getting the LM22676 to run 3 USBs with the attached schematic. I'll post the full project after everything is built and tested.
  6. Amberwolf


    May 2, 2008
    You could also build the Roman Black SMPS, found easily via Google. It's simple and should work well enough. I'm about to build a couple of them to power my ebike's 12V lighting systems off the 24V/36V traction batteries.
  7. Tahmid

    Active Member

    Jul 2, 2008
    Hi claddc4,
    Suitable chips include LM2575 from National and L4971 from ST.
    I have attached the L4971 circuit.
  8. djb


    May 17, 2008
    hey huys you forget one thing,...

    usb ports and chargers must have the 500mA current limit while regulates 5V.
    NOT a constant current regulation, this will burn up everything. current limiting! Just a power supply with regulated 5V output, but with current limiting @ 500mA. Otherwise is not usb compatible. Will be only usb interface.

    you need a circuit to do this. for example LM317 is adjustable voltage regulator, but you can find a current limiting application in it's datasheet together with voltage regulation using 1 extra tranzistor only. if you manage to do this, or another solution of power supply 5V with current limiting @ 500mA, then you have to connect the usb pinouts correctly and that's it.