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Driving LED displays from voltage source

Discussion in 'Electronic Design' started by Adam S., Jan 3, 2004.

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  1. Adam S.

    Adam S. Guest

    Can LEDs be driven from CMOS logic (HC family) without any
    added series resistance and expect reasonably stable current
    and repeatability ?

    I wanted to drive 8 x LED 7-seg displays from cascaded
    74HC164 serial-in parallel-out shift registers. Where there
    is one 74HC164 per digit. The display is programmed by
    serial clocking the data to the first register. The common
    pin from each display are tied together and held at some
    fixed voltage relative to supply. Using a one of those step
    down synchronous converter ICs the voltage at the LED
    commons can be adjusted to give desired brightness. I'm
    wondering how consistent LED brightness will be using this
    method.

    Adam
     
  2. Driving LED voltage mode is a very bad idea. The spread of the LEDs will
    make some dim, some bright and others dead. You need drop resistors on each
    LED or use the Max ICs, which are just on the spot for your application
    including current limiting:

    http://www.maxim-ic.com/quick_view2.cfm?qv_pk=3195&ln=

    It has internal registors for both the digits and dimming
    settings........:)

    Cheers

    Klaus
     
  3. No. Well, it depends on what you mean with "reasonable", but I guess the
    result would not qualify as reasonable in most people's opinion.
    Apart from brightness consistency, watch out for maximum current through
    the power supply pins of the 74HC164. You could very easily exceed that
    when all outputs are on.

    What are you trying to save? Expense for resistors? Power dissipation?
    Space on the board?

    Cheers
    Stefan
     
  4. Try Allegro A6275 or Toshiba TB62705 for driving 8 LEDs, especially 7-seg
    displays. One resistor sets the current sink for all. They make 16-driver
    versions, also. Serial in, daisy-chainable.
     
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