LM3914 LED Current Setting

Discussion in 'The Projects Forum' started by mw6spilot, Oct 14, 2010.

  1. mw6spilot

    Thread Starter New Member

    Oct 14, 2010
    I am having difficulty determining how to set the LED current.
    I have Rlo to 0V, RHi connected to RefOut, RefOut connected by resistor 1 to RefAdj, and RefAdj connected by resistor 2 to 0V.
    Using datasheet http://www.datasheetcatalog.org/datasheet/nationalsemiconductor/DS007970.PDF
    The current programming section (page 6) mentions 10 times the current drawn out of the reference voltage pin (pin 7) will be applied to the lit LED's.
    It also mentions taking into account the 10-resistor divider, external current and voltage setting divider in calculating LED drive current.
    I need assistance in how to take these items into account.
    Also the internal voltage reference section gives an equation of Vout = Vref ( 1 + (R2/R1)) + Iadj R2. I cannot get this to work. I am calculating as Vout = 1.25 ( R2 / R1) but getting a slight difference to a Duel LM3914 V3.0 basic application. Is this a math precision error or am I missing something e.g. Iadj R2, error terms (120microA mentioned), or divider resistors etc.?
    Your help would be appreciated.
  2. Ron H

    AAC Fanatic!

    Apr 14, 2005
    A schematic would help, with actual measurement values of the voltages in question.
  3. marshallf3

    Well-Known Member

    Jul 26, 2010
    The formulas included with most 3914 data sheets are rather confusing, try looking for an alternate data sheet release from an earlier year or something. I've run into the same problem with a couple of their chips.