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LCD Contrast Bias Problems

Discussion in 'Electronic Design' started by [email protected], Jan 29, 2007.

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  1. Guest

    I'm using an Okaya RC2004ARS-DSC-A (RC2004D, the baby 20x4) display in
    a 3.3V system. I originally planned on using a negative voltage charge
    pump driven by a PWM pin on a PIC (a circuit I've used before
    successfully.) When I couldn't get that to work I tried a ICL7660 neg
    chg pump. No luck on anything. Everything logic-wise appears to be
    operating normally, it init's fine, etc. When I put 5V on the system I
    can actually get the contrast up enough to start to see the

    There is an Okaya appnote that speaks of internal and external
    contrast adjustment on their modules. However, none of the jumpers
    they describe appear on the RC2004. So I'm assuming it doesn't have
    that capability (which could be causing the problem if somehow the
    internal contrast selection was jumpered on.)

    When I probe the Vo pin, I get an impedance of about 3K ohms. When I
    test the ICL7660 without it connected to the V0 pin I get a good V of
    about -3.2V from a 3.3V supply. However, when I connect the ICL7660
    output to the Vo pin all I get is -0.7V when scoped. The 4V bias
    obviously isn't enough when I need at least 5+ volts to start to see
    the characters when tested with a 5V Vdd. BTW, one would expect that
    when using the ICL7660 that would produce a bias of about 6.5V and the
    screen would be over contrasted and be dark. No such luck, it's as if
    there's no bias at all.

    Any ideas or suggestions would be hugely appreciated,

  2. Guest


    I've narrowed it down the Microchip MCP41010 SPI Digipot. If I replace
    the 41010 with a regular old pot everything works normally. I
    remembered there was a 1mA limit on the wiper of that part so I added
    a 4.7K current limiter. But that doesn't change anything.

    Any ideas?



  3. The Digipot needs all inputs to be between the power rails. Your
    negative voltage is being clamped by an internal diode to Gnd.
  4. Guest

    OK. That makes sense. Thanks for the info. I think I see where that is
    mentioned in the datasheet.

    Does anyone know of a digipot that isn't limited in this way?


  5. Sounds like you are creating a negative voltage with the 7660, then
    adjusting it with the digipot to feed the LCD.
    Do it the other way. Create an adjustable positive voltage, either from
    a PWM or otherwise, then feed that to the 7660 to invert it, and feed
    the output direct to the LCD.


    Adrian Jansen adrianjansen at internode dot on dot net
    Design Engineer J & K Micro Systems
    Microcomputer solutions for industrial control
    Note reply address is invalid, convert address above to machine form.
  6. Guest

    OK. That makes sense also. I had thought of maybe using the 7660 (as a
    voltage doubler) to supply the LCD and then use the digipot normally
    as a contrast adjust between Vss and Vdd, of course Vdd for the LCD
    now would be 2 x supply voltage to the 7660.

    Thanks again,

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