7 segment LCD's current consumption

Discussion in 'General Electronics Chat' started by nqchanh194, Nov 4, 2014.

  1. nqchanh194

    Thread Starter New Member

    Jul 5, 2013
    27
    1
    Hi all,
    I'm searching for 7 segment LCD. As what I've known, we need an LCD driver which is connected to 7 segment LCD through SEGMENT and COM pins. In fact, there's no DC component available between each pair of SEGMENT and COM pin, so I don't think there's any current consumption on these pins, the current consumption appears more on V_CC pin of LCD driver when it's running the corresponding module to drive LCD. Is that right ?
    Hope you light me up. I'm really not clear how 7 segment LCD consumes current, on SEGMENT and COM pins or somewhere else ?
     
  2. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
    2,503
    380
    hi,
    The Back plane [Common] of the LCD display is usually driven with a square wave voltage, at a typical frequency of 100HZ or so.
    The Segments are driven with the same frequency but either in the same phase or anti-phase to the Blanking drive.
    When a segments drive is in phase the segment it is not visible, when driven in anti-phase it is visible, so the drive to the LCD is AC.

    You can drive LCD's with DC, but very quickly the segments will start to turn black and remain black.

    E
     
    Last edited: Nov 5, 2014
  3. RichardO

    Well-Known Member

    May 4, 2013
    1,235
    384
    The current consumed by an LCD is from charging and discharging the capacitance of the LCD segments. Power consumed is equal to the voltage squared times the frequency times the capacitance: V^2 * f * C.

    For an LCD the voltage, frequency and capacitance are all small so the power consumed is also small.
     
    nqchanh194 likes this.
  4. nqchanh194

    Thread Starter New Member

    Jul 5, 2013
    27
    1
    Hi RichardO,

    I guess you've got experience on this kind of LCD. However, have you ever measured the increase in current consumption of LCD driver while driving LCD ?
    If already, hope you can share me some information.

    Thank you & Best regards.
     
  5. nqchanh194

    Thread Starter New Member

    Jul 5, 2013
    27
    1
    I've seen some LCD supplier release information about current consumption of this kind of LCD. For example, 5 nA/(mm^2), if I calculate the area of all segments (not the area of LCD), then multiply it by 5 nA/(mm^2), finally I get the maximum current consumption. Is that right ?
     
  6. nqchanh194

    Thread Starter New Member

    Jul 5, 2013
    27
    1
    I've read datasheet of MSP430 series which supports LCD driving module (please refer to the attached datasheet).

    And we've got two definition needing considering here. Those are LCD frequency and LCD frame rate. As what I've understand, the LCD frequency is the frequency you've mentioned in the last post above. Is that right ?

    Hope you light me up. Thanks. ^ ~ ^
     
  7. RichardO

    Well-Known Member

    May 4, 2013
    1,235
    384
    I have never measured the current in an operating circuit since there have always been other ways of getting the information.

    In one circuit, the current used to bias the LCD was the determining factor and not the current draw of the LCD. I only needed to know the current draw accurate enough to make sure that the bias current was high so as not to get an asymmetrical drive to the LCD causing it to fail. Since this was not a battery powered device, that was good enough.

    In all the other cases I have used an LCD, there was always a built-in LCD driver involved. The manufacturer of the LCD module does all of the hard work and specifies the current draw of the entire module.
     
  8. RichardO

    Well-Known Member

    May 4, 2013
    1,235
    384
    That sounds right to me. Keep in mind that if an LCD is specified this way, the specification must also give the voltage and frequency for the current specified.

    Edit: Reworded reply
     
    Last edited: Nov 8, 2014
  9. RichardO

    Well-Known Member

    May 4, 2013
    1,235
    384
    The frame rate (frequency) is what causes the dissipation. This is because that is how often the LCD segment capacitances are charged and discharged.
     
    nqchanh194 likes this.
Loading...