Engineering LCD display design guide
This article, a continuation in Focus Display Solutions LCD display design guide for engineers, covers optimum driving frequency ranges and segment ghosting for a static-segmented LCD Display module. This bulletin is valid for all types of standard and custom segment LCD Displays including 4:1 mux, 3:1 mux and direct drive interfaces.
Optimum driving frequency for a Segment LCD
Segmented LCD displays are driven by AC (Alternating Current). Optimum frequencies range between 60Hz to 150Hz. We recommend 64HZ for the majority of the static LCD displays that we supply.
The display can be operated above the 150Hz threshold, but is not recommended; one of the negative side effects of an excessive frequency is the increase in power consumption.
Operating below 60Hz could produce a flicker effect.
Segment Ghosting (Cross-talk) - Segment LCD Modules
Segment LCD cross-talk, (also referred to as segment ghosting) occurs when a segment that should be OFF (de-energized), is partially ON (energized) and produces an icon that is light grey in color.
It is believed that the smaller the LCD glass, the longer the segment will continue to ghost after the power has been turned OFF.
Cross-talk is generated from a combination of frequency and voltage. The photo below shows an example of segment ghosting on what looks to be a multiplexed, TN segment display.
Ghosting in a segment LCD: The solution
One solution to eliminate segment ghosting of a segment display is to actively drive the segments to an off state. Freescale semi-conductor provides an excellent article locate at http://cache.freescale.com/files/32bit/doc/eng_bulletin/EB762.pdf.
Segment LCDs: Avoid any DC component
In a perfect world, only AC would be applied across the segments of a static display, but we do not live in a perfect world and a DC sub component can be generated. An excessively high DC offset will slowly destroy the display and should be avoided.
As a general rule, the maximum allowable DC offset should not exceed 50mV.
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