Golden View Display wants you to make an informed choice among our LCD products. The tech center provides you with most of the information you will need to understand liquid crystal displays.
Driving LCD Displays
Static Drive LCD Technology
The configuration for Liquid Crystal Display Static Drive technique is that one
side of all of the Liquid Crystal Display segments are tied to a common, or
backplane, and the other side of each of the segments are routed to individual
connection points that are tied to the driver control circuitry. This method uses
a large number of interconnects and is not feasible for complex displays, but it
does produce the best looking display
LCD's require an AC drive voltage with virtually no DC component. Segments
are controlled by the magnitude of the AC voltage across the LCD segment,
but there must always be AC voltage across ALL segments of the LCD.
Prolonged DC operation may cause electrochemical reactions inside the
displays which will cause significantly reduced life. The initial indications of
display degradation because of excessive DC current is a loss of alignment
along the edges of some of the characters. The visual indication will be a "fuzzy" appearance of some of the characters.
The TN LCD is an RMS voltage responsive device, that is, the contrast of
a given segment is dependent upon the RMS value of the applied voltage
across it, measured with respect to the common plane. This fact, which
seems obvious now, is very important when discussing drive schemes.
Drive frequencies for direct drive displays are typically between 30Hz and
100Hz. Depending on the display size and design, displays can be operated
at higher frequencies, but this will result in increased power consumption.
LCDs portray a capacitive load, which reduces the load impedance as
frequency increases. However, operation below 30Hz typically results in
LCD's can be overdriven by a combination of voltage and frequency,
which will result in cross talk or "ghosting". Ghosting is the appearance
or partial activation of an "off" segment. This condition occurs when high
drive voltage and frequency are applied. Since the current is directly
proportional to the frequency, there is a voltage-frequency product
which must not be exceeded. These values are very dependent on the
design and layout of any given part, so proper display design and choice
of driving conditions is important. It is also very important that all unused
segments be connected to the backplane, and not allowed to float.
Multiplex Drive LCD Technology
The configuration for Liquid Crystal Display Multiplex Drive technique
differs from a Static Drive technique is that it uses more than a single"backplane" or segment common. With this configuration, each segment
control line can be connected to as many segments as there are backplanes,
providing that each of the segments that it is connected to are tied to a
separate backplanes. This method "Multiplexes" each of the segment
control lines and minimizes the number of interconnects. This is the
method used with complex displays that have limited interconnection
surface area or available drive circuits. This reduction in the number
of external connections enhances device reliability and increases the
potential display density. The liability of a higher multiplex rate will
effect display quality, operational temperature range, and the increased
complexity of drive circuitry (or perhaps microprocessor software) may
necessary for their operation.
The method of drive for multiplexed displays is essentially a time division
multiplex with the number of time divisions equal to twice the number of
common planes used in a given format. As is the case with conventional
LCDs, in order to prevent irreversible electrochemical action from
destroying the display, the voltage at all segment locations must be caused
to reverse polarity periodically so that zero net DC voltage is applied.
This is the reason for the doubling in time divisions: Each common plane
must be alternately driven with a voltage pulse of opposite polarity.
As is the case with non multiplexed displays, the drive frequency should
be chosen to be above the flicker-fusion rate, i.e. >30 Hz. Since increasing
the drive frequency significantly above this value increases current demand
by the CMOS drive electronics, and to prevent problems due to the finite
conductivity of the display segment and common electrodes, an upper drive
frequency limit of 60-90 Hz is recommended.