Liquid Crystal Displays (LCD) are widely used for commercial and industrial applications because of their good visual properties, low cost and, low power consumption. These properties make the LCD the standard solution for battery-operated devices, like portable instruments, calculators, watches, radios, etc.
However, to properly control what the LCD shows, the LCD’s electronic driver must generate appropriate voltage waveforms to LCD pins. The waveforms should be AC (alternate current) in nature because DC (direct current) voltages will permanently damage the device. The appropriate driver would source these signals to LCD at a minimum of power consumption.
Two types of LCDs exist, the Static, with only one backplane and one pin for individual segment control and, the Multiplexed, with multiple backplanes and multiple segments connected for each pin.
This project will present the design of one static LCD driver with an SLG46537V GreenPAK device. The designed LCD driver would drive up to 15 LCD’s segments, using a few microamperes of current from the power supply and offer an I²C interface for control.
In the following sections will be shown:
● basic knowledge information about LCDs;
● the SLG46537V GreenPAK LCD driver design in detail;
● how to drive a seven-segment, 4-digit static LCD with two GreenPAK devices.
Below we described steps needed to understand how the static LCD driver has been programmed. However, if you just want to get the result of programming, download GreenPAK software to view the already completed GreenPAK Design File. Plug the GreenPAK Development Kit to your computer and hit the program to design the solution.
Basics of Liquid Crystal Displays
Liquid Crystal Displays (LCD) is a technology that does not emit light, it only controls how an external light source passes through. This external light source could be the available ambient light, in the reflective display type, or the light from a backlight led or lamp, in transmissive display type.
LCDs are constructed with two plates of glass (upper and bottom), a thin layer of liquid crystal (LC) between them and two light polarizers. The polarizer is a light filter for the light electromagnetic field. Only the light components in the right electromagnetic field direction pass through the polarizer, while the other components are blocked. The liquid crystal is an organic material that rotates the electromagnetic field of the light 90 degrees or more. However, when an electrical field is applied to the LC it does not rotate the light anymore. With the addition of transparent electrodes in the upper and bottom display glass, it's possible to control when the light passes through, and when not, with an external source of the electrical field. Figure 1 below illustrates this operation control. In Figure 1, the display is dark when there isn’t an electrical field. This is because both polarizers filter the light in the same direction. If the polarizers are orthogonal, then the display will be dark when the electrical field is present. This is the most common situation for reflective displays.
However, it's important to note that the LCD can’t operate with a direct current (DC) voltage source for too long. The application of a DC voltage will cause chemical reactions in LC material, permanently damaging it. The solution is to apply an alternate voltage (AC) in LCDs electrodes.
In static LCDs, a backplane electrode is built in one glass and individual LCD’s segments, or pixels, are put in the other glass. This is one of the simplest LCD types and the one...Read more »