Segment LCD Driver Circuit Design

A segment liquid crystal display is one of the most durable and readable ways to show a few digits, and it works by chemistry rather than by light emission. The driver has to present the right waveform to each segment, and a direct current across the glass destroys it, so the waveform matters as much as the voltage.

How the Glass Responds

The liquid crystal between two transparent electrodes rotates when a voltage is applied and changes the polarisation of the light passing through it. Combined with polarisers, that produces the familiar dark segment on a light background.

The rotation depends on the root mean square voltage across the segment rather than on its instantaneous value, which is what makes a multiplexed drive possible. A segment sees the average of the waveforms applied to it, and the driver arranges for the wanted segments to see a higher value than the unwanted ones.

An average of zero over time is essential. A direct voltage across the glass drives an electrochemical reaction that permanently damages the display, and the damage is invisible until the display has been in use for some time.

bias voltage and Its Levels

A static drive uses two levels and needs one connection per segment. As soon as segments are multiplexed, intermediate bias voltage levels are needed so that the difference between the on and off states can be distinguished.

A one third bias with a three level waveform is the common arrangement for a display multiplexed by three. The number of levels follows from the multiplex ratio: a display with n backplanes uses n plus one levels, and the driver has an internal divider that generates them.

The divider is made of equal resistors, and their matching determines the symmetry of the waveform. A mismatch shows as a difference in contrast between segments that share a common line, which is why these resistors are integrated rather than external.

multiplexing and Duty

A multiplexed display shares the segment electrodes between several backplanes. With two backplanes, half the segments can be driven at any instant and each is on for half the time, so the contrast falls compared with a static display.

The ratio of on to off rms voltage falls as the multiplex ratio rises, which narrows the window between a segment that is clearly visible and one that is faintly visible. Beyond four backplanes the window becomes small enough that the display is sensitive to temperature and to the exact bias levels.

multiplexing is still worth it because it reduces the number of connections between the glass and the board. Every connection is a potential failure point, and a display with a hundred segments driven statically would need a hundred pins.

The charge pump and Supply

A display that needs more than the logic supply voltage uses a charge pump to generate the higher level. The pump needs flying capacitors, and their value and placement set the drive strength and the ripple.

The current drawn by the display is small, usually microamps, so the pump can be small. What matters is that the voltage is stable, because a change in the supply changes the root mean square voltage and therefore the contrast.

The pump and its capacitors should be placed away from sensitive analogue circuits. The switching waveform has fast edges, and although the currents are small, the voltage steps are the full supply level and couple capacitively into anything nearby.

contrast and Temperature

The contrast of a liquid crystal display falls as the temperature rises, because the viscosity of the material changes. A display that looks right at twenty degrees looks washed out at forty and too dark at zero.

Temperature compensation adjusts the drive voltage with the temperature, usually with a curve stored in the driver or a thermistor in the divider network. A simple linear slope of a few tenths of a percent per degree covers most displays over a modest range.

contrast is also the user visible result of every other setting. A display that is readable at one multiplex ratio and poor at another usually needs the bias or the drive voltage changed rather than a different glass.

Waveform Quality and Flicker

The drive waveform has a direct current component of zero, and any asymmetry appears as a flicker at the frame rate. An asymmetry of a few tens of millivolts is enough to be visible on a large display.

The frame rate is chosen to be high enough to avoid visible flicker, usually above fifty hertz, and not a multiple of the mains frequency. A frame rate close to the mains rate produces a slow beat between the two that is perceived as a flicker.

Segment to segment crosstalk is a second quality issue. It appears as a faint ghost on an off segment and comes from the resistance of the common electrode, which is a thin transparent conductor with a noticeable sheet resistance.

Layout and Connections

The connections to the glass are made with a conductive elastomer or a heat seal connector, and both are mechanical as well as electrical. The board has to press the connector evenly, because a variation in pressure produces a variation in resistance and therefore in contrast across the display.

Keep the driver close to the connector so that the tracks are short and similar in length. A long track to one backplane adds resistance that changes the waveform for every segment on that line.

Route the segment and backplane tracks away from switching circuits. The drive voltage is small and the display is a capacitive load, so a nearby switching node couples into the glass through the tracks and appears as flicker. The wider practice for keeping these nodes clean is described in our guide to mixed signal board design.

Verification and Faults

Verify the display with an oscilloscope on the segment and backplane pins. The waveform should be symmetrical about the mid level, with equal positive and negative areas, and the difference between the on and off levels should match the bias calculation.

A display with one dead segment is a connection or a driver output fault, while a display with a whole backplane dead is a common line fault. Testing the pins with a scope distinguishes the two in a few minutes.

A display that fades in a warm room is behaving normally and needs compensation. A display that shows a permanent dark patch after a year of operation has had a direct current through it, and the cause is in the waveform rather than in the glass. The release checks that keep these assemblies consistent are collected in our PCB design release checklist and the inspection points in judging PCB quality.

FAQ

Why has my LCD developed a dark patch? A net direct current through the glass has damaged it. Check the waveform symmetry and the bias levels at the driver pins.

Why does the contrast drop when the product warms up? The liquid crystal becomes less viscous with temperature. Add temperature compensation to the drive voltage.

How many backplanes can I multiplex? Up to four is comfortable. Beyond that the difference between the on and off levels becomes small and the contrast window narrows.