2/26

4-MULTIPLEXED LCD WITH A STANDARD ST62

1 LCD DRIVING PRINCIPLES

1.1 LCD REQUIREMENTS

An LCD segment can either be transparent (“off”) or opaque (“on”), depending on the voltage

applied to it. On Figure 1, this voltage is the difference between

COM and S voltages. On most

LCDs (reflective positive displays) an opaque segment is seen dark and a transparent seg-

ment is seen clear (same colour as the background).

Figure 1. Equivalent Electrical Schematic of an LCD Segment

If no voltage is applied to it, a segment is transparent. To make it opaque, the LCD driver must

apply an AC voltage which Root Mean Square (RMS) value is above a certain threshold. This

voltage threshold depends on the LCD characteristics.

Segment voltage must also comply with the following conditions:

– Its absolute DC (mean) value must be very low (under 100 mV typically). Otherwise, the life

time of the LCD can be shortened.

– Its frequency must be in the range 30 – 2000 Hz typically. If too low, the display flickers. If

too high, driving generates more power consumption.

1.2 DIRECT LCD DRIVE

Each LCD segment is located between a segment electrode and a backplane common to all

the segments (see Figure 2). Therefore, a display using

N segments contains (N+1) external

connections:

C

S

COM

segment

electrode

backplane

2