HD66740 Datasheet, PDF (52/95 Pages) Hitachi Semiconductor – 112 x 80-dot Graphics LCD Controller/Driver

HD66740
Contrast Adjuster
Software can adjust 64-step contrast for an LCD by varying the liquid-crystal drive voltage (potential
difference between V_LCD and V1) through the CT bits of the contrast adjustment register (electron volume
function). The value of a variable resistor between V_LCD and V1 (VR) can be precisely adjusted in a 0.05
x R unit within a range from 0.05 x R through 3.20 x R, where R is a reference resistance obtained by
dividing the total resistance.
The HD66740 incorporates a voltage-follower operational amplifier for each of V1 to V5 to reduce
current flowing through the internal bleeder resistors, which generate different liquid-crystal drive
voltages. Thus, CT5-0 bits must be adjusted so that the potential differences between V_LCD and V1 and
between V5 and GND are 0.4 V or higher when liquid-crystal drives, particularly when the VR is
small.
VLCD
HD66740
VR
CT
R
+-
V1
R
+-
V2
+-
V3
R0
R
R
GND
+-
V4
+-
V5
GND
Figure 27 Contrast Adjuster
52