HD66766R Datasheet, PDF (95/97 Pages) Hitachi Semiconductor – 132 x 176-dot Graphics LCD Controller/Driver for 65K Colors

HD66766R
Rev. 1.0-1 / September 2002
Additional Issue
HD66766R Contrast fluctuation on RAM access
Hitachi one-chip driver; HD66766R has contrast fluctuation while accessing the internal RAM. This
phenomenon occurs according to the structure of external circuit and the usage of HD66766R. Please
have a clear understanding of the phenomenon and measures described below before using HD66766R.
1. Contrast fluctuation
When HD66766R is mounted on glass, the grand terminal gets contact with resistance of ITO wiring.
HD66766R has 8 GNDs, 4 AGNDs for power supply circuit, and another 4 GNDs for RAM and Logic.
When all the GNDs are connected on glass with ITO, transferring display data to the internal RAM at
high-speed causes high current consumption. And resistance of ITO wiring connected with GND
terminal raises the voltage. The raised voltage is amplified with a step-up circuit, and it results in a
decrease of VCH/VCL voltage causing contrast fluctuation on display. Figure 1 shows the mechanism
of the phenomenon. Figure 2 shows decrease of VCH voltage according to RAM access frequency and
resistance of ITO wiring. (The value shown in Figure 2 is the actual data of a typical sample of
HD66766R measured by Hitachi’s jig.)
2. Measures
ITO patterns on glass must be separated as shown in Figure 3. (Even if GND bumps are separated on
LSI, if ITO patterns on glass are connected, GND level rises up in RAM circuit and Logic circuit.) Also,
GND resistor should be designed to be less than 10 Ω, considering the decrease of VCH voltage caused
by the raise of GND voltage.
LCD
RAM
HD66766R
Voltage
decrease
Voltage
generating
circuit
VCH
Voltage
decrease
RAM access
current
GND
Resistance of
ITO wiring
Figure 1 Influence of ITO wiring resistance
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