HD66420 Datasheet, PDF (38/52 Pages) Hitachi Semiconductor – (RAM-Provided 160 Channel 4-Level Grey Scale Driver for Dot Matrix Graphics LCD)

HD66420
Power Supply Circuits
HD66420 has following circuits for power supply circuit: operational amplifiers, resistive dividers, bias
control circuit and contrast control circuit. LCD driving voltage, VLCD, must be generated externally.
LCD Drive Voltage Power Supply Levels: To drive the LCD, a 6-level power supply is necessary. These
levels are generated internally or supplied from outside. When an internal voltage levels generator is
chosen, external capacitors are needed to stabilize these levels. AS the HD66420 incorporates operational
amplifiers to these levels, this circuit gives better quality of display with less power consumption. This
divided ratio is programmable.
Bias current of internal operational amplifier is determined with a resister which is inserted between
IREFM and GND. This resister value is between 1MΩ and 5MΩ. Larger resister value make less power
consumption at internal operational amplifier. However, too large value loose operational margin of
amplifiers.
Keep following relationship among voltage levels;
V_CC ≥ IREFMP > IREFM ≥ GND
VLCD > V_CC > GREF ≥ GND
VLCD ≥ V1O ≥ V2O ≥ V3O ≥ V4O ≥ V5O ≥ GREF
VLCD–V_CC ≥ 1.0V
IREFP–IREFM ≥ 1.0V
V_CC–GREF ≥ 1.0V
Contrast Control: Internal contrast control circuit can change the output voltage level of VLCD by setting
data to contrast control register, R16. VLCD adjustable range are showed below;
• 1/6 bias
0.75 * (VLCD-GND) ≤ VLCD ≤ 0.99 * (VLCD-GND)
• 1/9 bias
0.82 * (VLCD-GND) ≤ VLCD ≤ 0.993 * (VLCD-GND)
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