Another application note from Hantronix, Inc. on simple to digitally controlled efficient power supply for LCD display contrast. Link here (PDF)
Many LCD display modules require a negative or positive voltage that is higher than the logic voltage used to power an LCD. This voltage, called Vl, Vee or the bias voltage, would require a second power supply in the application device. If this power source is not available the LCD bias voltage must be generated from an existing voltage, either the logic voltage (+3.0-+5v) or a battery. This application note describes circuits for generating either a negative or positive LCD bias voltage from such a voltage source.
The LCD bias voltage is used to power the circuits that drive the LCD glass. This voltage sets the contrast level of the LCD. Since any changes in this voltage will cause a visible change in the contrast of the LCD it must be regulated to better than about 200mV. Any noise or ripple on this signal may cause visible artifacts on the LCD so they must be kept below about 100mV.