Voltage Generating Circuits for LCD Contrast Control Most Liquid Crystal Display modules require a Positive or Negative voltage that is higher than the logic voltage used to power the LCD. This voltage, called VI, VEE or Bias voltage, requires a second power supply. 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 illustrates circuits for generating either a Positive or Negative LCD Bias voltage from such a voltage source. The LCD Bias voltage is used to directly 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 more 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.

Charge Pump Circuits These simple circuits can be used to generate the bias voltage for character type displays and small graphics types. They have the advantage of being very low in cost but are not regulated and cannot deliver much current. They are also sensitive to variations in the source voltage (Vdd), so it cannot be driven directly from a battery. The driving signal is usually derived from an existing clock signal or generated directly by an I/O pin on a microprocessor. The frequency of the signal can be anywhere from about 1kHz to 50kHz or higher. If the signal is above about 5kHz the simple 1N4148 diodes should be replaced by Schottky diodes such as the 1N5817. The capacitors should also be upgraded to low ESR types. The device generating the signal must be capable of delivering the load current times the multiplication value. In the circuit in Figure 1 driving a small character display the input signal should be able to sink and source at least 4mA.