Voltage Generation Circuits for LCD Contrast Control
Most Liquid Crystal Display (LCD) modules require a contrast control voltage—referred to as VI, VEE, or bias voltage—that is either more positive or more negative than the logic voltage (typically +3.0V to +5V). Since this bias voltage usually cannot be sourced directly from the logic power rail, it must be generated using a separate supply or derived from an existing voltage source such as a logic rail or battery.
This application note presents several circuit options for generating positive or negative bias voltages suitable for driving LCD contrast, and outlines key considerations to ensure reliable and high-quality display performance.
The LCD bias voltage directly powers the internal circuits responsible for driving the LCD glass. It determines the contrast level of the display, making it essential that the voltage is well-regulated—fluctuations greater than ±200 mV can result in noticeable contrast shifts. Additionally, ripple or noise on this voltage must be kept below approximately 100 mV to prevent visible artifacts.
1. Charge Pump Circuits
Charge pump circuits are simple, low-cost solutions suitable for character displays and small graphic LCDs. However, they are unregulated and provide limited current output. These circuits are sensitive to source voltage variations, making them unsuitable for direct battery operation unless regulated upstream.
The driving signal for a charge pump is typically a square wave derived from an I/O pin or existing system clock. Frequencies may range from 1 kHz to 50 kHz or higher. For frequencies above 5 kHz, replace standard diodes (e.g., 1N4148) with low forward-voltage Schottky diodes (e.g., 1N5817), and use low ESR capacitors.
Example:
Figure 1 shows a basic charge pump circuit generating -4V from a +5V square wave. This setup is suitable for driving the VI line on an extended-temperature LCD character module. The signal source must be capable of sinking/sourcing at least 4 mA.
2. Adjustable Voltage Inverter
In applications where user-adjustable contrast is desired, an inverter circuit with a potentiometer can be used.
Figure 2 demonstrates a contrast voltage generator with a user-accessible potentiometer. The output voltage is adjustable from 0V to -12V. The total resistance of the pot (and any series resistors) should not exceed 50 kΩ.
If user control is not needed, a fixed resistor may replace the pot to set the optimum contrast voltage—simplifying production by removing the need for adjustment.
This circuit offers high efficiency, making it suitable for battery-powered applications and capable of driving displays up to approximately 240 × 64 pixels.
Control Features:
- ON/OFF Input (Pin 3): A logic low disables the contrast voltage. Tie to +5V if not used.
- Voltage Control (RVMAX): A single resistor sets the maximum output voltage. See Table 1 for selection values.
3. Digitally Adjustable Inverter
For systems where contrast control must be handled by a microcontroller rather than a potentiometer, a digitally controlled inverter is ideal.
Figure 3 presents a circuit enabling a microcontroller to control contrast via digital signals. A logic-controlled shutdown pin allows the controller to disable the negative voltage, facilitating safe power sequencing during startup and shutdown.
The internal 64-step DAC allows output voltage adjustment from 33% to 100% of maximum. On power-up or reset, the DAC defaults to 67% output. Each rising edge on the ADJUST line increments the DAC, and the value wraps around after reaching 100%.
Reset Functionality:
Apply a logic high to the ADJUST line, then pull the ON/OFF line low for more than 400 ns to reset the output to default.
4. High Voltage Circuits for Larger Panels
Larger displays, such as 1/4 VGA to full VGA (e.g., 320 × 240), often require bias voltages greater than ±20V. Typically, monochrome panels require a negative VEE, while colour panels need a positive bias voltage.
In battery-operated devices, these circuits must operate efficiently across a variable input voltage range as battery charge depletes.
Specialised ICs from various semiconductor manufacturers provide compact, efficient solutions for these requirements.
Figure 5 shows an application using the Linear Technology LT1615 series, configured to generate a positive voltage for a 1/4 VGA (320×240) colour display—ideal for handheld devices running platforms like Windows CE. The circuit runs from two AA batteries, producing a stable output across an input voltage range of 2.0V to 3.2V.
For negative output, the LT1617 variant is available.
5. General Circuit Considerations
For optimal performance:
- Place all passive components (resistors, capacitors, etc.) as close to the IC as possible.
- Especially ensure that decoupling capacitors are positioned close to the VIN and GND pins.
6. Power Sequencing Guidelines
Proper power sequencing is critical to prevent damage to the LCD module:
- Never allow VEE (or VL) to be present without VDD.
Doing so, even momentarily, can permanently damage the LCD.
Figure 6 shows the recommended power-on sequence for graphic LCDs with external controllers. For graphic LCDs with built-in controllers, the “signal” line timing is handled internally. For character displays, only the VDD and VEE (VL) lines require sequencing.
Most circuits (except the Figure 1 charge pump) feature a logic-controlled shutdown input, allowing VEE to be applied only after VDD is stable and the controller is initialised. Power-down should reverse the sequence—disable VEE before VDD is removed.