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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: The simplest and least expensive way of generating an LCD bias voltage is with a charge pump circuit. A charge pump generates a voltage that is some multiple of the peak to peak (P-P) voltage of the input square wave. The output can be either positive or negative.

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.

Figure 1 shows a simple charge pump circuit that generates a Negative 4V from a Positive 5V square wave. It is suitable for driving the VI line on an extended temperature LCD character module.

Adjustable Voltage Inverter It might be desirable to allow the end user of a product to have access to the contrast adjustment. The circuit in Figure 2 utilizes a pot to adjust the contrast voltage from 0V to -12V limits by adding one or two resistors in series with the pot. The total resistance of the pot and any added resistors should not exceed 50k. If the end user is not to have control of the contrast the pot can be substituted with a fixed resistor to set the voltage to the LCD to give the best contrast, which would also eliminate the need to adjust a pot during production. The efficiency is high enough to be used with battery operated equipment and the output can drive most small graphics displays, up to about 240 x 64px resolution.

Digitally Adjustable Inverter: Some applications require the user to have control of the contrast but do not lend themselves to using a pot to make the adjustment. The circuit in Figure 3 allows a micro controller to adjust the VL voltage in a very simple manner. It also provides an input to shut off negative voltage so the display can be shut down by the micro controller if desired. This shutdown signal can also be used to properly sequence the power to the display during power-up and power-down sequences.

On/OFF signal. A logic 0 on this pin will turn the display off by removing the VL voltage. If this signal is not needed, tie pin 3 to +5V. Output voltage control. The maximum voltage is set using a single resistor, RVMAX, See Table 1.

The output can then be adjusted from 33% to 100% of this value using the internal 64-step DAC / counter. On power-up or after a RESET command the output voltage is set to mid-range which is 67% of the maximum voltage. Each rising edge of the ADJUST signal increments the DAC output. When the DAC reaches 100% output, the next pulse will cause it to wrap-around to the 33% value.

Figure 4 A RESET is accomplished by setting the ADJUST line high and then setting the ON/OFF line low for longer than 400nS.

High Voltage Circuits for Larger Panels: Modern 1/4 VGA to full VGA size panels require VEE voltages above 20V. Most monochrome panels require a Negative VEE voltage while most color panels require a Positive voltage. Many of these panels require a Negative VEE voltage while most color panels require a Positive voltage. Many of these panels are used in handheld, battery operated applications and require very efficient conversion using a supply voltage that changes as the charge on the batteries is gradually depleted.

Several semiconductor manufactures have responded to this need with new devices made especially for this application. While the voltage converter can be done "in-house" it is usually not economical to do so because of the complexity of a circuit that has the regulation qualities and the efficiency required.

Figure 5 is a circuit based on the Linear Technologies LT1615 series chips. The circuit shown here generates a Positive voltage for a small 1/4 VGA (320x240) color graphics display that could be used in a palm sized PC running Windows CE. A Negative voltage version LT1617 is also available. The device in this example runs from a pair of AA batteries and must produce a stable output voltage with an input that varies from 2.0V ~ 3.2V.

General Circuit Considerations: All components associated with the circuits in Figure 2, Figure 3 and Figure 5 should be placed physically close to the IC. The decoupling capacitor on the input voltage line should be placed as close to the VIN and GND pins of the IC as possible.

Power Sequencing Considerations: The order in which the power supplies are applied to an LCD, power sequencing must be considered when designing an LCD bias power supply. The power sequencing requirement can be summarized by stating that the VEE (VL) must never be present without VDD also being present. If this condition exists, even for a short period of time, the display may be permanently damaged. The desired power on sequencing for graphics type LCDs with an external controller is shown in Figure 6. For graphics type LCDs with a built in controller you can ignore the "signal" line as this is taken care of in the controller at power on time. For character displays only the VDD and the VEE (VL) lines need to be considered.

All of the circuits described here, except the charge pump in Figure 1, have provisions to shut down the voltage generator with a logic signal. Using this signal the generator IC is kept in the shutdown mode until VDD is stable and the LCD controller has been initialized and has started to scan the display. At this time VEE can be applied to the display safely. The turn off procedure is just the reverse of the turn on procedure.

Sources: 1. Linear Technology, 1630 McCarthy Blvd. Milpitas, CA 95035 (408)432-1900 / http://www.linear.com/

2. Maxim Integrated Products, Inc. 120 San Gabriel Dr. Sunnyvale, CA 94086 (408)737-7600 / www.maxim-ic.com

Temperature Compensation for LCDs

The optimal contrast setting for LCDs varies with ambient temperature. For most applications this variation in contrast is tolerable over the "normal" temperature range of 0°C to +50°C. Most LCD modules are available with an extended temperature range option which allows the display to operate from -20°C to +70°C. The changes in contrast are NOT usually tolerable over this wide range of temperatures, which means a way of adjusting the contrast voltage as the ambient temperature changes must be provided.

As the temperature decreases the LCD fluid requires a higher operating voltage in order to maintain a given optical contrast. See Figure #1. One way to provide for this is to give the user control of the contrast. This is a simple solution but quite often its not desirable or practical.

Figure #1. The solid line describes Temperature compensated voltage provided by the circuit in Figure #2. The dashed line describes the way in which the LCD operating voltage varies with temperature.

The chart can be used to predict the voltage at VL needed to produce good contrast on the display by adding the "relative voltage" to the contrast voltage of the display at 25°C. If, for instance, a display looks good with -3V on VL at room temperature (25°C) this display will need -2.7V at 50°C.

The controlling microprocessor could measure the ambient temperature and supply the proper voltage to the LCD, but this is complicated and expensive.

The most common solution to the temperature compensation problem is to provide a circuit such as that in Figure #2 to adjust the contrast voltage automatically.

This circuit uses a negative temperature coefficient thermistor to sense the ambient temperature. It should be placed as physically close to the LCD module as possible. The PNP transistor is connected as an emitter follower to provide the drive current to the LCDs contrast voltage (VL) input.

The voltage VEE will very depending on the requirements of the LCD. NOTE: VL and VEE are measured in relation to the VDD supplied to the LCD. An extended temperature range character display will require about -7.8V at its VL input at 25°C or about -2.8V relative to ground. The VEE voltage will need to be about 25% higher than the actual voltage required at the VL input of the LCD. During development the VEE should be a variable voltage that can be used to adjust the contrast to an optimal level. The VEE can be made fixed or adjustable for the production units.

This circuit will work for all character modules and graphic modules up to 320 x 240. Modules larger than this are not available with the extended temperature option.