Using logic supply levels to regulate the vcom signal for thin-film transistor liquid crystal display (TFT LCD) flat panel displays

All thin-film transistor liquid crystal display (TFT LCD) flat panel displays require at least a properly regulated VCOM signal to provide a reference point for the flat panel display backplane. The exact value of the VCOM supply voltage varies from flat panel display to flat panel display, so the VCOM value must be set at the factory to match the individual characteristics of each display. Properly adjusting the VCOM value can reduce flicker and other undesirable effects.

In the voltage divider mode of operation, VCOM adjustment is usually achieved using a mechanical potentiometer or a trimmer potentiometer. In recent years, flat panel display manufacturers have begun to seek other solutions because mechanical potentiometers cannot provide the resolution required to meet the requirements of optimal image fidelity for large flat panel displays. In addition, manufacturers also require physical adjustments that are usually completed by technicians on the assembly line. This adjustment is not only time-consuming, but also prone to field failures caused by human error or mechanical vibration.

A simple way to achieve the resolution required to optimize the image quality of flat panel displays is to use digital potentiometers instead of mechanical potentiometers. By using digital potentiometers, flat panel display manufacturers can automate the VCOM adjustment process, thereby reducing production costs and improving product quality.

However, many flat panel displays operate at higher voltages, limiting the available supply voltages. If a 5 V power supply is available, the implementation is simple, as shown in Figure 1. If a 5 V supply is not available, the circuit becomes more complicated. This design idea shows a simple way to power the potentiometer with any available logic supply, as long as VCOM adjustment can be performed.

Figure 1. Configuration for providing a dedicated 5 V supply.

The device used to demonstrate this solution is the AD5258/59 6/8 bit nonvolatile digital potentiometer from Analog Devices. There is an I2C serial interface for control and to store the required potentiometer setting data in EEPROM flash memory. The AD5259 is manufactured using a 5 V submicron CMOS process to reduce power consumption. It is available in a 10-lead MSOP ultra-small package, which is an important feature for low-cost, small-size applications.

In systems where a 5 V supply is not available, many designers will simply try to take the tap off the 5 V in a resistor string. This is an undesirable solution because the AD5259 typically draws 35 mA during programming to write to the EEPROM. It cannot draw that much current due to the large voltage drop across R1. For this reason, the AD5259 provides a separate VLOGIC pin that can be connected to any logic supply. A digital potentiometer is used here to control the MCU's supply voltage. Now, the VLOGIC supply can draw a programmable 35 mA current, and the VDD supply only draws microampere (μA) supply current to provide bias current for the built-in switches of the digital potentiometer's built-in resistor string. If the flat panel display requires an increased VCOM voltage, two resistors can be added to the op amp's non-inverting gain configuration.

Figure 2. Taking the VDD tap from a resistor string.

The digital potentiometer has a ±30% tolerance on the resistance across the two terminals. Assuming that the tolerances of R1, R3, and Vi are negligible compared to the potentiometer, the following table lists the range of output values ​​achieved. As shown in the table, this circuit configuration guarantees an output range of 3.5 V to 4.5 V with an internal adjustment step of ±10 mV. In addition, since the logic power supply of the digital potentiometer matches the logic voltage of the MCU, data can be read back to the MCU if necessary.

Table 1. Output voltage range

Figure 3 shows a block diagram of a digital potentiometer.

Figure 3. AD5259 block diagram