Maxim’s Advanced Gamma Calibration System for TFT-LCD Panels

With the increasingly fierce market competition, the time to market of LCD products has become a key factor restricting the sales of products in the market. In the past, it took engineers a week to obtain the optimal gamma and VCOM voltage for a specific TFT-LCD panel, but with AGCS, the optimization process of gamma and VCOM voltage can be shortened to within seconds.

Users of LCD panels have very strict requirements on the gamma specification of the panel. Therefore, LCD manufacturers must establish a gamma curve for each panel module, and the difference in the gamma voltage of different panels during the production process exceeds the user’s requirements for gamma parameters. LCD module manufacturers can use AGCS to fine-tune each panel on the production line.

This paper presents a closed-loop feedback system capable of gamma and flicker calibration of TFT-LCD panels in engineering trials, with the ultimate goal of embedding an Advanced Gamma Calibration System (AGCS) into a TFT-LCD production line. The AGCS software runs on a personal computer and performs the following operations:

Display test template for liquid crystal display module (LCM); use Konica Minolta CA-210 to measure brightness and flicker; use fast algorithm software to build Gamma curve to suppress flicker; program Gamma and VCOM codes to Maxim’s MAX9669 16-channel programmable Gamma chip , while providing programmable VCOM.

AGCS can ultimately reduce flicker over a range of gray levels (rather than just one gray level).

Figure 1: Block diagram of the AGCS system.

Figure 1 shows the principle block diagram of the AGCS system. The software instruction controls the computer to send the video test template through the digital video interface (DVI). The TFT-LCD panel then displays the test template. Next, the software sends commands to the Konica Minolta CA-210 (LCD Color Analyzer) via the USB interface to measure the brightness and flicker of the panel (the CA-210’s far-end optical probe should be placed in front of the panel). The AGCS software calculates the Gamma and VCOM codes and loads them into the MAX9669’s digital-to-analog converter (DAC) registers via the USB and I2C interfaces.

With the increasingly fierce market competition, the time to market of LCD products has become a key factor restricting the sales of products in the market. In the past, it took engineers a week to obtain the optimal gamma and VCOM voltage for a specific TFT-LCD panel, but with AGCS, the optimization process of gamma and VCOM voltage can be shortened to within seconds.

Users of LCD panels have very strict requirements on the gamma specification of the panel. Therefore, LCD manufacturers must establish a gamma curve for each panel module, and the difference in the gamma voltage of different panels during the production process exceeds the user’s requirements for gamma parameters. LCD module manufacturers can use AGCS to fine-tune each panel on the production line.

This paper presents a closed-loop feedback system capable of gamma and flicker calibration of TFT-LCD panels in engineering trials, with the ultimate goal of embedding an Advanced Gamma Calibration System (AGCS) into a TFT-LCD production line. The AGCS software runs on a personal computer and performs the following operations:

Display test template for liquid crystal display module (LCM); use Konica Minolta CA-210 to measure brightness and flicker; use fast algorithm software to build Gamma curve to suppress flicker; program Gamma and VCOM codes to Maxim’s MAX9669 16-channel programmable Gamma chip , while providing programmable VCOM.

AGCS can ultimately reduce flicker over a range of gray levels (rather than just one gray level).

Figure 1: Block diagram of the AGCS system.

Figure 1 shows the principle block diagram of the AGCS system. The software instruction controls the computer to send the video test template through the digital video interface (DVI). The TFT-LCD panel then displays the test template. Next, the software sends commands to the Konica Minolta CA-210 (LCD Color Analyzer) via the USB interface to measure the brightness and flicker of the panel (the CA-210’s far-end optical probe should be placed in front of the panel). The AGCS software calculates the Gamma and VCOM codes and loads them into the MAX9669’s digital-to-analog converter (DAC) registers via the USB and I2C interfaces.

The MAX9669 has 16 programmable gamma buffer channels and a programmable VCOM amplifier. In addition, multi-time programming (MTP) memory is provided inside the device (shown in Figure 2). Using MTP memory, Gamma and VCOM codes can be written to nonvolatile memory and loaded into the DAC registers at power-up.

Figure 2: The MAX9669 programmable gamma buffer with MTP.

The following equations (1), (2) and (3) give expressions for the Gamma curve, luminance and luminance error.

where T(j) is the visibility of the LCD at gray level j, and for an 8-bit panel, j = 1 to 255; L(j) is the brightness of gray level j, ε(j) is the brightness error, and LT( j) is the target brightness calculated by equation (2). A successful gamma calibration occurs when the luminance error is within a user-defined tolerance. As can be seen from the Gamma calibration curve in Figure 3, using AGCS can reduce the maximum luminance error from 42% to 23%. The test bench described in this article is able to complete a Gamma calibration in 13 seconds.

Figure 3: Gamma curve and error function.

In order to reduce the residual DC voltage (which can cause image sticking), the voltage across the pixel needs to alternate between positive and negative polarity on a frame-by-frame basis. However, due to the presence of parasitic capacitance, the DC component will still reside, resulting in a change in brightness when switching from positive to negative polarity. This change in brightness is called flicker, and to reduce flicker, the common-mode voltage (VCOM) of each panel can be adjusted on the production line to remove the residual DC voltage on the pixels. The operator can observe the display on the production line and adjust the tap of the mechanical potentiometer to change the VCOM voltage.

Figure 4: VCOM DAC code and grayscale characteristics optimized for the MAX9669’s 10-bit DAC and 8-bit grayscale.

Programmable VCOM buffers allow automatic calibration, eliminating the need for mechanical potentiometers. AGCS uses the fast tracking algorithm to correct flicker, and the calibration operation for a certain gray level can be completed within 2 seconds. Figure 4 shows the optimization of VCOM for different gray levels. AGCS can equalize the VCOM, and obtain the best VCOM voltage in the whole gray scale range by adjusting the Gamma voltage. Figure 5(a) shows the calibration result of VCOM equalization on a 40″ LCD panel. The calibrated VCOM DAC code is shown as a green dot in the figure, the target VCOM value is a pink circle, and the initial VCOM value is a blue circle. Calibration The difference between the post VCOM and the target VCOM will not exceed ?2LSB, while the initial error of the 10-bit DAC is ?6LSB (1LSB is approximately equal to 16mV).

The AGCS GUI shown in Figure 5 contains four tabs: Hardware, Equalized VCOM, Flicker, and Gamma. The Hardware tab provides functions to manually program Gamma and VCOM voltages; the Flicker tab contains flicker measurement functions with different test templates and white and green color options. The Gamma tab allows the user to measure and calibrate the Gamma curve; the Equalized VCOM tab provides a range of Gamma calibration and VCOM equalization.

Click the Calibrate button in the Gamma tab to start Gamma calibration, AGCS draws a red curve, representing the user-defined Gamma curve that must be satisfied. AGCS measurements show the Gamma curve of the panel, shown in blue. If the Gamma value cannot meet the tolerance requirements, the AGCS will use the tracking algorithm to calibrate the Gamma voltage and obtain the required Gamma curve. The green curve shows whether the calibration is complete.

The flicker measurement includes the following process: setting the flicker template, programming the VCOM voltage, reading the flicker level, and starting a fast algorithm to calculate the optimal VCOM voltage to reduce the flicker level.

VCOM equalization will perform a series of AGCS and equalized VCOM (or EVCOM) calibration procedures. In EVCOM, flicker is minimized by adjusting the gamma voltage and measuring the flicker level. The target VCOM voltage is shown in the pink circle, the calibrated VCOM voltage is shown in the blue circle, and the final VCOM voltage is shown in the green circle. AGCS and EVCOM may require several heavy-duty processes to get the final result. Thus, the AGCS software allows the user to customize the AGCS and EVCOM procedures. Since the vast majority of LCD panels can only be set to one VCOM level, the feature of the Equalized VCOM tag ensures the best gamma and flicker metrics.