Illuminating Automotive Displays

In recent years, there has been a substantial improvement in automotive displays. Automotive manufacturers now integrate these displays in various applications within the automobile, such as in mirror replacements, central information displays or CIDs, instrument clusters, entertainment displays for the rear seats, and many more. Sometimes, such displays can total up to twelve per vehicle.

Most of these displays use thin-film transistor or TFT driving liquid crystal displays or LCD for enhanced brightness and reliability at reasonable costs. However, there is an increasing need for better picture quality, as the display must also handle real-time video feed from cameras. In addition, an increase in display size is necessary due to the merging of CID and instrument clusters. With these requirements, the displays consume more power, and often, the timing controller or internal power supply source drivers cannot provide this. Therefore, there is a need for a suitable power supply that can handle these high-quality automotive displays.

Although many solutions exist for such TFT-Bias power supply devices, the display panel options are highly variable, and most have an expanded set of feature requirements. This leads to power supply designers considering a few needs for TFT-LCD display systems—high-quality display, source driving, functional safety, EMI mitigation, fast turn-on time, and low total solution cost.

Manufacturers use several TFT technologies for display panel solutions. However, each of them has its own set of benefits, needs, and limitations. For instance, two very common TFT technologies are the Low-Temperature Polysilicon LTPS and the Amorphous Silicon or A-SI panels. While driving an A-Si panel requires a unipolar source driver, the LTPS panels typically require a bipolar source driver.

Manufacturers bond unipolar A-Si source drivers to the edges of the display panel. Multiple digital to analog converters—one for each display column—drive the sources of the TFTs. Depending on the received video signal, the output voltage of the DACs varies, thereby setting the transmissivity of the LC panels. With the common rail for the display backplane set at approximately half the supply voltage, the source driver voltage is free to alternate between zero and full supply voltage.

Using LTPS technology, manufacturers can implement all the necessary circuits, including the bipolar LTPS source driver, directly on the glass of the display panel. This precludes the requirement of a storage capacitor in parallel with the subpixels, The higher carrier mobility in the transistors leads to higher performance in LTPS panels, and subsequently to advanced features.

Optimal display panel performance differentiates the high display quality necessary and requires support from consistent pixel response. However, imperfections in materials and processes often lead to deviations in parameter performance in physical manifestations of display solutions. These imperfections change the electrical characteristics of materials, which the design must account for while stabilizing the performance of the end product. This requires calibration methods where the voltage of the common rail is set.

The common rail voltage must also be compensated for temperature variations. This is because panel characteristics change over temperature, and the common rail voltage must also adjust for the display panel functionality to remain consistent.