In the automotive display system, TFT LCD displays dominate in the automotive display panel. Furthermore, OLED and micro-LED displays are drawing more and more attention in the market. The power techniques need to be developed for different display technology. The edge-lit backlight and direct-lit backlight are commonly used in TFT LCD displays. The local dimming based on the direct-lit backlight with mini-LEDs is developed for improving the display performance. Although OLED displays are more common in smartphones, the automotive OLED and micro-LED displays are still in development. This article comprehensively describes the automotive display system and the power techniques.
Introduction
With the electrification of vehicles, customers expect the vehicle to deliver more features across safety, convenience, entertainment, productivity, comfort, and sustainability. Meeting this demand will require new solutions from OEMs and Tier 1 suppliers. One of the challenges is the increased power consumption with the incorporation of more hardware in the vehicle. A sound solution is proactively considering the power management capabilities in the early design stage. This article describes power techniques for automotive display systems.
Automotive Display System
Figure 1 shows a simplified automotive display system, which comprises a head unit, display panel, and shielded twisted pair (STP) or coaxial cables. The automotive head unit, also called the infotainment system, is the central control interface for audio, entertainment, navigation, and connectivity features in a vehicle. Advanced human-machine interfaces, such as precise voice recognition, smooth and responsive touch screens, and sophisticated gesture control, have become common features in modern vehicles.
Also read: Automotive Displays – Basics of TFT LCD, OLED, and Micro-LED Displays
Inside the display panel, the timing controller (TCON) serves as the interface between the video or image stream and the display pattern. The connectivity between the head unit and TCON is established by a pair of serializer (SER) and deserializer (DES) chips, which enable high speed chip-to-chip communication via up to 10 meter shielded STP or 15 meter coaxial cables. For a 45 inch to 60 inch 7680 × 2160 pixels pillar-to-pillar display, the data transmission needs to be up to 28 Gbps payload.
Figure 1. Analog Devices ’Gigabit Multimedia Serial Link(GMSL™) solution is a cost-efficient, simple, and scalable SERDES technology
Powering Automotive TFT-LCD Displays
As an example, Figure 2 demonstrates the TFT-LCD display panel. The entire display panel system consists of a TCON, microcontroller (MCU), LED panel (backlight), and TFT bias PMIC.
Figure 2. Block diagram of the LCD display system.
Backlight
The backlight power techniques can be classified as two common types: edge-lit and direct-lit backlight, as shown in Figure 3. The edge-lit backlights are the conventional method to power the backlight, which allocates LEDs at the edge of the display panel. Typically, four to eight LED strings are used in an edge-lit backlight. The method to control LED string brightness is called global dimming, where the whole backlight or a specific LED string dimming is performed. As shown in Figure 4, the LED strings are powered by the boost converter. The dimming function can be performed by the external PWM, or an internal 18-bit register T_ON. The MAX25512 also supports hybrid dimming, which combines analog dimming and PWM dimming. The dimming ratio with 200 Hz dimming frequency is 16,667:1. With hybrid dimming, it can be doubled to 33,333:1.
Figure 3. Backlight power techniques: edge-lit and direct-lit.
As shown in Figure 3, the mini-LEDs are allocated uniformly at the LED matrix where they are located directly behind the LCD panel. Each mini-LED brightness can be controlled individually and dynamically to adopt the image content. This dimming method is called local dimming, which improves the contrast ratios. In comparison with the edge-lit display, the direct-lit backlight has low power consumption. When the dark content in the image is displayed, the direct-lit backlight can turn off the LEDs at the dark image region. It can also mitigate the light leakage when the dark content image is displayed.
The display size, resolution, and brightness depend on the number of LED strings and LEDs per string in edge-lit backlight. In direct-lit backlight, the large size, high resolution, and contrast ratio of the display panel are achieved by the LED zones. Figure 5 shows the configuration of one LED in series and two LEDs in parallel (1S2P). The backlight driver and backlight requirement decide the LED configuration, such as 1S2P or 2S1P. The VLED is provided for the LED forward voltage and VSINK is the voltage to maintain the desired constant current for the LEDs.
Figure 4. The MAX25512 backlight driver.
Figure 5. 1S2P LED zone
The two different types of local dimming LED drivers are shown in Figure 6. The matrix LED driver uses time-sharing multiplexing technology. As shown in Figure 6a, four LEDs share the same current source with four PMOS. The LEDs in the direct drive LED driver are powered by the same VLED and individual current source as shown in Figure 6b. The direct drive LED driver could have higher current driving capability than the matrix LED driver. Fewer matrix LED drivers can be used to drive the same number of LED zones than the direct drive LED driver.
Figure 6. Local dimming LED driver:(a) a matrix LED driver, and (b)a direct drive LED driver
The MAX2550x is a matrix LED driver that integrates the PMOS, and VLED can operate at 14 V. Thus, it can support up to four LEDs in series (4S1P) in one zone. The internal PMOS improves the current loop for better EMI performance and eliminates ghost images. Furthermore, it reduces system complexity and size due to the internal control of the PMOS.
Also, the MAX2550x is featured with a feedback control function. The forward voltage of the LEDs exhibits a monotonical drop as the junction temperature increases. If the VLED is various for the full range of the operating temperature, the VSINK voltage increases, and it causes higher power dissipation, especially under 85°C. The feedback control function optimizes the VSINK voltage by adjusting VLED. Ideally, VLED is the sum of the LED forward voltage and VSINK. As shown in Figure 7, the FB in the MAX2550x backlight driver sinks the current and increases the VLED when the VSINK voltage is at a lower voltage.
Both the matrix LED driver MAX2550x and the direct drive LED driver MAX21610 are featured with the dot correction function. LED dot calibration is pixel-by-pixel correction technology for the LED backlight display. It calibrates the individual LED outputs for the same brightness. To achieve display uniformity performance as same display color and brightness, the MAX2550x has two methods to set the individual LED current: each of the LED zones have the individual 5-bit current setting and the individual 17-bit PWM setting. The 5-bit current setting can be used for each LED brightness calibration and the 17-bit PWM setting can be used with a global setting when all LED zones have the same brightness setting.
Furthermore, the MAX2550x can achieve the tell-tale indicator, which indicates the malfunction or operating system by an illuminated symbol, such as low fuel/charge. With specific current settings and PWM settings, the peak current of the MAX2550x can be boosted by the SCALE_SEL registers.
TFT Bias
External power circuits for TFT bias are required for display panels larger than 6 inches with higher resolutions. For smaller display panels, the power circuit is integrated into the glass. Typically, the supply voltage is 3.3 V and 5 V. Furthermore, the 5 V rail is mostly used in automotive display panels for a reliable and robust lifetime.
As shown in Figure 8, an integrated TFT power circuit (MAX25221) powers the source driver (also called the column driver) with AVDD and NAVDD, which are supplied by a boost converter. The gate driver (also called the row driver) is powered by VGH and VGL, which are generated by two separate charge pumps. This circuit also integrates VCOM and VCOM temperature compensation.
The targeted voltages for the column driver, row driver, VCOM, and power ON/OFF sequence can be programmed and stored in nonvolatile memory. This functionality guarantees a reference backplane voltage for TFT LCDs that can be tuned for different panels, helping to reduce display aberrations and improve system robustness.
OLED/Micro-LEDDriver
In comparison between OLED/micro-LED and LCD displays, the power circuitry is similar, which includes the TFT bias PMIC and OLED/micro-LED power supply. At present, the technology of OLED displays in consumer electronics is more mature. A 5 V rating PMIC, which combines the TFT bias power and OLED/micro-LED power and integrates with two boost converters and one inverting buck-boost converter, is typically powering the OLED display. For the automotive OLED/micro-LED display, the power PMIC is still under development. The supply current required by OLED/micro-LED in automotive displays is higher than in consumer electronics due to the large size. Also, more reference voltages are needed for TFT bias PMIC.
Conclusion
Display technology is evolving rapidly with the emergence of new display types, such as OLED and micro-LED. These new display types require more advanced pixel drivers and TFT bias PMICs to achieve optimal performance. As LED drivers and TFT PMICs become more efficient, better performing, more intelligent, and cooler, they can better meet the demands of these new display technologies.
The article has been written by YujieBai, Senior Applications Engineer, Analog Devices
Yujie Bai is a senior applications engineer at Analog Devices and is responsible for support and application of automotive power products. Yujie joined Maxim Integrated (now part of Analog Devices) in 2020 and holds a master’s degree in electrical engineering from Miami University (Ohio).
