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Local Dimming with Functional Safety for Automotive Displays
High-resolution TFT-LCDs (thin-film transistor liquid-crystal display) have become ubiquitous in modern life and their use in automotive has grown at 20% for the past 5 years. Evidence suggests high-resolution, high contrast ratio, large displays attract potential car purchasers. A possible alternative to TFT-LCDs to attain stunningly high contrast is OLED (Organic Light Emitting Diode), seen in high-end TVs today. Unfortunately OLED still has a long way to go before being used in automotive, due to reduced lifetime when subjected to the automotive temperature range, not enough maximum luminance, and higher cost. In order to bridge this gap, a local-dimming TFT display with arrays of white LEDs (light emitting diode) behind the TFT panel is the solution to obtain high contrast ratio as of OLED at an affordable price and at automotive ruggedness levels. TFT-LCDs require a large amount of electronics to render them functional: video reception circuitry, a display power supply, a backlight power supply and row/column drivers for the individual display pixels. In this paper, we will concentrate on the white LED backlight driver. The LED driver generally consists of a DC/DC converter which provides the correct drive voltage for the LEDs and a number of constant-current sources to drive the LEDs. Since, to achieve local dimming, we will need to use multiple LED drivers each with multiple output channels, a centralized DC/DC converter will be more cost effective. Because in local dimming each LED zone has its own individual brightness setting, the multi-channel backlight driver must permit individual dimming values for each channel. This is fundamental to local dimming to achieve a 'black is true black' effect especially important during night-time operation. To render local dimming more effective, many LED zones are needed (where each zone may consist of 1-4 LEDs). Considering that it is uneconomic to make an integrated circuit (IC) with an arbitrarily large number of output channels, a local-dimming system will consist of multiple ICs. As an example daisy-chain capability of the system interface will be advantageous. One such interface is the serial peripheral interface (SPI), chosen for this application for its high-speed capability. With multiple LED Drivers in the module, a cyclic redundancy check (CRC) code can be used to ensure error-free communication. In this paper, we will concentrate on a local-dimming implementation with daisy-chaining for a large number of LED zones and show the effect with and without local dimming. Fully digital instrumental clusters are gaining popularity, so that functionally-safe displays are essential to guarantee driver and passenger safety as well as the safety of other road users. Clearly a malfunctioning backlight driver could make the screen completely dark and constitute a safety hazard. The relevant features of the LED driver which help in attaining ASIL B include: - Detection of open or shorted LED strings - Redundant reference to detect over-voltage or under-voltage - Thermal warning and shutdown In this paper, we will give a detailed implementation of the diagnostics and their communication via SPI to enable ASIL B.
--- Date: 27.02.2019 Time: 15:55 - 16:15 Location: Conference Counter NCC Ost