Implementing Analog Thermal Foldback with the LP8867-Q1 and TMP61-Q1
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1 Introduction
The LED backlight driver is a key part of this LCD panel system and some engineers may encounter various problems when choosing the right devices and designing such circuits. These LED drivers regulate the output current to the LED’s on the panel. In automotive environments the LED’s on the display panel can get very hot. Temperature sensors are often used to monitor the LED temperature and provide brightness regulation. Additionally, temperature sensors are also used to monitor the LED Driver thermals. With the remote thermistor embedded in the backlight enclosure/panel there are a couple ways to achieve thermal sensing and foldback with an LED Driver. The integrated thermistor can be used with the foldback pin of an LED Driver or provide thermal foldback in a discrete implementation to the LED Driver. This application report shows how to implement TI’s TMP6 family of thermistors with TI’s LP886x family of low EMI Automotive LED Drivers, specifically the LP8867-Q1.
2 Conventional Backlighting vs. Local-Dimming backlighting
There are two types of technologies for controlling the dimming and brightness of an LCD screen, conventional and local dimming. In conventional backlighting black is reproduced as dark gray and details are lost in the shadows. With local dimming techniques, backlighting is optimized for specific areas of the image, resulting in deeper blacks. A local-dimming backlight technology is a direct-lit architecture where the LEDs are directly behind the LCD panel as shown in Figure 2-1.
Figure 2-1 LCD Panel ArchitectureIn local dimming backlight, the LCD panel is divided into many small “zones”. The brightness of each zone is adjusted according to different display content. A local dimming backlight allows the display to have “darker blacks” and “brighter whites”.
Figure 2-2 Local Dimming vs. Conventional
Backlight zonesTo optimize the control of the brightness of these small zones, temperature feedback is critical to provide to the system. The LP8869-Q1 has two pins and that can be used in concert with thermistor divider circuits to provide thermal information to the system and allowing for a thermal foldback profile for the LED’s, limiting the amount of current after a certain temperature threshold thus protecting them.
