SNLA457 May   2024 TLC6C5748-Q1 , TPS552892-Q1

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1Introduction
  5. 2Typical Local Dimming System
  6. 3ASIL-B Design for LED Power
  7. 4Functional Safety Design for LED Driver
  8. 5Summary
  9. 6Reference

4 Functional Safety Design for LED Driver

For system-level ASIL-B requirement, vehicle manufacturers usually require that automotive display information is visible. For the local dimming LCD display backlight unit, the LED driver should ideally have the following features:

  • Communication interface for device setting and monitoring
  • Malfunction detection for each LED channel LED

The TLC6C5748-Q1 driver can meet this requirement. In Figure 4-1, you can see the display controller (TCON) can control and adjust TLC6C5748-Q1 each channel’s LED grayscale by SPI interface (SIN, SCLK, LAT, GSCLK) on time, while the TCON can also detect the TLC6C5748-Q1 driver’s channel LED malfunctions using the LED open detection (LOD) and LED short detection (LSD) functions through SOUT interface.

Local Dimming Typical Application Daisy-Chained TLC6C5748-Q1 Devices Figure 4-1 Local Dimming Typical Application Daisy-Chained TLC6C5748-Q1 Devices

The TLC6C5748-Q1 driver has an internal LED open and short detection circuit (LOD and LSD) and truth table is shown in Figure 4-2 and Table 4-1. LOD detects a fault caused by an LED open circuit or a short from OUTXn to ground with low resistance by comparing the OUTXn voltage to the LOD detection threshold voltage (0.3V, typically). LSD data detects a fault caused by a shorted LED between LED terminals by comparing the OUTXn voltage to the LSD detection threshold voltage level set by internal register LSDVLT in device.

TLC6C5748-Q1 LOD and LSD Circuit Figure 4-2 TLC6C5748-Q1 LOD and LSD Circuit
Table 4-1 TLC6C5748-Q1 LOD and LSD Truth Table
SID Data Condition
LOD LSD
0 LED is not opened (VOUTXn > VLOD) LED is not shorted (VOUTXn > VLOD)
1 LED is open or shorted to GND (VOUTXn > VLOD) LED is shorted between anode and cathode, or shorted to higher voltage side (VOUTXn > VLOD)

Detailed safety mechanism as below:

  • For LOD, if the OUTXn voltage is lower than the threshold voltage when OUTXn is on, then output LOD bit is set to 1 to indicate an open LED. TCON gets this information from the SPI interface (SOUT) to perform the corresponding safety action (dependent on safety goal requirement). Usually, if a certain percentage of LEDs LOD happens, the TCON stops sending to the GSCLK and alerts the MCU to cut off VCC and VLED, close display backlight and let display into a black screen safe state.
  • For LSD, if the OUTXn voltage is higher than the programmed threshold when OUTXn is on, then the corresponding output LSD bit is set to 1 to indicate a shorted LED. TCON gets this information from the SPI interface (SOUT) to perform the corresponding safety action (dependent on safety goal requirement). Usually, if a certain percentage of LEDs LSD happens, the TCON stops sending to the GSCLK and alerts the MCU to cut off VCC and VLED, close display backlight and let display into a black screen safe state.