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

3 ASIL-B Design for LED Power

For local dimming LCD applications, LED cathodes are connected to LED drivers and LED anodes are connected to LED power, as shown in Figure 2-1. For LED power, the input typically connects directly to the car battery. When you enter your car, the display is already turned on to welcome you. When you start the engine in cold weather, VIN may go down to 3V during cold crank, as shown in Figure 3-1. However, the display still needs to be on or not blinking during cold crank to give the best user experience. You can put a pre-boost in front of the LED power, however, this can increase cost and lower efficiency. TI’s buck-boost TPS552892-Q1 converter can handle as low as 3VIN after starting up with no external pre-boost or extra power supply.

Typical Cold Crank Profile Figure 3-1 Typical Cold Crank Profile

The TPS552892-Q1 is a synchronous buck-boost converter that is optimized for converting battery voltage or adapter voltage into power supply rails. The TPS552892-Q1 converter integrates four MOSFET switches that can operate from 3.0V to 36V wide input voltage. For a 6V LED power application, the TPS552892-Q1 converter can deliver up to 36W from a 12VIN.

TPS552892-Q1 Typical Application Circuit Figure 3-2 TPS552892-Q1 Typical Application Circuit

How can you design functional safety for LED power? Per the ISO26262-5 D.2.6 description, noted in Table 3-1, there are two methods to implement a diagnostic safety mechanism for power supply. The first method is to detect voltage or current from the input side. In this method, the diagnostic coverage is low (60%). The other method is to detect the voltage or current from the output side, where the diagnostic coverage is high (99%).

Table 3-1 Power Supply Safety Mechanism From ISO26262
Safety Mechanism/Measure See Overview of Techniques Typical Diagnostic Considered Achievable Notes
Voltage or current control (input) D.2.6.1 (1) Low –
Voltage or current control (output) D.2.6.2 (2) High –
(1) Aim: To detect as soon as possible wrong behavior of input current or voltage values.
Description: Monitoring of output voltage or current.
(2) Aim: To detect as soon as possible wrong behavior of input current or voltage values.
Description: Monitoring of output voltage or current.

For LED power, the TPS552892-Q1 converter system design, which is compliant with ASIL-B design, has two common output detection methods:

  • Using a safety MCU ADC pin to monitor the TPS552892-Q1 output.

    The MCU ADC pin is independent to the TPS552892-Q1 converter output. This means that the diagnostic coverage is relatively high, and it will not add additional BOM cost. From Figure 3-3, you can see the TPS552892-Q1 VOUT pin connected to the MCU ADC pin for output voltage monitoring and a CDC pin connected to a MCU ADC pin for output current monitoring. The TPS552892-Q1 converter senses the output current with ISP/ISN pin and CDC pin voltage = 20 × (VISP-VISN), IOUT= (VISP-VISN)/ R4. The MCU could monitor the CDC pin voltage with ADC to then calculate the buck-boost IOUT. The PG (power good) pin connects to the MCU GPIO pin for power monitoring. The PG acts as an auxiliary safety mechanism can help improve diagnostic coverage. If a TPS552892-Q1 converter over voltage (OV)or under voltage (UV) fault happens, the MCU ADC pin will detect this fault and then cut off (pulling EN pin to low) the TPS552892-Q1 converter output by MCU GPIO, enabling the display to achieve a black screen safe state.

    TPS552892-Q1 Output Detection by MCU Mechanism Figure 3-3 TPS552892-Q1 Output Detection by MCU Mechanism
  • Using external power supervisor to monitor TPS552892-Q1 output.

    If the display MCU does not have sufficient ADC pins, you can use an external power supervisor to monitor the TPS552892-Q1 converter output OV and UV. The supervisor is independent to the power output, so there is no common cause failure and high performance and accurate detection means that the diagnostic coverage is high.

TI has a variety of voltage supervisors, from monitoring 1 channel to multi channels and supporting input voltage from 5.5V to 65V. For LED power, the VLED is equal to VF (LED forward voltage) and VH (LED Driver headroom voltage), with the voltage usually around 6V. In this paper the TPS37A-Q1 high input voltage supervisor is used as an example to achieve voltage monitoring, as shown in Figure 3-4.

TPS552892-Q1 Output Detection by supervisor TPS37A-Q1 Figure 3-4 TPS552892-Q1 Output Detection by supervisor TPS37A-Q1

TPS552892-Q1: Input 3V~36V, output 6.0V

TPS37A: Acts as safety mechanism to monitor TPS552892-Q1(VLED) power output. If a TPS552892-Q1 OV (overvoltage > 6.5V) or UV (undervoltage < 5.8V) fault happens, the TPS37A-Q1 detects this using the SENSE1(OV) or the SENSE2(UV) pin and send an alert to the MCU through RESET1(OV) or RESET2(UV) signals. The MCU responds to this signal and performs the corresponding safety action (action depends on the safety goal requirement). For example, to cut off buck-boost output, a black screen is allowed and considered as a safe state.