Meeting ASIL Requirements for LIDAR Systems Using Remote Temperature Sensors
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1 Introduction
LIDAR systems contain several critical components such as the laser arrays, time-of-flight sensors, and an MCU or processor. Monitoring the temperature of these components is essential for proper operation of the system. For example, LIDAR designs typically have two or more laser arrays and at temperatures above 70°C, the performance of each laser array will vary and require compensation. The time-of-flight sensor relies on optics, which has a different focus as temperatures change, making temperature monitoring crucial. Lastly, processors often become temperature sensitive around 105°C or higher. At high temperatures, designers may choose to lower the clock rate of the processor or shut it down to prevent overheating. Monitoring the temperatures of these components enhances safety and reliability and helps meet the ASIL requirements of LIDAR systems.
ASILs are established in ISO 26262 and specify the applicable requirements and safety measures needed to avoid unreasonable residual risk. The four ASIL levels range from A to D, with A representing the least stringent level and D the most stringent level. Depending on the specific function of the system within the vehicle, LIDAR systems can require up to an ASIL-D grade. Because temperature sensors used in these applications are often single-function devices, these safety standards are met using redundant and diverse temperature sensors. Redundancy means there are at least two temperature sensors where monitoring is needed, and the devices at each location may also connect to separate communication buses. Diversity ensures that the temperature sensors are as different as possible for safety reasons, and it can be introduced into the design in many ways (for example, differences in fabrication, packaging, die, common mode faults, and so forth). TI offers several options to achieve various levels of diversity depending on the requirements of the design.
There are different methods for meeting these temperature monitoring challenges, two of which are discussed in this application report. Option 1 is to use two thermistors or analog temperature sensor ICs at each location where temperature monitoring is needed. Option 2 is to use remote temperature sensors, which incorporate a local temperature sensor and remote channels for monitoring the temperature of another location. As is discussed, option 2 optimizes this design by using TI’s automotive remote temperature sensors to monitor several temperatures with high accuracy and fewer components.
2 LIDAR Temperature Sensing Solution Using Thermistors or Analog Temperature Sensor ICs
Thermistors are commonly used to meet the temperature monitoring requirements of LIDAR systems. Designers can use two TI thermistors—for example, one TMP61-Q1 and one TMP63-Q1—at each location where temperature monitoring is needed to satisfy redundancy, as shown in Figure 2-1.
Redundancy is achieved by having two temperature sensors monitoring the time-of-flight sensor, two monitoring the MCU or processor, and two monitoring the left and right laser modules. The sensors at each location connect to channels of the ADCs which are compared by the MCU or processor. Depending on system architecture, the thermistors may connect to the same ADC, separate ADCs, or both ADCs.
The TMP61-Q1 and the TMP63-Q1 are linear thermistors with different R25 values of 10 kΩ and 100 kΩ, respectively, which contribute to the diversity of the design. Using different package types of the TMP61-Q1 and the TMP63-Q1 can also add to the diversity of the sensors. The TMP61-Q1 Functional Safety FIT Rate and FMD application report shows the TMP61-Q1 has a FIT rate of 3 and failure mode of "open", which increases the safety of the system and makes it easier to detect failures.
Although a design using only thermistors is inexpensive, some disadvantages include the use of more external circuitry to bias the device and the inability to use any integrated thermal transistors that may be in the components of the LIDAR system. Additional work is also required to achieve high accuracy because thermistors are discrete devices and the total temperature sensing accuracy depends on the tolerances and PPM error of the other components in the circuit. However, if a designer still prefers a low-cost analog temperature sensing solution and wants more diversity, another option is to use one TMP61-Q1 and an integrated analog temperature sensor like TI’s TMP235-Q1, as shown in Figure 2-2. Analog temperature ICs provide the reduction of external components, assuring accuracy to the data sheet specification without calibration, and an integrated output driver. Additional information is available in the TMP23x-Q1 Functional Safety FIT Rate and FMD application report.
3 LIDAR Temperature Sensing Solution Using Remote Temperature Sensors
TI’s digital remote temperature sensors measure the local temperature of the device and sense the junction temperature of either an NPN or PNP bipolar junction transistor (BJT). The BJT can be an integrated transistor in an MCU, GPU, ASIC, FPGA, or a discrete transistor. Figure 3-1 shows a typical application of TI’s TMP451-Q1 automotive remote temperature sensor.
Figure 3-1 Typical Application of
TMP451-Q1 Automotive Remote Temperature SensorThis device and TI’s other remote sensors are ideal for multi-location, high-accuracy temperature measurements. Using remote temperature sensors is particularly beneficial in LIDAR applications because the MCU or processor, time-of-flight sensor, and other components may have built-in thermal transistors or diodes that can be used to measure their temperature. Remote channels of the temperature sensors allow the use of these built-in transistors, which prevent the need for another external sensor and have a better response time because they measure the die temperature directly.
For components that do not have integrated thermal transistors, the local temperature sensor or the remote channel with a discrete thermal transistor can still be used for temperature monitoring. When using the remote channel of the temperature sensor with an integrated or discrete thermal transistor, it is important to ensure that the sensor is optimized for the most accurate temperature readings. For more information on optimizing remote temperature sensors, see the Optimizing Remote Temperature Sensor Design application report. Like the TMP61-Q1, TI also provides the TMP451-Q1 Functional Safety FIT Rate and FMD functional safety information to aid in system-level functional safety certification.
