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.