A SPAD Macro-pixel Based D-TOF Ranging Sensor Design

Unmanned driving delegates the direction of future automobile industry.Lidar detects obstacles such as vehicles,roads and pedestrians around the vehicle,the location information would be provided to the unmanned system.Lidar will be one of the important systems of unmanned system for the high accuracy of Lidar detection and small influence of illumination.Time of Flight is a ranging method,which is basing on CMOS sensors.The sensor emits a laser pulse at a specific wavelength,and the laser will be reflected when it reaches the obstacle.TOF measures the distance between sensor and obstacle by measuring the time difference or phase difference of the laser pulse.Thus,the sensor obtain the depth information and the 3D contour of the object will be present in the form of depth image.Compared with phased TOF ranging,the characteristic of direct TOF ranging is that the return signal received by the sensor is a single photon signal,avoiding continuous echo crosstalk,and the measurement distance would not be limited by laser wavelength.Direct TOF ranging can achieve high-speed,lowpower,high-precision measurement for medium-to-long-range and low-echo energy targets,thus showing good prospects in automotive driving.This paper presents a SPAD macro-pixel based D-TOF ranging sensor design.Scheme is based on the uncertainty characteristic of photons in ambient light and the characteristics of single photon avalanche diodes.Multiple SPADs are used to form macro pixels,and macro pixels are used as the basic unit of laser detection to reduce the error caused by ambient light to SPAD.The chip uses a multi-phase phase-locked loop and a time-to-digital conversion circuit for flight time measurement with an accuracy of 200 picoseconds.To meet the requirement of automotive applications,the frame rate is set to 20 fps and the range of detecting is set to 200 m.This chip is designed and verified under the SMIC 0.18 μm process.