Compact Macro-Cell With OR Pulse Combining for Low Power Digital-SiPM

High-density digital-Silicon Photomultipliers call for high-performance Single Photon Avalanche Diodes (SPAD) front-ends. Power consumption and fill factor are significant concerns in this kind of sensors. This paper presents a compact and power-efficient macro-cell where several SPADs share the active recharge circuitry for increased fill factor. Integrated with 110nm technology for image sensors, the array of macro-cells has 30% fill factor. Also, following the first firing of any SPAD during the same macro-pixel dead-time, the other SPADs are disabled for power saving. Of course, subsequent triggers are lost. However, they would have been masked by the OR pulse combining scheme. Besides this event-driven disabling feature, the macro-cell includes circuitry to disable noisy devices - similar to other SiPM cells. Also, the macro-cell features control of the dead-time. This paper describes the macro-cell concept, its associated analysis and design equations. Key parameters of the design are discussed to optimize power consumption. Design scalability is contemplated as well. Experimental results proved that the power efficiency of the proposed scheme depends on the illumination power. Also, power efficiency is linked to the pulse overlapping probability. For example, power saving up to 30% is obtained with 4 sub-cells per macro-cell, when pulse overlapping is about 11% for correlated light or the pulse rate per sub-cell is about 100kHz for uncorrelated light.