Depth sensing technology has developed rapidly recently due to its broad applications. Time-of-Flight (ToF) is a popular technology in depth sensing because of its advantages in high measuring accuracy and low complexity in image processing. ToF technologies are divided into two kinds: indirect time-of-flight (iToF) and direct Time-of-Flight (dToF). Compared to the iToF, dToF can detect targets at a longer distance by directly calculating the flight time of the laser. To obtain time information, the dToF system requires a Time-to-Digital Converter (TDC). This thesis proposes a system-level model to optimize the TDC parameters for a dToF system used in an autofocus system of a mobile phone. According to the simulation result of the system modeling, a ring-based TDC is chosen and built using a shared and duty-cycled Voltage-Controlled Oscillator (VCO). To decrease the DNL of the TDC, chopping comparators are used in this design. In addition, to save power and area, a new method called double sampling is used to align the coarse and fine phases of the ring based TDC. The proposed TDC achieves a resolution of 100 ps and a DNL of −0.27/+0.37 and an average power of 0.179 mW/TDC, which are smaller than that of state-of-the-art TDCs used in dToF systems. SMIC 55BSC technology is used to design and tape out the TDC.