A Time Over Threshold (TOT) system has advantage over pulse height measurements on its high integrity and low power dissipation because of its binary readout and circuit simplicity. However the relation between TOT and input charge is strongly nonlinear and dynamic range is limited. We propose a new dynamic TOT system which converts the pulse height to pulse width with a dynamically changing threshold. This kind of TOT system can enable wider dynamic range and improves linearity since the threshold follows the input signal and even shorten the width of TOT pulse. We show the concept of dynamic TOT system and results with discrete circuits. It can improve the dynamic range and theoretically it is possible to desired relation between TOT and input charge by using dedicated threshold function. We also designed and fabricated 48channel dynamic TOT ASIC with 0.25um TSMC CMOS technology. ©2009 IEEE.