Optimization of transistor size and operating point for the LVDS driver of the ALICE ITS pixel chip

The ALICE Inner Tracker System (ITS) will be upgraded during Long Shutdown 2. The tracker layers will be equipped with monolithic pixel sensors chips. A Low Voltage Differential Signalling (LVDS) driver is required for the off chip data transmission. A current mode 1.2 Gb/s LVDS driver based on H-bridge scheme has already been implemented and tested. Although the present driver meets the specifications, a decrease of its power consumption is beneficial for the reduction of the material required for the detector powering and cooling. This report presents the study of a current mode LVDS driver based on H-bridge scheme where the switches are replaced with current sources that can deliver either ON level or OFF level currents. The ON current is the main static power contributor, and its value is set to 4 mA by specifications to have a differential signal of 400 mV over the 100 Ω termination resistor. The second contributor for the static power is the OFF power, which has to be optimized together with the dynamic power. Detailed simulations of the transistor current-voltage characteristics for different transistor sizes are done to find the minimum for the sum of the dynamic and OFF power consumption. From this study transistor sizes and operating voltages are defined for the proposed LVDS driver design.