Serial powering is the baseline choice for low mass power distribution for the CMS and ATLAS HL-LHC pixel detectors. Two 2.0 A Shunt-LDO regulators are integrated in a prototype pixel chip implemented in 65-nm CMOS technology and used to provide constant supply voltages to its power domains from a constant input current. Performance results from testing prototype Shunt-LDO regulators are shown, including their behaviour after x-ray irradiation. The system level simulation studies, which had been performed with a detailed regulator design in a serially powered topology, have been validated.