The TPS56121 devices are designed to operate from an input voltage supply between 4.5 V and 14 V. This supply must be well regulated. These devices are not designed for split-rail operation. The VIN and VDD terminals must be the same potential for accurate high-side short circuit protection. Proper bypassing of input supplies and internal regulators is also critical for noise performance, as is PCB layout and grounding scheme. See the recommendations in Layout Guidelines.