Fixed 1.2V rail — what the 1.3V dropout ceiling means for your rail budget
The LD1117ADT12TR is a fixed 1.2V output LDO regulator from STMicroelectronics, delivering up to 1A continuous current. The 1.3V maximum dropout at full load means the input rail must stay above 2.5V to maintain regulation — a common gotcha when the input is a 3.3V bus that sags under transient load. PSRR is 80dB at 120Hz, which makes this part a strong candidate for cleaning up rectified mains ripple in audio or precision analog supplies. The 10 mA quiescent current is typical for a 1A LDO of this generation — not a low-Iq part, so not ideal for battery-powered always-on rails.
DPAK package and thermal design-in
The tab is the ground (VSS) connection and the primary heat path — the board copper area under the tab sets the junction-to-ambient thermal resistance. For 1A continuous at 1.3V dropout (1.3W dissipation), a 1-inch-square copper pad on a 2-layer board keeps the junction below the 125°C maximum operating temperature. If the board sees ambient below freezing or above 85°C, this part is out of spec and a -40°C rated sibling (like the LD1117ADT12TR's industrial-temp variant) would be needed.
Protection features and output configuration
Built-in over-current and over-temperature protection are on-chip. The output is fixed positive 1.2V — no external resistor divider needed, but also no adjustability. Input voltage maximum is 15V, so the part can run from 5V, 12V, or unregulated rectified rails as long as the dropout headroom is met.