LD1117S18CTR

The LD1117S18CTR from STMicroelectronics is a low-dropout (LDO) positive voltage regulator providing a fixed 1.8V output at up to 800mA in a compact SOT-223 surface-mount package. It is part of the widely-used LD1117 family, which offers fixed output voltages of 1.2V, 1.8V, 2.5V, 2.85V, 3.0V, 3.3V, and 5.0V, as well as an adjustable version (VREF = 1.25V).

The LD1117 uses an NPN pass transistor architecture, which is a critical differentiator from many modern LDOs that use PNP pass transistors. In an NPN-based regulator, the quiescent current flows mostly into the load rather than being wasted as ground current. This means the LD1117 is particularly efficient in applications where the load draws significant current, as the quiescent current contributes to the load rather than being dissipated as heat. The trade-off is a slightly higher dropout voltage (1.0V typical) compared to PNP-based LDOs, which can achieve dropout voltages below 0.5V.

The 1.8V output voltage makes the LD1117S18CTR suitable for powering low-voltage digital ICs such as FPGA core voltages, DSP core supplies, and DDR memory termination voltages. The plus/minus 1% output voltage tolerance (C suffix) ensures that the supply voltage stays within the tight tolerances required by modern digital circuits.

One of the key advantages of the LD1117 family is its simplicity of use. Only a single 10uF output capacitor is required for stability, and no external resistors or compensation components are needed for the fixed-voltage versions. This minimizes the bill of materials and board space. The internal current limiting and thermal shutdown protect the device and the load from fault conditions.

The SOT-223 package is optimized for surface-mount applications, providing a good balance between small footprint and thermal performance. The tab (large rear pin) is electrically connected to VOUT and serves as both an electrical connection and a heat dissipation path to the PCB copper. Proper PCB layout with a copper pad under the tab is essential for achieving the maximum current rating.

The LD1117S18CTR is the tape-and-reel, plus/minus 1% tolerance version. The LD1117S18TR is the tape-and-reel standard tolerance version. The LD1117DT18CTR is the DPAK package version with the same electrical specifications. All LD1117 variants in different packages (SOT-223, DPAK, SO-8, TO-220) share the same die and electrical characteristics, differing only in thermal performance and footprint.

The LD1117S18CTR operates as a series-pass linear voltage regulator using an NPN pass transistor with a feedback control loop.

NPN Pass Transistor Architecture: The LD1117 uses a Darlington-connected NPN pass transistor between VIN and VOUT. The NPN architecture has a fundamental advantage over PNP-based LDOs: the quiescent current (ground current) flows through the pass transistor into the load rather than being shunted to ground. This means the ground pin current is relatively independent of the load current, and the total input current approximately equals the output current plus the small base drive current. The NPN architecture also provides inherent stability without the pole associated with the PNP output stage.

The trade-off of the NPN architecture is the dropout voltage. For the NPN pass transistor to remain in the active region, the base voltage must be approximately 1.5-2V above the emitter (VOUT), and the base drive circuitry requires additional headroom. This results in a typical dropout voltage of 1.0V at 800mA, compared to 0.3-0.5V for PNP-based LDOs. The minimum input voltage for 1.8V output at full load is therefore approximately 2.8V.

Voltage Reference and Feedback: An internal bandgap voltage reference provides a stable reference voltage. For the fixed 1.8V version, an internal resistor divider scales the output voltage down to the reference voltage. The error amplifier compares the scaled output voltage with the reference and drives the base of the pass transistor to maintain the output at 1.8V. The output voltage is trimmed during manufacturing (on-chip trimming) to achieve plus/minus 1% accuracy at 25C.

Dropout Operation: When the input voltage drops below the minimum required for regulation (VOUT + VDO), the pass transistor saturates and the device enters dropout mode. In dropout, the output voltage tracks the input voltage minus the saturation voltage of the pass transistor. The dropout voltage varies with load current and temperature, increasing at higher currents and lower temperatures.

Stability: The LD1117 requires a minimum 10uF output capacitor for stability. The capacitor provides the dominant pole in the feedback loop. Larger capacitance values (up to 100uF) improve transient response and output noise. Both ceramic and electrolytic capacitors can be used, but the ESR of the capacitor must be within the stable range specified in the datasheet. Very low-ESR ceramic capacitors (below 0.1 ohm) may cause instability in some cases.

Current Limiting: An internal current sense circuit monitors the pass transistor current. When the current exceeds the limit (typically 800mA-1.3A), the drive to the pass transistor is reduced, limiting the output current. The current limit protects the device from damage during output short-circuit conditions.

Thermal Shutdown: A temperature sensor on the die monitors the junction temperature. When TJ exceeds approximately 150C, the pass transistor is turned off, preventing thermal runaway. The thermal shutdown has hysteresis to prevent oscillation. The device restarts when the junction temperature drops below the hysteresis point.