STMicroelectronics LD2981CU18TR

Overview

LD2981CU18TR from STMicroelectronics is a 1.8 V fixed-output ultra-low-dropout linear voltage regulator in SOT23-5L package, designed for battery-powered and low-noise systems. It delivers up to 50 mA output current with 7 mV typical dropout at 1 mA load, 80 µA typical quiescent current in active mode, and <1 µA shutdown current. Its stable operation with low-ESR ceramic capacitors enables use in cellular phones and portable medical sensors.

For engineers reviewing the LD2981CU18TR datasheet, LD2981CU18TR pinout, LD2981CU18TR application, or LD2981CU18TR equivalent, key selection criteria include dropout voltage at light load, inhibit logic thresholds, output noise (160 µVRMS), thermal shutdown behavior (170 °C), and compatibility with 1 µF ceramic output capacitors.

Technical Context

The LD2981CU18TR employs a PNP pass transistor architecture enabling ultra-low dropout-just 7 mV at 1 mA-and supports fast transient response to load steps (e.g., 1–100 mA) with minimal output voltage deviation. Its inhibit input (Pin 3) accepts TTL-compatible logic levels (VIH ≥ 1.3 V, VIL ≤ 0.18 V) and draws ≤15 µA at 5 V, enabling precise system-level power gating.

It integrates internal current limiting and thermal shutdown (170 °C), operates across –40 to +125 °C junction temperature, and achieves 63 dB supply voltage rejection at 1 kHz with a 10 µF output capacitor. The device requires no external compensation and remains stable with 1 µF X7R ceramic output capacitors for 1.8 V output.

Key Specifications

Pinout & Package

SOT23-5L is a 5-pin, surface-mount plastic package (2.95 × 1.60 mm footprint) with exposed thermal pad (not electrically connected), optimized for high thermal resistance efficiency (R_thJA = 255 °C/W) in space-constrained PCB layouts.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar low-dropout regulator applications.

Compared with AP2210K-18W5 and MCP1700T-1802E/TT, LD2981CU18TR uniquely combines sub-10 mV dropout at light load, integrated TTL-compatible inhibit, and 1.8 V ±0.5% accuracy-making it optimal for precision, power-gated, battery-sensitive designs.

Availability

LD2981CU18TR is available at Aetrix Electronics and suitable for wearable health monitors, industrial sensor nodes, smartphone camera modules, and automotive body control units requiring stable component supply across extended temperature ranges and long production lifecycles.

Supply support for LD2981CU18TR includes scheduled delivery planning, volume procurement assistance, BOM continuity management, traceable sourcing, and lifecycle availability coordination for OEM customers, industrial embedded developers, connected-device designers, and electronics production programs.

Manufacturer

STMicroelectronics is a global semiconductor leader headquartered in Geneva, Switzerland, designing and manufacturing microcontrollers, power management ICs, sensors, and analog chips for industrial, automotive, and consumer markets.

The LD2980 series targets ultra-low-power, low-noise local regulation in portable and harsh-environment electronics-emphasizing dropout minimization, capacitor flexibility, and robust thermal behavior over cost-driven general-purpose LDO trade-offs.

FAQ

What is the minimum input voltage required for regulation at full 50 mA load?

The LD2981CU18TR requires VIN ≥ VO + VDROP. At 50 mA, typical dropout is 120–150 mV, so minimum input is 1.8 V + 0.12 V = 1.92 V. Absolute maximum dropout is 225 mV over temperature, so design margin requires VIN ≥ 2.025 V for guaranteed regulation across –40 to +125 °C.

Can the INHIBIT pin be left unconnected?

No. The INHIBIT pin has no internal pull-up or pull-down. Leaving it floating risks undefined output state and potential oscillation. If not used, tie it directly to VIN to ensure permanent enable; if using shutdown, drive it actively to ≤0.18 V for OFF or ≥1.3 V for ON per datasheet thresholds.

Is an input capacitor mandatory, and what type is recommended?

Yes-a minimum 1 µF ceramic capacitor is required at VIN, placed within 12.7 mm of Pin 1 and returned to clean analog ground. X7R or C0G dielectrics are preferred; tantalum or film types are acceptable but offer no advantage over low-ESR ceramics for this application.

How does reverse current behave when VOUT exceeds VIN?

Unlike NMOS-based LDOs, the LD2981CU18TR's PNP pass element lacks a body diode. When VOUT > VIN, no significant reverse current flows through the pass transistor. Measured reverse current is ~160 µA, sourced through internal feedback resistors to GND-well below levels that would disrupt upstream supplies or cause latch-up.