STMicroelectronics LD2981ABM50TR

Overview

LD2981ABM50TR from STMicroelectronics is a 100 mA fixed-output low-dropout linear voltage regulator with 5.0 V output, ultra-low dropout (180 mV typ. at 100 mA), 80 µA typical quiescent current in active mode, and <1 µA shutdown current. It features TTL-compatible inhibit control on pin 3, ±0.75% output voltage tolerance, and 160 µV_RMS output noise - designed for battery-powered portable electronics requiring stable, low-noise local regulation.

For engineers reviewing the LD2981ABM50TR datasheet, LD2981ABM50TR pinout, LD2981ABM50TR application, or LD2981ABM50TR equivalent, key selection criteria include dropout behavior under 100 mA load, inhibit logic thresholds (V_IL = 0.18 V, V_IH = 1.6 V), thermal shutdown at 170 °C, SOT23-5L package compatibility, and ceramic capacitor stability requirements (min. 3.3 µF, low ESR).

Technical Context

The LD2981ABM50TR employs a PNP pass transistor architecture enabling ultra-low dropout voltage - critical for operation with input voltages as low as 5.18 V while maintaining 5.0 V output at full 100 mA load. Its internal feedback network uses trimmed thin-film resistors (R1 = 42.6 kΩ, R2 = 51.15 kΩ) to achieve ±0.75% initial output tolerance and stable line/load regulation (0.032 %/V max over temperature).

Shutdown is implemented via a dedicated INHIBIT pin (pin 3) with no internal pull-up; device disables when pulled ≤0.18 V, drawing <1 µA. The regulator remains stable with ceramic output capacitors ≥3.3 µF and ESR < 1 Ω, and includes internal current limiting (150 mA short-circuit current) and thermal shutdown (170 °C junction).

Key Specifications

Pinout & Package

SOT23-5L package: 5-pin surface-mount, 1.3 mm × 2.9 mm footprint, 1.45 mm height, with exposed thermal pad (not electrically connected). Designed for high-density portable PCB layouts and efficient heat dissipation via soldered pad.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar fixed-output LDO regulator applications.

Compared with MCP1700T-5002E/TT and TLS850B2TE/V33, the LD2981ABM50TR uniquely balances ultra-low dropout (180 mV), sub-µA shutdown, and precise ±0.75% tolerance in a compact SOT23-5L package - making it optimal for space-constrained, battery-sensitive consumer and medical devices where discrete enable control is required.

Availability

LD2981ABM50TR is available at Aetrix Electronics and suitable for cellular phone power management, portable medical sensor modules, and laptop subsystem regulation requiring stable component supply across production lifecycles.

Supply support for LD2981ABM50TR 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 components for industrial, automotive, and consumer markets.

The LD2981 series belongs to ST's ultra-low-dropout linear regulator product line, engineered specifically for energy-efficient portable electronics where minimal voltage overhead, low quiescent current, and tight output tolerance are critical.

FAQ

What is the minimum input voltage required for stable 5.0 V output at 100 mA load?

The LD2981ABM50TR requires a minimum input voltage of 5.18 V (5.0 V + 180 mV dropout) to maintain regulation at 100 mA load across −40 to 125 °C. At room temperature, typical dropout is 180 mV; maximum specified dropout at full load and temperature is 375 mV - so 5.375 V ensures margin. Input below 5.18 V causes output to droop linearly with input.

Can the INHIBIT pin be left unconnected or tied to VOUT?

No - the INHIBIT pin has no internal pull-up and must never be left floating. Leaving it unconnected risks erratic enable/disable behavior due to noise coupling. If shutdown control is unused, tie INHIBIT directly to VIN to ensure continuous operation. Tying to VOUT is unsafe: VOUT may be absent during startup or fault, causing unintended disable.

Is an output capacitor mandatory, and what happens if its ESR exceeds 1 Ω?

Yes - a minimum 3.3 µF ceramic output capacitor with ESR < 1 Ω is mandatory for stability. Exceeding 1 Ω ESR risks oscillation, especially at light loads or temperature extremes, as confirmed by Figures 18–19 in the datasheet. Instability manifests as output voltage ringing (>100 kHz) or sustained overshoot during load transients - potentially damaging downstream logic.

Does the LD2981ABM50TR support reverse current protection when VOUT > VIN?

Yes - the LD2981ABM50TR uses a PNP pass transistor without an inherent body diode, providing intrinsic reverse-current blocking. When VOUT exceeds VIN, only leakage flows through internal feedback resistors (typ. 160 µA), not through the pass element. This prevents backfeeding into upstream supplies - critical in multi-rail systems with independent power domains.