STMicroelectronics LD2981CU50TR

Overview

LD2981CU50TR 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, internal thermal/current limiting, and operates across –40 to +125 °C for use in battery-powered portable electronics requiring stable low-noise power.

For engineers reviewing the LD2981CU50TR datasheet, LD2981CU50TR pinout, LD2981CU50TR application, or LD2981CU50TR equivalent, key selection criteria include dropout voltage at 100 mA load, inhibit logic thresholds (0.18 V low / 1.6 V high), output noise (160 µVRMS), thermal shutdown (170 °C), and compatibility with low-ESR ceramic capacitors (min. 2.2 µF).

Technical Context

The LD2981CU50TR uses a PNP pass transistor architecture enabling ultra-low dropout and minimal quiescent current across load and temperature. Its inhibit input directly controls the bias network of the internal reference and error amplifier, enabling full shutdown with sub-µA leakage.

Stability is optimized for ceramic output capacitors ≥2.2 µF with ESR <1 Ω; the regulator's loop compensation is internally fixed and not user-adjustable. Reverse-current protection is inherent due to absence of body diode between VIN and VOUT.

Key Specifications

Pinout & Package

LD2981CU50TR is packaged in SOT-23-5L - a 5-pin surface-mount package with exposed pad for thermal dissipation (R_thJA = 255 °C/W). Pin 4 is NC (not connected); pin 3 is INHIBIT with no internal pull-up - must be actively driven.

Key Features

Applications

Equivalent & Alternatives

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

Compared with MCP1700T-5002E/TT and TLC7705IPK, LD2981CU50TR uniquely combines sub-µA shutdown, reverse-current blocking, and 100 mA capability in SOT-23-5L - making it optimal for space-constrained, battery-critical, and multi-rail sequenced systems.

Availability

LD2981CU50TR is available at Aetrix Electronics and suitable for portable medical sensors, industrial PLC I/O modules, wearable fitness trackers, and automotive cabin infotainment peripherals requiring stable component supply with guaranteed long-term manufacturability.

Supply support for LD2981CU50TR 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, analog chips, and MEMS sensors for industrial, automotive, and consumer markets.

LD2981CU50TR belongs to ST's ultra-low-quiescent-current LDO family, engineered specifically for energy-constrained portable and battery-backed systems where micropower operation and precise voltage regulation are mandatory.

FAQ

What is the minimum input voltage required for LD2981CU50TR to maintain 5.0 V output at 100 mA?

The typical dropout voltage is 180 mV at 100 mA, so the minimum input voltage is 5.0 V + 0.18 V = 5.18 V. At worst-case (375 mV per datasheet), it rises to 5.375 V. Input must remain ≥2.5 V per absolute maximum rating, but regulation fails below dropout threshold.

Can LD2981CU50TR be used without connecting the INHIBIT pin?

No - the INHIBIT pin has no internal pull-up and must not be left floating. For always-on operation, tie it directly to VIN. Leaving it unconnected risks unpredictable enable/disable behavior and potential oscillation due to noise coupling.

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 the pin and returned to clean analog ground. Tantalum or film capacitors are acceptable, but ceramic offers lowest ESR and best high-frequency decoupling for transient immunity.

Does LD2981CU50TR require a specific output capacitor ESR range for stability?

Yes - stability requires output capacitance ≥2.2 µF with ESR <1 Ω when using X5R/X7R ceramics. The datasheet provides ESR vs. capacitance curves (Figures 18–19); exceeding the upper ESR limit causes oscillation, while too-low ESR may degrade phase margin in some layouts.