STMicroelectronics LD3985M27R

Overview

LD3985M27R from STMicroelectronics is a 2.7 V fixed-output, 150 mA ultra-low-dropout linear regulator in SOT23-5L package, featuring 60 mV typical dropout at full load, 30 µV_RMS output noise (10 Hz–100 kHz), and logic-controlled shutdown. It operates from 2.5 V to 6 V input, supports ceramic capacitors as low as 1 µF, and targets power-sensitive battery-operated systems such as mobile radios and wearable sensors.

For engineers reviewing the LD3985M27R datasheet, LD3985M27R pinout, LD3985M27R application, or LD3985M27R equivalent, key selection criteria include its 85 µA no-load quiescent current, 60 dB PSRR at 1 kHz, thermal shutdown at 160 °C, and compatibility with low-ESR ceramic output capacitors down to 5 mΩ.

Technical Context

The LD3985M27R uses BiCMOS process technology to achieve ultra-low dropout and sub-100 µA quiescent current across temperature. Its internal architecture integrates a precision bandgap reference, error amplifier, PMOS pass transistor, and independent bypass-capacitor path for noise filtering - enabling stable regulation even under fast load transients.

Shutdown is controlled via a dedicated VINH pin with defined logic thresholds (≤0.4 V = OFF, ≥1.2 V = ON), and the BYPASS pin connects to a 10 nF capacitor to suppress high-frequency noise on the reference node. Power supply rejection remains >50 dB up to 10 kHz, critical for RF subsystems powered from noisy shared rails.

Key Specifications

Pinout & Package

LD3985M27R is housed in a RoHS-compliant SOT23-5L package (2.95 mm × 1.60 mm × 1.30 mm), with exposed pad for thermal enhancement and standard tape-and-reel packaging per JEDEC J-STD-020.

Key Features

Applications

Equivalent & Alternatives

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

Compared with XC6206P272MR and AP2210K-2.7TRG1, LD3985M27R delivers superior noise performance and thermal safety at the expense of slightly higher quiescent current - making it optimal for medical and industrial edge devices where signal integrity and reliability are non-negotiable.

Availability

LD3985M27R is available at Aetrix Electronics and suitable for mobile radio front-ends, wearable sensor nodes, industrial IoT edge sensors, and medical patch monitors requiring stable component supply with guaranteed long-term availability.

Supply support for LD3985M27R 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 silicon solutions for automotive, industrial, and consumer markets since 1987.

The LD3985 series belongs to ST's high-efficiency, low-noise LDO portfolio targeting battery-powered portable electronics - engineered specifically for minimal dropout, ultra-low IQ, and robust transient response in space-constrained designs.

FAQ

What is the minimum output capacitor required for stable operation?

The LD3985M27R requires a minimum 1 µF ceramic output capacitor with ESR between 5 mΩ and 5000 mΩ. Stability is maintained across temperature and capacitor tolerance variations, including ±30% capacitance deviation. X7R dielectrics are recommended for consistent performance.

Can the BYPASS pin be left unconnected?

No - the BYPASS pin must be connected to a 10 nF capacitor referenced to GND. Leaving it open degrades PSRR by up to 20 dB and increases output noise by >2×. The capacitor provides AC grounding for the internal reference, directly impacting noise and ripple rejection performance.

Does LD3985M27R support reverse polarity protection?

No - the device lacks integrated reverse-voltage protection. Applying reversed input voltage may damage the internal PMOS pass transistor. External protection (e.g., series Schottky diode or ideal diode controller) is required if reverse polarity exposure is possible in the application.

What is the turn-on time and how is it measured?

Turn-on time is 200 µs typical, measured from VINH crossing 1.2 V (logic high threshold) to VO reaching 95% of 2.7 V. This timing assumes CO = 1 µF, CBYP = 10 nF, and IO = 1 mA. Load current and capacitor values directly affect observed startup behavior in actual circuits.