STMicroelectronics LD3985M18R

Overview

LD3985M18R from STMicroelectronics is a 1.8 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, draws only 85 µA quiescent current at no load, and is optimized for battery-powered RF and sensor subsystems requiring clean, stable low-voltage bias.

For engineers reviewing the LD3985M18R datasheet, LD3985M18R pinout, LD3985M18R application, or LD3985M18R equivalent, key selection criteria include dropout voltage under light/heavy load, PSRR performance at 1–10 kHz, shutdown leakage (<1.5 µA), ceramic capacitor stability (1 µF min), and thermal shutdown threshold (160 °C).

Technical Context

The LD3985M18R uses a BiCMOS architecture to achieve ultra-low quiescent current while maintaining fast transient response and high power supply rejection-60 dB at 1 kHz and 50 dB at 10 kHz-without requiring external compensation. Its internal current and thermal limits enable robust operation in compact, thermally constrained layouts.

It integrates a dedicated BYPASS pin for 10 nF noise-filtering capacitor, enabling sub-30 µV_RMS noise performance; the VINH inhibit pin accepts TTL/CMOS-compatible logic thresholds (≤0.4 V OFF, ≥1.2 V ON) with <1 nA input current, supporting precise system-level power sequencing.

Key Specifications

Pinout & Package

LD3985M18R is housed in a RoHS-compliant, surface-mount SOT23-5L package (1.45 mm × 1.60 mm × 1.30 mm height), optimized for high-density PCB layouts and automated assembly. Thermal resistance is R_thJA = 255 °C/W (JEDEC standard board).

Key Features

Applications

Equivalent & Alternatives

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

Compared with XC6206P182MR and MIC5205-1.8YM5, the LD3985M18R delivers superior noise performance (30 µV_RMS) and PSRR (60 dB @ 1 kHz) due to its dedicated BYPASS pin and BiCMOS design-critical for RF and precision analog applications where noise coupling cannot be tolerated.

Availability

LD3985M18R is available at Aetrix Electronics and suitable for wireless sensor nodes, portable audio devices, industrial PLC I/O modules, and automotive body control units requiring stable component supply across extended temperature ranges and long production lifecycles.

Supply support for LD3985M18R 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.

The LD3985 belongs to ST's ultra-low-noise, low-quiescent-current LDO family-engineered specifically for battery-powered RF, sensor, and portable computing applications demanding high PSRR, fast transient response, and minimal standby power loss.

FAQ

What is the minimum output capacitor required for stable operation of LD3985M18R?

The LD3985M18R requires a minimum 1 µF ceramic output capacitor (X5R or X7R dielectric) with ESR between 5 mΩ and 5 Ω. Stability is maintained across temperature and voltage variations even with ±30% capacitance tolerance. Larger values (e.g., 2.2 µF) further improve transient response without compromising stability.

Can LD3985M18R be used with input voltages below 2.5 V?

No. The LD3985M18R is specified for input voltages from 2.5 V to 6 V. Operation below 2.5 V is outside absolute maximum ratings and may result in unregulated output or undefined behavior. Dropout voltage specification does not apply below 2.5 V per datasheet Section 5 footnote 3.

How does the BYPASS pin function, and what capacitor value is recommended?

Pin 4 (BYPASS) connects to a 10 nF ceramic capacitor to filter internal bandgap reference noise, directly reducing output noise to 30 µV_RMS. This capacitor must be placed close to the pin with short traces; using values significantly larger or smaller degrades PSRR and noise performance as characterized in Figures 18 and 21 of the datasheet.

Is LD3985M18R pin-compatible with other variants in the LD3985 family?

Yes. All LD3985 variants-including LD3985M122R, LD3985M25R, and LD3985M33R-share identical SOT23-5L pinout, electrical interface, and thermal characteristics. Only the output voltage differs; no layout or schematic changes are needed when substituting across the family for different rail requirements.