STMicroelectronics LD3985G18R

Overview

LD3985G18R 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, supports low-ESR ceramic capacitors, and targets power-sensitive applications including mobile baseband processors and RF transceivers.

For engineers reviewing the LD3985G18R datasheet, LD3985G18R pinout, LD3985G18R application, or LD3985G18R equivalent, key selection criteria include dropout voltage under light/heavy load, quiescent current in ON/OFF modes, PSRR performance at 1–10 kHz, thermal shutdown threshold, and bypass capacitor dependency for noise suppression.

Technical Context

The LD3985G18R employs a BiCMOS architecture to achieve ultra-low quiescent current (85 µA typ. at no load) while maintaining fast transient response and high PSRR (60 dB @ 1 kHz). Its internal reference and error amplifier are optimized for stability with 1 µF output capacitance and 10 nF bypass capacitor.

Shutdown is controlled via VINH pin with precise logic thresholds (≤0.4 V for OFF, ≥1.2 V for ON), and thermal protection activates at 160 °C with 20 °C hysteresis. Dropout behavior is linearly scalable-0.4 mV typ. at 1 mA, rising to 60 mV typ. at 150 mA-enabling extended battery runtime in brownout conditions.

Key Specifications

Pinout & Package

SOT23-5L package: 5-pin surface-mount, 2.95 mm × 1.60 mm footprint, 1.45 mm height, thermal resistance R_thJA = 255 °C/W.

Key Features

Applications

Equivalent & Alternatives

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

Compared with XC6206P182MR and AP2204K-18, the LD3985G18R uniquely combines ultra-low dropout (60 mV), industry-leading 30 µV_RMS noise with BYPASS control, and sub-100 µA quiescent current-making it optimal for noise-critical, battery-constrained applications where input voltage margin is tight.

Availability

LD3985G18R is available at Aetrix Electronics and suitable for mobile communications, wearable health monitors, and automotive body electronics requiring stable component supply across extended temperature ranges and long production lifecycles.

Supply support for LD3985G18R 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 LD3985 belongs to ST's ultra-low-noise, low-quiescent-current LDO family, engineered specifically for battery-powered portable electronics where efficiency, noise immunity, and thermal robustness are design-critical.

FAQ

What is the minimum output capacitor required for stability?

The LD3985G18R requires a minimum 1 µF ceramic output capacitor (X7R or NP0 dielectric) with ESR between 5 mΩ and 5 Ω. Stability is maintained across temperature and tolerance variations, including ±30% capacitor value drift. Using smaller values risks oscillation; larger values improve transient response but add cost and board area.

Can the BYPASS pin be left unconnected?

No. The BYPASS pin must be connected to a 10 nF ceramic capacitor referenced to GND. Leaving it floating degrades PSRR above 1 kHz and increases output noise by up to 2×. The capacitor provides a low-impedance AC path for internal reference noise, directly enabling the specified 30 µV_RMS performance.

How does thermal shutdown behave during overload?

Thermal shutdown activates at 160 °C junction temperature with ~20 °C hysteresis. When triggered, the LDO disables output and draws <1.5 µA. Recovery occurs only after junction cools below ~140 °C, preventing thermal cycling. This protects against sustained short-circuit or excessive ambient + power dissipation combinations.

Is the LD3985G18R AEC-Q100 qualified?

The LD3985G18R is not AEC-Q100 qualified. However, its -40 °C to +125 °C operating junction temperature range, internal thermal shutdown, and robust ESD protection (±2 kV HBM) make it suitable for non-safety-critical automotive body electronics (e.g., door modules, lighting controllers) where formal qualification is not mandated.