STMicroelectronics LD3985G33R

Overview

LD3985G33R from STMicroelectronics is a 3.3 V, 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 with <1.5 µA OFF-mode current. It operates from 2.5 V to 6 V input and is optimized for battery-powered RF and sensor subsystems requiring stable low-noise bias.

For engineers reviewing the LD3985G33R datasheet, LD3985G33R pinout, LD3985G33R application, or LD3985G33R equivalent, key selection criteria include dropout voltage under light/heavy load, PSRR performance at 1–10 kHz, thermal shutdown threshold, bypass capacitor dependency, and compatibility with 1 µF ceramic output capacitors without external compensation.

Technical Context

The LD3985G33R uses a BiCMOS architecture to achieve ultra-low quiescent current (85 µA typ. at no load) while maintaining fast turn-on (200 µs typ.) and high PSRR (60 dB @ 1 kHz). Its internal reference and error amplifier are designed for stability with low-ESR ceramic capacitors down to 1 µF.

Shutdown is controlled via VINH pin with precise logic thresholds (ON ≥1.2 V, OFF ≤0.4 V), and the BYPASS pin enables noise reduction via a dedicated 10 nF capacitor. Thermal protection activates at 160 °C with ~20 °C hysteresis, and dropout is defined as the VI–VO difference when VO drops 100 mV below nominal.

Key Specifications

Pinout & Package

SOT23-5L surface-mount package (1.45 mm × 1.30 mm × 0.95 mm height), RoHS-compliant, ECOPACK2 qualified, with exposed pad not connected internally.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar low-noise, logic-enabled LDO applications.

Compared with XC6206P332MR and MIC5205-3.3YM5, the LD3985G33R delivers superior noise performance, lower dropout, and integrated logic shutdown - making it optimal for battery-sensitive, RF-critical designs where BYPASS-capacitor-based noise suppression is required.

Availability

LD3985G33R is available at Aetrix Electronics and suitable for mobile RF front-ends, wearable sensor nodes, and industrial IoT edge controllers requiring stable component supply, long-lifecycle support, and guaranteed parametric compliance.

Supply support for LD3985G33R 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 analog, MCU, power, and sensor solutions for automotive, industrial, and consumer markets.

The LD3985 belongs to ST's ultra-low-noise LDO family, engineered specifically for power-sensitive portable and wireless applications where battery life, RF cleanliness, and thermal robustness are critical design constraints.

FAQ

What is the minimum output capacitor value required for stability?

The LD3985G33R is stable with a minimum 1 µF ceramic output capacitor (X7R or X5R dielectric), and remains stable even with ±30% capacitance tolerance across temperature. ESR must be between 5 mΩ and 5000 mΩ; typical recommended value is ≤5 Ω. No external compensation network is needed.

Can the BYPASS pin be left unconnected?

No - the BYPASS pin must be connected to a 10 nF ceramic capacitor to ground. Leaving it floating degrades output noise performance and may cause instability. The capacitor provides AC bypass for the internal reference, reducing 30 µV_RMS noise to its specified value; omitting it increases noise significantly.

What happens if VINH is left floating?

VINH must never be left floating - the pin has no internal pull-up or pull-down. An unconnected VINH causes unpredictable ON/OFF behavior and potential shoot-through or oscillation. It must be driven actively to ≥1.2 V for ON or ≤0.4 V for OFF, or tied via resistor to VI or GND if fixed mode is required.

Is the LD3985G33R qualified for automotive applications?

The LD3985G33R is not AEC-Q200 qualified. While it operates from -40 °C to +125 °C junction temperature and includes thermal shutdown, it lacks automotive-specific qualification testing (e.g., HTOL, ESD, vibration). For automotive cabin modules, designers should verify system-level qualification and consider ST's AEC-Q200 LDO portfolio (e.g., LD39050 series) instead.