STMicroelectronics LD3985M33R

Overview

LD3985M33R 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. It operates from 2.5 V to 6 V input, supports low-ESR ceramic capacitors, and targets power-sensitive battery-operated systems such as mobile phone baseband ICs and wireless sensor nodes.

For engineers reviewing the LD3985M33R datasheet, LD3985M33R pinout, LD3985M33R application, or LD3985M33R equivalent, key selection criteria include dropout voltage under light/heavy load, shutdown quiescent current (<1.5 µA), PSRR performance (60 dB @ 1 kHz), thermal shutdown threshold (160 °C), and stability with 1 µF output capacitance.

Technical Context

The LD3985M33R uses BiCMOS process technology to achieve ultra-low quiescent current (85 µA typ. at no load) while maintaining fast transient response and high PSRR across frequency. Its internal architecture integrates a precision bandgap reference, error amplifier, and PMOS pass transistor with dedicated bypass pin for noise filtering.

It implements logic-controlled enable via VINH pin (1.2 V threshold), thermal shutdown with 20 °C hysteresis, and internal current limiting (600 mA short-circuit peak). The device remains stable with 1 µF ±30% X7R ceramic output capacitor and requires only 10 nF on BYPASS for optimal noise suppression.

Key Specifications

Pinout & Package

LD3985M33R is housed in a RoHS-compliant, ECOPACK®-qualified SOT23-5L package (2.95 mm × 1.60 mm × 1.30 mm), optimized for space-constrained portable PCB layouts and reflow-compatible assembly.

Key Features

Applications

Equivalent & Alternatives

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

Compared with XC6206P332MR and ADP160AUJZ-3.3-R7, the LD3985M33R uniquely balances ultra-low dropout (60 mV), sub-µA shutdown, and integrated BYPASS filtering - making it optimal for space- and battery-constrained RF/analog systems where both efficiency and spectral purity are critical.

Availability

LD3985M33R is available at Aetrix Electronics and suitable for mobile communications, industrial IoT edge nodes, and medical wearables requiring stable component supply, long-term lifecycle support, and traceable sourcing.

Supply support for LD3985M33R 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 belongs to ST's ultra-low-noise, low-quiescent-current LDO family targeting battery-powered portable electronics - engineered specifically for RF subsystems, precision analog, and always-on sensor applications.

FAQ

What is the minimum input voltage required for regulation at 3.3 V output?

The LD3985M33R maintains regulation down to an input voltage of 3.36 V (3.3 V + 60 mV dropout) at 150 mA load. At lighter loads, dropout falls to 0.4 mV - enabling operation from 3.3004 V input at 1 mA. Input below 2.5 V is outside absolute maximum ratings and not supported.

Can the BYPASS pin be left unconnected?

No. The BYPASS pin must be connected to a 10 nF capacitor to GND. Leaving it floating degrades PSRR by up to 20 dB and increases output noise by >2×. The capacitor provides AC shunt path for internal reference noise and improves high-frequency rejection - omitting it violates the stable operating condition defined in the datasheet.

Is an external pull-down required on the VINH pin?

Yes. VINH has no internal pull-down; leaving it floating risks undefined enable state and potential shoot-through or oscillation. A 100 kΩ–1 MΩ resistor to GND ensures reliable OFF-state behavior when the enable signal is inactive or disconnected - critical for predictable power sequencing in multi-rail systems.

Does the LD3985M33R require a minimum load current to maintain regulation?

No. The LD3985M33R regulates stably from 0 mA to 150 mA output current. Quiescent current rises only slightly in dropout (not proportional to load), and line/load regulation specs are guaranteed across the full 0–150 mA range - eliminating need for dummy loads in low-power sleep modes.