STMicroelectronics LD3985M28R

Overview

LD3985M28R from STMicroelectronics is a 2.8 V fixed-output, ultra-low-dropout linear regulator (LDO) with 150 mA output capability, 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 and is optimized for battery-powered mobile phones and wireless systems requiring stable low-noise biasing.

For engineers reviewing the LD3985M28R datasheet, LD3985M28R pinout, LD3985M28R application, or LD3985M28R equivalent, key selection criteria include dropout voltage under light/heavy load, shutdown quiescent current (<1.5 µA), ceramic capacitor compatibility (≥1 µF, ESR ≤5 Ω), thermal shutdown threshold (160 °C), and supply voltage rejection (60 dB @ 1 kHz).

Technical Context

The LD3985M28R uses BiCMOS process technology to achieve ultra-low quiescent current (85 µA typ. at no load) while maintaining fast transient response and high PSRR. Its internal architecture integrates a precision bandgap reference, error amplifier, pass transistor, and thermal/current protection circuitry - all designed for operation across –40 °C to +125 °C junction temperature.

It features dual-capacitor stabilization: a 1 µF output capacitor (CO) for regulation stability and a dedicated 10 nF bypass capacitor (CBYP) on the BYPASS pin to suppress high-frequency noise. The inhibit (VINH) input enables precise system-level power sequencing with TTL/CMOS-compatible thresholds (ON ≥1.2 V, OFF ≤0.4 V).

Key Specifications

Pinout & Package

LD3985M28R is housed in a compact SOT23-5L package (2.95 mm × 1.60 mm × 1.30 mm), optimized for space-constrained portable designs and compatible with standard reflow processes.

Key Features

Applications

Equivalent & Alternatives

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

Compared with XC6206P282MR and BD3985FP-E2, LD3985M28R uniquely combines ultra-low dropout (60 mV), industry-leading low-noise performance (30 µV_RMS), and a dedicated BYPASS pin for PSRR enhancement - making it optimal for RF and precision analog subsystems where both efficiency and spectral purity are critical.

Availability

LD3985M28R is available at Aetrix Electronics and suitable for mobile phone power management, wearable sensor node biasing, and industrial IoT edge controllers requiring stable component supply, long-term lifecycle support, and AEC-Q100-compliant alternatives.

Supply support for LD3985M28R 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 devices for automotive, industrial, 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 extended runtime, clean power, and small form factor are essential.

FAQ

What is the minimum output capacitor value required for LD3985M28R stability?

The LD3985M28R requires a minimum 1 µF ceramic output capacitor (CO) with ESR between 5 mΩ and 5 Ω. Stability is maintained across temperature and tolerance variations, including ±30% capacitance deviation. X7R or X5R dielectrics are recommended; no external compensation network is needed.

Can LD3985M28R operate with an input voltage below 2.5 V?

No. The absolute minimum operating input voltage is 2.5 V per datasheet specifications. Operation below this level is outside guaranteed parameters and may result in unregulated output, increased dropout, or failure to maintain regulation - especially under load or elevated temperature.

Is the BYPASS pin mandatory for proper operation?

Yes. The BYPASS pin must be connected to a 10 nF ceramic capacitor to ground. Omitting this capacitor degrades high-frequency PSRR by up to 20 dB and increases output noise above 10 kHz, compromising performance in RF or high-speed digital applications.

How does the thermal shutdown behave during continuous overload?

LD3985M28R activates thermal shutdown at 160 °C junction temperature and remains latched off until junction cools by ~20 °C (hysteresis). During overload, it cycles between regulation and shutdown - limiting average power dissipation and preventing permanent damage without requiring external reset circuitry.